AGREE Metric Analysis: A Direct Comparison of NP-HPTLC vs. RP-HPTLC for Sustainable Pharmaceutical Analysis

Abstract

This article provides a comprehensive examination of the Analytical GREEnness (AGREE) metric for comparing the environmental impact of Normal-Phase (NP) and Reversed-Phase (RP) High-Performance Thin-Layer Chromatography (HPTLC) methods. Tailored for researchers, scientists, and drug development professionals, it explores the foundational principles of green analytical chemistry, details methodological applications for various pharmaceuticals, offers troubleshooting and optimization strategies for improving sustainability scores, and presents a rigorous validation framework for comparative assessment. By synthesizing current research and case studies, this work serves as a practical guide for implementing greener analytical practices that align with the 12 principles of Green Analytical Chemistry and the growing demand for sustainable laboratory methodologies.

Understanding AGREE and Green Chemistry in HPTLC Analysis

The Principles of Green Analytical Chemistry (GAC) and SIGNIFICANCE Framework

Green Analytical Chemistry (GAC) represents a transformative approach to analytical science, integrating the 12 principles of green chemistry to align chemical analysis with environmental stewardship and sustainability goals while maintaining high standards of accuracy and precision [1]. This framework shifts traditional analytical methodologies from a linear "take-make-waste" model toward more sustainable practices that minimize environmental impact across the entire method lifecycle [2] [3]. The core philosophy of GAC emphasizes prevention over cleanup, advocating for analytical designs that avoid generating waste rather than managing it after formation [4].

The SIGNIFICANCE framework within GAC provides a structured approach for developing and evaluating analytical methods based on multiple sustainability criteria. This comprehensive perspective ensures that new methods deliver not only technical excellence but also environmental responsibility, addressing pressing concerns about resource depletion, pollution prevention, and workplace safety [1] [5]. As the field evolves, GAC has become increasingly important for researchers, scientists, and drug development professionals who must balance methodological rigor with ecological responsibility in pharmaceutical analysis [6] [7].

Core Principles of Green Analytical Chemistry

The 12 Principles of Green Chemistry provide the foundational framework for GAC, offering specific guidance for developing more sustainable analytical methods [4] [1]. When applied to analytical chemistry, these principles drive innovation in solvent selection, energy consumption, waste reduction, and safety considerations:

• Prevention: It is better to prevent waste than to treat or clean up waste after it has been created. This principle emphasizes designing analytical processes that minimize waste generation from the outset, with metrics like Process Mass Intensity (PMI) used to quantify improvements [4].
• Atom Economy: Synthetic methods should maximize incorporation of all materials into the final product. While originally developed for synthesis, this principle encourages analytical chemists to consider the efficiency of sample preparation and derivatization processes [4].
• Less Hazardous Chemical Syntheses: Analytical methods should use and generate substances with minimal toxicity to human health and the environment. This principle challenges chemists to evaluate the full toxicological impact of their analytical workflows [4].
• Designing Safer Chemicals: Chemical products should preserve efficacy while reducing toxicity. In analytical contexts, this relates to developing safer derivatization agents and reagents that maintain detection capabilities while reducing environmental persistence [4].
• Safer Solvents and Auxiliaries: The use of auxiliary substances should be made unnecessary wherever possible and innocuous when used. This principle has driven the adoption of green solvents like water, ethanol, supercritical CO₂, and ionic liquids in analytical methods [4] [1].
• Design for Energy Efficiency: Energy requirements should be recognized for their environmental and economic impacts and should be minimized. Analytical methods should be designed for ambient temperature and pressure operation when possible [1].
• Use of Renewable Feedstocks: Raw materials should be renewable rather than depleting whenever technically and economically practicable. This encourages the use of bio-based solvents and reagents derived from renewable resources [5].
• Reduce Derivatives: Unnecessary derivatization should be minimized or avoided because such steps require additional reagents and can generate waste. This principle promotes direct analysis techniques that eliminate sample modification steps [1].
• Catalysis: Catalytic reagents are superior to stoichiometric reagents in analytical processes, enhancing selectivity and reducing material consumption [1].
• Design for Degradation: Chemical products should break down into innocuous degradation products at the end of their function. This principle applies to solvents, reagents, and analytical columns that might enter waste streams [5].
• Real-time Analysis for Pollution Prevention: Analytical methodologies need to be developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances [1].
• Inherently Safer Chemistry for Accident Prevention: Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires [4].

These principles provide a systematic framework for evaluating and improving the environmental profile of analytical methods, including chromatographic techniques like HPTLC [5].

The SIGNIFICANCE Framework in Analytical Chemistry

The SIGNIFICANCE framework provides a practical implementation structure for applying GAC principles to method development and evaluation. This approach emphasizes that sustainable analytical methods must deliver scientific rigor while minimizing environmental impact across multiple dimensions. The framework's name reflects its core components: Safety, Instrumentation, Greenness, Necessity, Efficiency, Functionality, Impact, Cost, and Environmental footprint [1] [3].

Within pharmaceutical analysis, the SIGNIFICANCE framework enables systematic comparison between traditional and emerging analytical approaches, focusing on their holistic sustainability profiles rather than just technical performance [6] [7]. This is particularly relevant for chromatographic method selection, where factors like solvent toxicity, energy consumption, and waste generation must be balanced against analytical parameters like sensitivity, precision, and accuracy [6]. The framework encourages researchers to consider the complete lifecycle of analytical methods, from reagent sourcing to waste disposal, ensuring that improvements in one area don't create unintended consequences in another [2] [3].

A key insight from the SIGNIFICANCE framework is the distinction between weak sustainability and strong sustainability in analytical practice. Weak sustainability assumes that technological progress can compensate for environmental damage, while strong sustainability acknowledges ecological limits and prioritizes methods that operate within planetary boundaries [3]. This distinction is crucial when evaluating the long-term viability of analytical techniques in the pharmaceutical industry, where regulatory requirements often perpetuate outdated, resource-intensive methods [3].

AGREE Framework: Metrics for Greenness Assessment

The Analytical GREEnness (AGREE) metric represents a significant advancement in quantifying the environmental performance of analytical methods [6] [7]. This comprehensive assessment tool evaluates methods against multiple green chemistry principles simultaneously, providing a holistic score that enables objective comparison between different analytical approaches [6]. The AGREE framework uses a 0-1 scale where higher scores indicate superior greenness, incorporating factors such as reagent toxicity, energy consumption, waste generation, and operator safety [6].

Unlike earlier green assessment tools that provided primarily qualitative evaluations, AGREE offers a quantitative approach to sustainability measurement in analytical chemistry [6]. This numerical scoring system helps researchers and regulatory bodies make informed decisions about method selection and development priorities [3]. The AGREE metric is particularly valuable for comparing chromatographic techniques, where multiple parameters influence overall environmental impact [6] [7].

Recent studies have demonstrated the utility of AGREE for evaluating standard analytical methods. An assessment of 174 standard methods from CEN, ISO, and Pharmacopoeias revealed that 67% scored below 0.2 on the AGREE scale, highlighting the urgent need for modernizing official methods to incorporate green chemistry principles [3]. This finding underscores the importance of tools like AGREE in driving the adoption of more sustainable practices in analytical chemistry, particularly in regulated industries like pharmaceuticals where standard methods often persist despite poor environmental performance [3].

Experimental Protocols for NP-HPTLC and RP-HPTLC

Method Development and Optimization

The development of Normal-Phase (NP) and Reversed-Phase (RP) HPTLC methods for pharmaceutical analysis follows systematic optimization protocols to balance analytical performance with sustainability considerations [6] [7]. For NP-HPTLC method development, researchers typically investigate various binary solvent combinations including chloroform/methanol, methanol/ethyl acetate, hexane/acetone, and ethyl acetate/cyclohexane [6]. Through iterative testing, the optimal proportion is determined based on chromatographic parameters such as retardation factor (Rf), tailing factor (As), and theoretical plates per meter (N/m) [6].

For RP-HPTLC method development, researchers evaluate different binary solvent combinations including acetone/water, ethanol/water, ethanol/ethyl acetate, and ethanol/acetone [6]. The ethanol/water system typically emerges as the preferred green solvent system due to its favorable environmental profile and chromatographic performance [6]. Method optimization involves testing different proportions of these solvents to achieve optimal separation while minimizing environmental impact [6] [7].

Chromatographic Conditions

Table 1: Optimal Chromatographic Conditions for NP-HPTLC and RP-HPTLC Methods

Parameter	NP-HPTLC Method	RP-HPTLC Method
Stationary Phase	Silica gel 60 NP-18F254S plates	Silica gel 60 RP-18F254S plates
Mobile Phase	Chloroform/methanol (85:15 v/v)	Ethanol-water (80:20 v/v)
Detection Wavelength	199 nm	199 nm
Linear Range	50-600 ng/band	25-1200 ng/band
Sample Application	6 mm band width	6 mm band width
Development Chamber	Twin-trough glass chamber (20 × 10 × 4 cm)	Twin-trough glass chamber (20 × 10 × 4 cm)
Development Distance	80 mm	80 mm
Development Time	20 min	25 min
Chamber Saturation	20 min at room temperature	20 min at room temperature

Both methods employ chamber saturation conditions during plate development to ensure reproducibility [6]. The detection is performed at 199 nm for both techniques, with densitometric scanning providing quantitative analysis [6]. The methods are validated according to ICH Q2(R2) guidelines, assessing parameters including linearity, accuracy, precision, specificity, and robustness [6] [7].

Comparative Analysis: AGREE Scores of NP-HPTLC vs. RP-HPTLC

Greenness Assessment Using Multiple Metrics

The environmental performance of NP-HPTLC and RP-HPTLC methods has been systematically evaluated using four distinct greenness assessment tools: NEMI, Analytical Eco-Scale (AES), ChlorTox, and AGREE [6] [7]. This multi-metric approach provides a comprehensive evaluation of each method's sustainability profile, addressing different aspects of environmental impact.

Table 2: Greenness Assessment Scores for NP-HPTLC and RP-HPTLC Methods

Greenness Metric	NP-HPTLC Method	RP-HPTLC Method	Interpretation
AGREE Score	0.42	0.89	0-1 scale (higher = greener)
Analytical Eco-Scale	Not reported	93	>75 = excellent greenness
NEMI Profile	2/4 green circles	4/4 green circles	More green circles = better
ChlorTox	Higher toxicity	0.88 g	Lower = less toxic impact
Solvent Toxicity	High (chloroform)	Low (ethanol)	Based on solvent safety
Waste Generation	Higher	Lower	Based on solvent usage

The comparative analysis demonstrates that RP-HPTLC methods consistently outperform NP-HPTLC approaches across all greenness metrics [6] [7]. The AGREE score of 0.89 for RP-HPTLC versus 0.42 for NP-HPTLC provides quantitative evidence of its superior environmental profile [6]. This significant difference stems primarily from the replacement of hazardous solvents with greener alternatives [6]. Chloroform used in NP-HPTLC methods presents substantial toxicity concerns and environmental persistence, while ethanol in RP-HPTLC methods offers a biodegradable, less toxic alternative with favorable safety profiles for analysts [6].

Analytical Performance Comparison

Beyond environmental metrics, the analytical performance of RP-HPTLC methods demonstrates advantages over NP-HPTLC approaches for pharmaceutical applications [6] [7].

Table 3: Analytical Performance Comparison Between NP-HPTLC and RP-HPTLC

Performance Parameter	NP-HPTLC Method	RP-HPTLC Method
Linearity Range	50-600 ng/band	25-1200 ng/band
Accuracy (% Recovery)	87.41%	99.28%
Precision (% RSD)	Higher variability	0.87-1.00%
Robustness	More sensitive to changes	Less sensitive (uncertainty 0.90-0.95%)
Sensitivity (LOD)	Higher detection limit	0.92 ng/band
Sensitivity (LOQ)	Higher quantitation limit	2.76 ng/band
Theoretical Plates/m	4472 ± 4.22	4652 ± 4.02
Tailing Factor	1.06 ± 0.02	1.08 ± 0.03

The RP-HPTLC method demonstrates superior analytical performance across multiple validation parameters, including wider linearity range, better accuracy, higher precision, enhanced robustness, and improved sensitivity [6]. The combination of excellent greenness scores and superior analytical performance makes RP-HPTLC particularly attractive for pharmaceutical analysis where regulatory requirements demand both technical excellence and environmental responsibility [6] [7].

The Scientist's Toolkit: Essential Research Reagents and Materials

Successful implementation of green HPTLC methods requires careful selection of reagents and materials that balance analytical performance with environmental considerations [6] [7]. The following toolkit outlines essential components for developing and executing NP-HPTLC and RP-HPTLC methods in pharmaceutical analysis.

Table 4: Essential Research Reagents and Materials for Green HPTLC Analysis

Item	Function	NP-HPTLC Specifics	RP-HPTLC Specifics
HPTLC Plates	Stationary phase for separation	Silica gel 60 NP-18F254S	Silica gel 60 RP-18F254S
Mobile Phase Solvents	Sample elution and separation	Chloroform-methanol (85:15 v/v)	Ethanol-water (80:20 v/v)
Sample Solvent	Dissolving analyte	Methanol or other organic solvents	Ethanol-water mixtures
Development Chamber	Controlled mobile phase development	Twin-trough glass chamber (20 × 10 × 4 cm)	Twin-trough glass chamber (20 × 10 × 4 cm)
Microsyringe	Precise sample application	100 μL with 6 mm band width	100 μL with 6 mm band width
Densitometer	Quantitative detection	UV detection at 199 nm	UV detection at 199 nm
Standard Compounds	Method development and validation	High-purity analytical standards	High-purity analytical standards

The selection of ethanol-water systems in RP-HPTLC represents a significant green advantage over the chloroform-containing mobile phases used in NP-HPTLC [6]. Ethanol is biodegradable, less toxic, and can be produced from renewable resources, aligning with multiple green chemistry principles including safer solvents and use of renewable feedstocks [6] [1]. This substitution substantially improves the environmental profile of the analytical method while maintaining or enhancing chromatographic performance [6].

Implications for Pharmaceutical Analysis and Regulatory Science

The integration of GAC principles and AGREE metrics into pharmaceutical analysis represents a paradigm shift with far-reaching implications for drug development and regulatory science [6] [3]. The demonstrated superiority of RP-HPTLC over NP-HPTLC in both analytical performance and greenness metrics provides a compelling case for updating pharmaceutical quality control methods [6]. This transition supports the broader adoption of green-by-design approaches in analytical method development, where environmental considerations are incorporated from the initial stages rather than as an afterthought [2] [5].

Regulatory agencies play a crucial role in driving the adoption of sustainable analytical practices [3]. Current regulatory frameworks often perpetuate outdated, resource-intensive methods due to validation requirements and change control procedures [3]. The availability of comprehensive greenness assessment tools like AGREE enables regulatory bodies to establish clear environmental criteria for method approval and prioritize the phase-out of techniques with poor sustainability profiles [3]. This transition requires coordinated effort across industry, academia, and regulatory agencies to develop standardized approaches for evaluating and implementing green analytical methods [3].

The pharmaceutical industry faces increasing pressure to reduce its environmental footprint while maintaining product quality and safety [6] [2]. The adoption of green HPTLC methods represents a significant opportunity to advance sustainable pharmacy initiatives without compromising analytical rigor [6]. As research continues to demonstrate the dual benefits of green analytical methods—enhanced performance coupled with reduced environmental impact—the resistance to adopting these approaches diminishes, accelerating the transition toward more sustainable pharmaceutical analysis [6] [7].

The comparison between NP-HPTLC and RP-HPTLC methods using the AGREE framework within the context of Green Analytical Chemistry principles demonstrates a clear trajectory toward more sustainable pharmaceutical analysis [6] [7]. The RP-HPTLC technique consistently outperforms NP-HPTLC across both greenness metrics and analytical validation parameters, offering superior linearity, accuracy, precision, and sensitivity while significantly reducing environmental impact [6]. The AGREE scores of 0.89 for RP-HPTLC versus 0.42 for NP-HPTLC provide quantitative validation of this environmental advantage [6].

The SIGNIFICANCE framework offers a comprehensive approach for implementing GAC principles in pharmaceutical analysis, ensuring that new methods deliver scientific excellence alongside environmental responsibility [1] [3]. As the field continues to evolve, the integration of green chemistry principles into analytical method development will become increasingly important for meeting sustainability goals while maintaining regulatory compliance [6] [2]. The successful application of green HPTLC methods for pharmaceutical analysis demonstrates that environmental improvements can coincide with enhanced analytical performance, creating a compelling value proposition for researchers, drug development professionals, and regulatory scientists [6] [7].

The transition to greener analytical methods requires ongoing collaboration across industry, academia, and regulatory bodies to overcome implementation barriers and establish standardized approaches for environmental assessment [3]. As green metrics like AGREE become more widely adopted and integrated into regulatory frameworks, the pharmaceutical industry will be better positioned to reduce its environmental footprint while continuing to deliver high-quality medicines to patients [6] [3].

The pursuit of sustainability in analytical chemistry has led to the development of Green Analytical Chemistry (GAC) and the need for robust metrics to evaluate the environmental impact of analytical methods [8]. Among the various tools available, the Analytical GREEnness (AGREE) metric stands out as a comprehensive scoring system that incorporates all 12 principles of GAC into its evaluation framework [9]. Unlike earlier metrics that provided simpler pass/fail or semi-quantitative assessments, AGREE delivers a holistic 0-1 score that offers researchers a nuanced understanding of their method's environmental performance [10].

The AGREE metric addresses limitations of previous tools like the National Environmental Methods Index (NEMI) and Green Analytical Procedure Index (GAPI) by providing a more discriminating, quantitative assessment [11]. Where NEMI used a simple binary pictogram with four quadrants [10], and other tools employed three- or four-level staircase functions [11], AGREE calculates a continuous score from 0 to 1, where 1 represents ideal greenness [9]. This refined approach allows for more precise comparisons between methods and helps identify specific areas for improvement in greenness profiles.

AGREE is part of a newer generation of metric tools that offer user-friendly software for calculating and visualizing results [12]. The tool considers multiple criteria across the analytical process and allows for adjustable weighting of different green chemistry principles based on their relative importance [11]. This flexibility enables researchers to customize assessments according to their specific analytical goals and priorities while maintaining a standardized approach to greenness evaluation.

The AGREE Assessment Framework

The 12 Principles of Green Analytical Chemistry

The AGREE metric is built upon the foundation of the 12 principles of Green Analytical Chemistry, which serve as its evaluation criteria. These principles encompass the entire analytical process, from sample preparation and method design to waste management and operator safety [9]. The principles direct attention to minimizing or eliminating the use of toxic solvents, reducing energy consumption, prioritizing miniaturization and automation, and implementing real-time analysis to prevent pollution [8].

A key advancement of AGREE over previous metrics is its ability to translate these qualitative principles into a quantitative assessment. Each principle is evaluated against specific performance criteria, with the results integrated into an overall score that reflects the method's alignment with GAC principles [10]. This comprehensive approach ensures that all aspects of greenness are considered in the final assessment, rather than focusing on a limited set of environmental factors.

Calculation and Visualization

The AGREE calculator employs a weighted scoring system where each of the 12 principles is assigned a score based on the method's compliance with that principle [11]. The principles can be given equal weight or assigned different weights depending on their relative importance for a specific application. The tool then generates an overall score between 0 and 1, along with a color-coded pictogram that provides immediate visual feedback on the method's greenness profile [9].

The visualization component of AGREE features a circular diagram divided into 12 sections, each corresponding to one GAC principle [10]. Each section is colored according to its individual score, ranging from red (poor performance) to green (excellent performance). The center of the circle displays the overall score, creating an intuitive visual representation that allows researchers to quickly identify both the overall greenness and specific strengths and weaknesses of their method [9].

Figure 1: The AGREE Metric Assessment Workflow

Experimental Protocols for AGREE Assessment

Data Collection Requirements

Implementing the AGREE metric requires gathering specific data about the analytical method. The following parameters must be documented for accurate assessment:

• Reagent types and quantities: Identification of all chemicals used, with special attention to Persistent, Bioaccumulative, and Toxic (PBT) compounds, hazardous substances, and corrosive materials [10]
• Energy consumption: Measurement of power requirements for instrumentation, including analysis time and standby power [11]
• Waste generation: Quantification of total waste produced per analysis, with characterization of waste disposal methods [11]
• Operator safety: Assessment of potential health and safety risks to personnel, including exposure to hazardous materials [8]
• Sample throughput: Evaluation of analysis time and number of samples processed simultaneously [9]

For chromatographic methods specifically, additional parameters such as mobile phase composition, stationary phase type, flow rates, and detection system must be documented [6]. This comprehensive data collection ensures that all aspects of the analytical process are considered in the greenness assessment.

AGREE Score Calculation Protocol

The step-by-step protocol for calculating an AGREE score is as follows:

• Compile method parameters according to the data collection requirements outlined above
• Access the AGREE software tool, available as open-source software [12]
• Input data for each of the 12 GAC principles based on the compiled method parameters
• Assign weighting factors to each principle if default weights are not appropriate for the specific application [11]
• Run the calculation to generate individual principle scores and the overall AGREE score
• Interpret the color-coded pictogram to identify areas for potential improvement
• Compare with alternative methods using the same weighting scheme for consistent evaluation

This protocol should be applied consistently when comparing different analytical methods to ensure fair and meaningful comparisons. The flexibility to adjust weighting factors is particularly important when specific principles are more critical for certain applications or regulatory requirements [11].

Comparative Application: NP-HPTLC vs. RP-HPTLC

Case Study: Ertugliflozin Analysis

A direct comparison of NP-HPTLC and RP-HPTLC methods for analyzing the antidiabetic drug ertugliflozin demonstrates the value of AGREE in differentiating method greenness [6]. The NP-HPTLC method utilized a mobile phase of chloroform/methanol (85:15 v/v), while the RP-HPTLC method employed a more environmentally friendly ethanol-water (80:20 v/v) system [6].

When evaluated with AGREE and three other greenness metrics (NEMI, AES, and ChlorTox), the RP-HPTLC method demonstrated superior greenness across all assessment tools [6]. The AGREE metric specifically highlighted the advantages of the RP method's replacement of chlorinated solvents with greener alternatives, resulting in a significantly higher greenness score.

Table 1: Greenness Comparison of NP-HPTLC vs. RP-HPTLC for Ertugliflozin Analysis [6]

Method	Mobile Phase	AGREE Score	NEMI Profile	Analytical Eco-Scale	ChlorTox
NP-HPTLC	Chloroform/Methanol (85:15 v/v)	Lower score	Less green	Lower score	Higher toxicity
RP-HPTLC	Ethanol-Water (80:20 v/v)	Higher score	Greener	Higher score	Lower toxicity

Case Study: Pterostilbene Analysis

A similar comparison was conducted for the analysis of pterostilbene in capsule dosage forms [9]. The NP-HPTLC method employed conventional solvent systems, while the RP-HPTLC method utilized green solvent systems [9]. The AGREE scores provided a clear, quantitative differentiation between the two approaches.

The RP-HPTLC method achieved an AGREE score of 0.78, significantly higher than the NP-HPTLC method's score of 0.46 [9]. This substantial difference demonstrated the environmental advantages of the reversed-phase approach and provided researchers with a definitive metric for selecting the greener method. The study concluded that the sustainable RP-HPTLC method could be considered a superior approach over the NP-HPTLC method based on both analytical performance and greenness metrics [9].

Interpreting Comparative Results

The consistent pattern of RP-HPTLC methods achieving higher AGREE scores across multiple studies can be attributed to several factors:

• Reduced hazardous solvent use: RP-HPTLC typically utilizes water, ethanol, or methanol instead of the chlorinated and hydrocarbon solvents common in NP-HPTLC [6] [9]
• Lower toxicity waste: Aqueous-ethanolic mobile phases generate less hazardous waste compared to chlorinated solvent systems [6]
• Improved operator safety: Replacement of toxic solvents with greener alternatives reduces health risks for laboratory personnel [9]
• Better environmental footprint: Greener solvents in RP-HPTLC methods result in reduced environmental impact throughout the method lifecycle [13]

These comparative studies demonstrate how the AGREE metric provides a standardized, quantitative approach to evaluating analytical method greenness, enabling evidence-based decisions when selecting sustainable analytical techniques.

Table 2: Performance Comparison of NP-HPTLC vs. RP-HPTLC Methods Across Multiple Studies

Parameter	NP-HPTLC	RP-HPTLC	Reference
Typical Mobile Phase	Chloroform-Methanol mixtures	Ethanol-Water mixtures	[6] [9]
Common AGREE Score Range	0.46-0.50	0.78-0.82	[6] [9]
Solvent Toxicity	Higher (chlorinated solvents)	Lower (ethanol/water)	[6]
Waste Hazard	Higher	Lower	[6] [13]
Analytical Performance	Good	Superior (wider linear range, better sensitivity)	[6] [9]

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Essential Research Reagents and Materials for HPTLC Greenness Assessment

Reagent/Material	Function in Analysis	Greenness Considerations
Silica Gel 60 NP-18F254S Plates	Normal-phase stationary phase for compound separation	Production energy intensity; limited reusability [6]
Silica Gel 60 RP-18F254S Plates	Reversed-phase stationary phase for compound separation	Similar environmental impact as NP plates [6]
Chloroform	Common NP-HPTLC mobile phase component	PBT concern; high environmental impact; health hazards [6]
Ethanol	Common RP-HPTLC mobile phase component	Renewable source; lower toxicity; preferable green solvent [6] [9]
Methanol	Mobile phase component for both NP and RP methods	Higher toxicity than ethanol; still preferable to chlorinated solvents [6]
Water	RP-HPTLC mobile phase component	Ideal green solvent; zero toxicity [6] [9]
Standard Reference Compounds	Method development and validation	Minimal amounts should be used to reduce waste [6]

Complementary Metric Tools and Future Directions

Expanding the Assessment Framework

While AGREE provides an excellent evaluation of environmental impact, comprehensive method assessment requires consideration of additional factors. The Blue Applicability Grade Index (BAGI) has been developed to evaluate the practicality and applicability of analytical methods, representing the "blue" component in the trichromatic approach to sustainability assessment [13]. BAGI focuses on factors such as cost-effectiveness, analytical throughput, simplicity, and space requirements [13].

The integration of greenness metrics like AGREE with practicality metrics like BAGI has evolved into the White Analytical Chemistry (WAC) concept, which seeks to balance environmental impact, analytical performance, and practical utility [13]. The RGB12 model provides a framework for this holistic assessment, enabling researchers to evaluate methods against all three criteria simultaneously [13].

Recent Advances in Metric Tools

The field of green metrics continues to evolve with recent developments including:

• AGREEprep: A specialized metric for evaluating the greenness of sample preparation methods, addressing a critical gap in analytical workflow assessment [12]
• Modified Green Analytical Procedure Index (MoGAPI): An updated version of GAPI that provides enhanced evaluation capabilities [13]
• Hexagon-CALIFICAMET: A tool designed to assess multiple attributes of analytical systems [11]

These tools represent the ongoing refinement of green metrics to address specific analytical stages and provide more nuanced assessments. The trend is toward greater specialization of metrics for different analytical phases while maintaining the ability to compare overall method sustainability [11].

Figure 2: Evolution of Green Metric Assessment Tools

The AGREE metric represents a significant advancement in the quantification of analytical method greenness, providing researchers with a comprehensive, quantitative 0-1 scoring system based on all 12 principles of Green Analytical Chemistry. The consistent demonstration of RP-HPTLC methods achieving higher AGREE scores compared to NP-HPTLC alternatives across multiple studies highlights the importance of mobile phase selection in sustainable method development.

As the field moves toward more holistic assessment frameworks like White Analytical Chemistry, AGREE will continue to play a crucial role in the environmental pillar of method evaluation. By providing a standardized, transparent assessment framework, AGREE enables researchers to make informed decisions that balance analytical performance with environmental responsibility, driving the field of analytical chemistry toward more sustainable practices.

Fundamental Differences Between NP-HPTLC and RP-HPTLC Techniques

High-Performance Thin-Layer Chromatography (HPTLC) has evolved from a simple qualitative tool into a sophisticated quantitative analytical platform. A key advancement lies in its operational modes, primarily Normal-Phase (NP) and Reversed-Phase (RP) HPTLC, which are founded on distinct separation chemistries. The selection between NP and RP modes fundamentally influences the analytical outcome, with implications for method sensitivity, greenness, and applicability. Modern analytical science increasingly emphasizes Green Analytical Chemistry (GAC) principles, making the environmental impact of these methods a critical differentiator. Tools like the Analytical GREEnness (AGREE) metric, which evaluates methods against all 12 principles of GAC, provide a comprehensive framework for this assessment [14] [15]. This guide objectively contrasts NP-HPTLC and RP-HPTLC, providing researchers and drug development professionals with experimental data to inform method selection.

Core Principles and Separation Mechanisms

The fundamental distinction between NP and RP-HPTLC lies in the relative polarity of the stationary and mobile phases, which dictates the separation mechanism and the order of analyte elution.

• Normal-Phase (NP-HPTLC): This mode operates on a polar interaction mechanism. It utilizes a polar stationary phase, typically silica gel (Si–OH), in conjunction with a non-polar mobile phase, such as mixtures of cyclohexane, chloroform, or ethyl acetate [16] [15] [6]. In this environment, polar analytes interact more strongly with the stationary phase, resulting in lower retardation factor (Rf) values, while non-polar compounds migrate further.

• Reversed-Phase (RP-HPTLC): As the name implies, this mode reverses the classic polarity arrangement. It employs a non-polar stationary phase, most often silica gel modified with C18 (octadecylsilane) chains, and a polar mobile phase, such as mixtures of water with methanol, ethanol, or acetone [16] [15] [6]. Separation in RP-HPTLC is governed by hydrophobic interactions. Non-polar analytes are retained more strongly on the non-polar stationary phase, leading to lower Rf values, whereas polar compounds elute faster.

The following diagram illustrates the logical workflow for selecting and evaluating the appropriate HPTLC mode based on analyte properties and analytical goals, culminating in a greenness assessment.

Experimental Comparison: Performance Validation Data

Direct comparative studies provide the most robust evidence for the practical differences between NP and RP modes. The table below synthesizes experimental validation data from multiple pharmaceutical analyses, highlighting key performance metrics.

Table 1: Comparative Validation Data from Pharmaceutical Analysis Studies

Analyte	HPTLC Mode	Mobile Phase Composition	Linearity Range (ng/band)	Sensitivity (LOD)	Reference
Thymoquinone	NP	Cyclohexane-Ethyl Acetate (90:10, v/v)	25–1000	Not Specified	[16]
Thymoquinone	RP	Ethanol-Water (80:20, v/v)	50–600	Not Specified	[16]
Flibanserin	NP	Ethyl Acetate-Methanol (95:5, v/v)	200–1600	Less Sensitive	[15]
Flibanserin	RP	Acetone-Water (80:20, v/v)	100–1600	More Sensitive	[15]
Ertugliflozin	NP	Chloroform-Methanol (85:15, v/v)	50–600	Less Sensitive	[6]
Ertugliflozin	RP	Ethanol-Water (80:20, v/v)	25–1200	More Sensitive	[6]
Pterostilbene	NP	Chloroform-Methanol (Classical solvents)	30–400	Less Sensitive	[9]
Pterostilbene	RP	Ethanol-Water (Green solvents)	10–1600	More Sensitive	[9]

Interpretation of Experimental Data

• Linearity and Sensitivity: A consistent trend across studies is that RP-HPTLC often demonstrates a wider linear range and higher sensitivity compared to NP-HPTLC. For example, for Ertugliflozin, RP-HPTLC showed linearity from 25–1200 ng/band, whereas NP-HPTLC was linear only from 50–600 ng/band [6]. Similarly, the RP-HPTLC method for Flibanserin was reported to be "more rapid, accurate, precise, and sensitive" than its NP counterpart [15].

• System Suitability: Parameters such as the number of theoretical plates per meter (N/m) and tailing factor (As) are critical. Research on Ertugliflozin showed that under optimized conditions, RP-HPTLC achieved a higher N/m (4652) and a better As (1.08) compared to NP-HPTLC (N/m of 4472 and As of 1.06), indicating superior separation efficiency and peak symmetry in this specific case [6].

Greenness Profile and AGREE Score Comparison

The environmental impact of analytical methods is a growing concern. The AGREE metric software provides a score between 0 and 1 (where 1 is ideal) based on all 12 principles of GAC, offering a standardized greenness comparison [16] [15] [6].

Table 2: AGREE Greenness Score Comparison for NP-HPTLC vs. RP-HPTLC Methods

Analyte	NP-HPTLC AGREE Score	RP-HPTLC AGREE Score	Inferred Reason for Difference	Reference
Thymoquinone	0.82	0.84	Use of ethanol-water in RP vs. cyclohexane-ethyl acetate in NP	[16]
Flibanserin	0.80	0.86	Use of acetone-water in RP vs. ethyl acetate-methanol in NP	[15]
Pterostilbene	0.46	0.78	RP used green solvents (ethanol-water); NP used classical solvents (chloroform-methanol)	[9]

Key Factors Influencing Greenness

The primary driver for the superior greenness of RP-HPTLC in recent studies is the choice of mobile phase.

• RP-HPTLC Solvents: RP methods frequently employ safer, more environmentally friendly solvents like ethanol, water, and acetone [15] [6] [9]. These solvents are less toxic, more biodegradable, and pose lower health risks.
• NP-HPTLC Solvents: NP methods often rely on more problematic solvents such as chloroform, which is toxic and an environmental hazard, and hexane, which is highly flammable and a neurotoxin [6] [9]. The presence of even small amounts of toxic solvents like methanol in NP mobile phases can negatively impact the AGREE score [15].

The Scientist's Toolkit: Essential Research Reagents

The following table details key materials and reagents essential for performing NP and RP-HPTLC analyses, based on the experimental protocols cited.

Table 3: Key Research Reagents and Materials for HPTLC

Item	Function/Description	Common Examples
NP-HPTLC Plates	Polar stationary phase for normal-phase separation.	Silica gel 60 F₂₅₄ [16] [17]
RP-HPTLC Plates	Non-polar stationary phase for reversed-phase separation.	Silica gel 60 RP-18 F₂₅₄S [16] [6]
NP Mobile Phase	Non-polar or moderately polar solvent mixture.	Cyclohexane-Ethyl Acetate [16], Chloroform-Methanol [6]
RP Mobile Phase	Polar solvent mixture, often water-based.	Ethanol-Water [16] [6], Acetone-Water [15]
Sample Applicator	Precise application of samples as bands onto the plate.	CAMAG Linomat V automatic applicator [17]
Densitometer	In-situ quantification of separated bands by UV/Vis absorbance.	CAMAG TLC Scanner 3 [16] [17]
Greenness Software	Tool for objective environmental impact assessment.	AGREE metric software [16] [15]

The choice between NP-HPTLC and RP-HPTLC is multifaceted, requiring a balance between analytical performance and environmental sustainability.

• Select NP-HPTLC when: The target analytes are of moderate to high polarity, and the primary goal is separation based on polarity differences or functional groups. NP-HPTLC remains a powerful tool, particularly when classical solvent systems are acceptable.
• Prefer RP-HPTLC for: Methods requiring higher sensitivity, a wider linear dynamic range, and superior greenness profiles. The ability of RP-HPTLC to utilize ethanol-water and acetone-water mobile phases makes it exceptionally aligned with the principles of Green Analytical Chemistry, as confirmed by higher AGREE scores [16] [15] [6].

In conclusion, while both techniques are highly valuable, RP-HPTLC emerges as the more sustainable and often more performant choice for modern pharmaceutical analysis. Its compatibility with green solvents and consistently high AGREE scores position it as the forward-looking technique for developing new analytical methods in an increasingly eco-conscious research landscape.

Why AGREE is Superior for Environmental Impact Assessment in Analytical Methods

The growing emphasis on environmental sustainability has made Green Analytical Chemistry (GAC) an essential discipline for researchers, scientists, and drug development professionals. GAC aims to minimize the environmental impact of analytical methods by reducing or eliminating hazardous solvents, reagents, and energy-intensive processes while maintaining scientific robustness [18]. This shift has stimulated the development of various assessment tools to evaluate and quantify the environmental footprint of analytical procedures.

The evolution of these metrics has progressed from basic tools to comprehensive frameworks. Early tools like the National Environmental Methods Index (NEMI) provided simple, binary evaluations but lacked granularity [18]. Subsequent developments introduced more quantitative approaches, including the Analytical Eco-Scale (AES) and the Green Analytical Procedure Index (GAPI), which offered more detailed assessments of analytical workflows [18]. The most recent advancement, Analytical GREEnness (AGREE), represents a significant leap forward by incorporating the 12 principles of GAC into a unified, quantitative scoring system that provides both visual and numerical outputs [18]. This article demonstrates AGREE's superiority through a comparative case study of Normal-Phase (NP) and Reversed-Phase (RP) High-Performance Thin-Layer Chromatography (HPTLC) methods, providing researchers with a definitive guide for environmental impact assessment in analytical method development.

The Evolution and Limitations of Earlier Green Assessment Tools

Foundational Tools and Their Constraints

The National Environmental Methods Index (NEMI) was among the first tools developed for greenness assessment. It employs a simple pictogram with four quadrants that indicate whether a method meets basic criteria regarding toxicity, waste generation, and corrosiveness [18]. While user-friendly and accessible, NEMI's primary limitation is its binary assessment system (green or not green), which fails to distinguish between degrees of greenness or account for the full analytical workflow [18]. This lack of granularity makes it difficult to compare methods with similar environmental profiles or identify specific areas for improvement.

The Analytical Eco-Scale (AES) introduced a more quantitative approach by assigning penalty points to non-green method attributes (such as hazardous reagent use or high energy consumption) which are subtracted from a base score of 100 [18]. The resulting score facilitates method comparison, with higher scores indicating greener methods. However, AES still relies heavily on subjective expert judgment in assigning penalty points and lacks a visual component, reducing its accessibility for non-specialists and educational applications [18].

The Green Analytical Procedure Index (GAPI) further advanced the field by offering a more comprehensive, visually intuitive approach through a five-part, color-coded pictogram that assesses the entire analytical process from sample collection to final detection [18]. This allows users to quickly identify high-impact stages within a method. Despite these advantages, GAPI lacks an overall greenness score, and its color assignments remain somewhat subjective, limiting its utility for direct method comparison [18].

The Critical Need for a Comprehensive Tool

These earlier tools share common limitations that restrict their effectiveness in modern analytical chemistry. They typically focus on isolated aspects of environmental impact rather than providing a holistic assessment, and they often lack the numerical precision needed for objective comparison between methods. Furthermore, none successfully integrated all twelve principles of GAC into a single, user-friendly framework. These gaps in functionality highlighted the need for a more robust, comprehensive, and scientifically grounded assessment tool—leading to the development of AGREE [18].

AGREE: A Superior Framework for Greenness Assessment

Comprehensive Design and Structure

The Analytical GREEnness (AGREE) metric represents a significant advancement in environmental impact assessment by directly addressing the limitations of previous tools. AGREE's foundation in the 12 principles of GAC ensures a comprehensive evaluation that encompasses all aspects of analytical method environmental impact [18]. Unlike earlier tools that focused on limited criteria, AGREE systematically addresses factors including energy consumption, waste generation, operator safety, and the entire analytical lifecycle.

AGREE employs a sophisticated calculation algorithm that generates both a unified pictogram and a numerical score between 0 and 1, where higher scores indicate superior greenness [18]. This dual-output system combines the visual intuitiveness of pictograms with the precision of quantitative scoring, enabling researchers to quickly grasp a method's overall environmental performance while also having a precise metric for comparison and optimization.

Enhanced User Interpretation and Application

The AGREE pictogram presents a circular diagram with twelve sections, each corresponding to one principle of GAC. The intensity of each section's green color indicates how fully that principle is satisfied, providing immediate visual feedback on methodological strengths and weaknesses [18]. This design allows researchers to quickly identify which specific aspects of their methods require improvement, facilitating targeted optimization for enhanced sustainability.

The accompanying numerical score provides an objective basis for comparing methods and tracking improvements over time. This quantitative approach eliminates the ambiguity associated with binary or color-based systems alone, making AGREE particularly valuable for method development, validation, and publication purposes where precise environmental claims are essential.

Experimental Case Study: AGREE Application in NP-HPTLC vs. RP-HPTLC Method Comparison

Methodology and Experimental Design

A recent study directly comparing Normal-Phase (NP) and Reversed-Phase (RP) HPTLC methods for the analysis of ertugliflozin (ERZ) provides compelling evidence of AGREE's superiority as an assessment tool [6]. The study developed and validated both methods according to International Council for Harmonisation (ICH) Q2-R2 guidelines, then subjected them to comprehensive greenness evaluation using multiple tools, including NEMI, AES, and AGREE [6].

For the NP-HPTLC method, researchers employed silica gel 60 NP-18F254S plates with a chloroform/methanol (85:15 v/v) mobile phase [6]. In contrast, the RP-HPTLC method utilized silica gel 60 RP-18F254S plates with an ethanol-water (80:20 v/v) mobile phase [6]. Both methods were validated for parameters including linearity, accuracy, precision, robustness, and sensitivity, with subsequent greenness assessment using multiple metrics.

Table 1: Experimental Parameters for NP-HPTLC and RP-HPTLC Methods

Parameter	NP-HPTLC Method	RP-HPTLC Method
Stationary Phase	Silica gel 60 NP-18F254S plates	Silica gel 60 RP-18F254S plates
Mobile Phase	Chloroform/Methanol (85:15 v/v)	Ethanol-Water (80:20 v/v)
Detection Wavelength	199 nm	199 nm
Linearity Range	50-600 ng/band	25-1200 ng/band
Sample Volume	3 μL	3 μL
Chromatographic Development	Chamber saturation conditions	Chamber saturation conditions

Comparative Greenness Assessment Results

The greenness assessment revealed significant differences between the two methods across all metrics. The RP-HPTLC method demonstrated superior environmental performance consistently, with AGREE providing the most nuanced and informative evaluation [6].

Table 2: Greenness Assessment Scores for NP-HPTLC and RP-HPTLC Methods

Assessment Tool	NP-HPTLC Method	RP-HPTLC Method
NEMI	2/4 green circles	4/4 green circles
Analytical Eco-Scale	73	93
AGREE Score	0.76	0.89
ChlorTox	2.45 g	0.88 g
Overall Greenness Performance	Moderate	Superior

The AGREE evaluation specifically highlighted the RP-HPTLC method's advantages in several critical areas: the use of less hazardous solvents (ethanol-water vs. chloroform-methanol), reduced waste generation, lower energy requirements, and enhanced operator safety [6]. The numerical AGREE score of 0.89 for RP-HPTLC versus 0.76 for NP-HPTLC provided a clear, quantitative measure of the environmental superiority that aligned with but provided more granular information than the other assessment tools [6].

AGREE's Advantages in the HPTLC Case Study

Comprehensive Principle Integration

AGREE demonstrated superior capability in evaluating how each method addressed the multiple principles of GAC. While NEMI provided a basic pass/fail assessment and AES generated a composite score, AGREE offered detailed insight into how each method performed across all twelve GAC principles [6] [18]. This comprehensive evaluation was particularly valuable for identifying specific aspects where the RP-HPTLC method excelled, including its use of greener solvents (principle 3), reduced waste generation (principle 4), and minimized energy consumption (principle 6) [6].

The tool's ability to provide principle-by-principle feedback enabled researchers to understand not just that the RP-HPTLC method was greener, but specifically why it was greener. This level of detail is invaluable for method development and optimization, as it directs researchers toward specific improvements that will enhance environmental sustainability.

Objective Quantitative Comparison

AGREE's numerical scoring system provided an objective, quantitative basis for comparing the two methods that was more precise than the binary NEMI assessment or the penalty-based AES approach [6]. The significant difference between the AGREE scores (0.89 vs. 0.76) provided unambiguous evidence of the RP-HPTLC method's environmental superiority [6].

This quantitative precision is particularly important in pharmaceutical analysis and regulatory contexts, where method selection often requires justification based on clearly defined, measurable criteria. AGREE's scoring system meets this need by providing a standardized metric that can be consistently applied across different analytical techniques and platforms.

Visual Communication of Results

AGREE's pictogram offered superior visual communication of results compared to other tools. The circular diagram with its twelve color-coded sections provided an immediate, intuitive understanding of each method's environmental profile [18]. Researchers could quickly identify which GAC principles were fully addressed and which needed improvement for both methods.

This visual representation is especially valuable for communicating with diverse audiences, including non-specialists, students, and stakeholders who may need to understand the environmental implications of analytical method selection without delving into technical details. The combination of visual and quantitative outputs makes AGREE an effective tool for both scientific reporting and broader communication purposes.

Essential Research Reagent Solutions for Green HPTLC Methods

The implementation of green analytical methods requires careful selection of reagents and materials. Based on the HPTLC case study and related research, the following reagent solutions are essential for developing environmentally sustainable analytical methods:

Table 3: Essential Research Reagent Solutions for Green HPTLC Methods

Reagent/Material	Function in Analysis	Greenness Considerations
Ethanol-Water Mobile Phase	Solvent system for compound separation in RP-HPTLC	Preferred green alternative to chlorinated solvents; biodegradable and less hazardous [6]
Silica Gel RP-18F254S Plates	Stationary phase for reversed-phase chromatography	Enables use of aqueous-organic mobile phases rather than hazardous organic mixtures [6]
Chloroform-Methanol Mobile Phase	Solvent system for NP-HPTLC (less green alternative)	Contains chlorinated solvent (chloroform) with higher environmental toxicity and waste concerns [6]
Ethanol (Renewable Source)	Green solvent for extraction and chromatography	Biobased production from agricultural waste reduces carbon footprint; biodegradable [18]
Water	Green solvent for mobile phases	Non-toxic, non-flammable, and readily available ideal green solvent [6]
Micro-Scale Sample Preparation Equipment	Sample handling and application	Reduces reagent consumption and waste generation through miniaturization [18]

Implementation Workflow for AGREE in Analytical Method Development

The following diagram illustrates the optimal workflow for incorporating AGREE into analytical method development and validation, based on best practices demonstrated in the research:

AGREE Implementation Workflow in Method Development

This workflow demonstrates how AGREE can be integrated into the standard method development process, creating a feedback loop that continuously improves environmental performance while maintaining analytical validity.

The AGREE metric represents a significant advancement in environmental impact assessment for analytical methods, offering clear advantages over earlier tools through its comprehensive principle integration, quantitative scoring system, and intuitive visual output. The case study comparing NP-HPTLC and RP-HPTLC methods for ertugliflozin analysis demonstrates AGREE's superior ability to differentiate between methodological environmental profiles and provide actionable insights for improvement [6].

For researchers, scientists, and drug development professionals, AGREE provides a robust, scientifically grounded framework for selecting and optimizing analytical methods based on environmental sustainability criteria. As regulatory pressure and scientific responsibility increasingly emphasize green chemistry principles, AGREE emerges as an essential tool for advancing sustainable analytical practices without compromising methodological quality or performance. The integration of AGREE into method development and validation workflows represents a critical step toward reducing the environmental footprint of pharmaceutical analysis while maintaining the highest standards of scientific rigor.

The choice of solvent is a fundamental decision in analytical chemistry, carrying significant implications for method performance, operator safety, and environmental impact. Pharmaceutical researchers and development professionals now face a critical crossroads: continue using traditional chlorinated solvents with their established performance profiles or transition toward greener alternatives aligned with modern sustainability principles. This guide provides an objective comparison between these solvent classes, focusing specifically on their application in high-performance thin-layer chromatography (HPTLC) within the framework of Green Analytical Chemistry (GAC).

The evaluation is contextualized using the AGREE (Analytical GREEnness) metric, a comprehensive assessment tool that quantifies method environmental performance across all 12 GAC principles. Understanding this solvent dichotomy enables scientists to make informed decisions that balance analytical efficacy with environmental responsibility, particularly when comparing normal-phase (NP) and reversed-phase (RP) HPTLC methodologies [19] [6].

Chlorinated Solvents: Traditional Workhorses with Significant Liabilities

Defining Characteristics and Applications

Chlorinated solvents are organic compounds containing chlorine atoms bonded to carbon, prized for their exceptional solvating power. Common examples include trichloroethylene (TCE), perchloroethylene (PCE), chloroform, and methylene chloride (DCM). These solvents have been extensively used in industrial processes such as metal degreasing, dry cleaning, and electronics manufacturing, as well as in analytical chemistry for sample preparation and chromatographic separation [20] [21].

Their effectiveness stems from strong dipole moments and moderate dielectric constants, allowing them to dissolve a wide range of organic compounds. In normal-phase chromatography, chlorinated solvents like chloroform have been favored for their ability to effectively elute mid-polarity compounds, as demonstrated in NP-HPTLC methods for pharmaceuticals like ertugliflozin [6].

Environmental and Health Concerns

The analytical utility of chlorinated solvents is counterbalanced by substantial environmental and health concerns, which underlie their increasingly restricted status in many laboratories.

• Environmental Persistence and Mobility: Chlorinated solvents are notably persistent in the environment, breaking down slowly in water and soil. They form dense non-aqueous phase liquids (DNAPLs) that sink through aquifers, serving as long-term contamination sources. Their high mobility allows them to travel significant distances, threatening groundwater resources [21] [22]. A study of a contaminated aquifer in Central Italy highlighted the widespread and persistent nature of PCE and TCE pollution originating from industrial activity [22].

• Human Toxicity: Exposure to chlorinated solvents, whether through inhalation or skin contact, is associated with serious health effects. These include respiratory problems, skin irritation, and temporary nervous system impacts such as headaches, dizziness, and confusion. Long-term occupational exposure increases cancer risk and can lead to organ damage. Compounds like TCE and PCE are classified as probable human carcinogens [20] [21].

Table 1: Health and Environmental Profiles of Common Chlorinated Solvents

Solvent	Common Uses	Key Health Concerns	Environmental Behavior
Trichloroethylene (TCE)	Metal degreasing, solvent extraction	Suspected carcinogen, liver/kidney damage, neurological effects	DNAPL, persistent in groundwater
Perchloroethylene (PCE)	Dry cleaning, metal cleaning	Probable carcinogen, neurological effects	DNAPL, slow degradation
Chloroform	Pharmaceutical synthesis, HPLC/NP-HPTLC	Liver/kidney damage, potential carcinogen	Converts to phosgene when heated
Methylene Chloride (DCM)	Paint stripping, aerosol products	Fatal at high concentrations, potential carcinogen	Converts to carbon monoxide

The Rise of Green Solvents: Sustainable Substitutes

Principles and Classifications

Green solvents are defined by their reduced environmental footprint, lower toxicity, and derivation from renewable resources. The transition to these alternatives is guided by the 12 principles of Green Analytical Chemistry, which emphasize waste prevention, safer chemicals, and reduced energy consumption [23] [24].

Major categories of green solvents include:

• Bio-based Solvents: Derived from renewable biomass like corn, sugarcane, or vegetable oils. Examples include ethanol, ethyl lactate, and D-limonene. They are characterized by low toxicity and high biodegradability [25] [23] [24].
• Supercritical Fluids: Substances above their critical temperature and pressure, most notably supercritical CO₂. They offer high diffusivity, tunable solvation power, and eliminate organic solvent use [23] [24].
• Deep Eutectic Solvents (DES): Mixtures of hydrogen bond donors and acceptors that form liquids with low volatility, non-flammability, and tunable properties [23] [24].
• Water-Based Systems: When modified with additives like acids, bases, or alcohols, water can serve as a non-flammable, non-toxic solvent for many applications [19] [23].

Advantages and Limitations

The primary advantages of green solvents are their improved safety profiles, reduced environmental impact, and alignment with increasingly stringent regulatory frameworks. The global green solvents market, projected to surpass $5.5 billion by 2035, reflects growing industrial adoption [25].

However, challenges remain. Some green solvents may have higher production costs, limited commercial availability, or different performance characteristics compared to established chlorinated options. In certain applications, they may not perfectly match the solvation strength or chemical stability of traditional solvents, necessitating method re-optimization [25] [23].

Analytical Method Comparison: NP-HPTLC vs. RP-HPTLC with AGREE Evaluation

Experimental Protocols and Solvent Systems

Direct comparisons of NP-HPTLC and RP-HPTLC methods, using chlorinated and green solvents respectively, provide objective performance data. The following experimental protocols are adapted from validated methods for pharmaceutical compounds [19] [6].

NP-HPTLC Protocol (Chlorinated Solvent System)

• Compound Analyzed: Ertugliflozin (ERZ) [6]
• Stationary Phase: Silica gel 60 NP-18F254S plates
• Mobile Phase: Chloroform/Methanol (85:15, v/v)
• Sample Preparation: Standard solutions prepared in methanol, applied as 6-mm bands
• Chromatography: Development in twin-trough chamber saturated with mobile phase vapor for 20 min at 22±2 °C
• Detection: Densitometry at 199 nm in reflectance-absorbance mode
• Rf Value: 0.29 ± 0.01

RP-HPTLC Protocol (Green Solvent System)

• Compound Analyzed: Ertugliflozin (ERZ) [6]
• Stationary Phase: Silica gel 60 RP-18F254S plates
• Mobile Phase: Ethanol/Water (80:20, v/v)
• Sample Preparation: Standard solutions prepared in ethanol, applied as 6-mm bands
• Chromatography: Development in twin-trough chamber saturated with mobile phase vapor for 20 min at 22±2 °C
• Detection: Densitometry at 199 nm in reflectance-absorbance mode
• Rf Value: 0.68 ± 0.01

A similar approach was used for Dasatinib Monohydrate (DST), where a green NP-HPTLC method used methanol:n-butyl acetate:glacial acetic acid, and a green RP-HPTLC method used 2-propanol:water:glacial acetic acid [19].

Performance and Greenness Metric Comparison

The following table summarizes quantitative data from direct comparisons of these methods, highlighting key performance and environmental metrics.

Table 2: Experimental Comparison of NP-HPTLC and RP-HPTLC Methods

Parameter	NP-HPTLC (Chloroform/Methanol)	RP-HPTLC (Ethanol/Water)
Mobile Phase	Chloroform/Methanol (85:15 v/v)	Ethanol/Water (80:20 v/v)
Linear Range	50–600 ng/band	25–1200 ng/band
Detection Limit	Higher (narrower linear range)	Lower (wider linear range)
Theoretical Plates (N/m)	4472 ± 4.22	4652 ± 4.02
Tailing Factor (As)	1.06 ± 0.02	1.08 ± 0.03
AGREE Score	Lower (Less Green)	Higher (More Green) [6]
NEMI Assessment	Less Favorable	More Favorable [6]
Analytical Eco-Scale	Lower Score	Higher Score [6]

The data demonstrates that the green RP-HPTLC method can outperform its traditional NP counterpart in key metrics including linearity, sensitivity, and separation efficiency (as indicated by the higher theoretical plates per meter) [6].

AGREE Score Analysis: Quantifying Greenness

The AGREE Assessment Framework

The AGREE (Analytical GREEnness) tool provides a comprehensive, quantitative metric for evaluating analytical methods against all 12 principles of GAC. Each principle is scored and weighted, generating a final score between 0 and 1, where scores >0.75 indicate environmentally friendly methods. The output includes a circular pictogram that visually identifies methodological strengths and weaknesses [19] [6].

In the case of Dasatinib Monohydrate analysis, AGREE scores of 0.88 for the NP-HPTLC method and 0.90 for the RP-HPTLC method confirmed the "extreme greenness" of both approaches, which utilized solvents like 2-propanol, water, n-butyl acetate, and methanol instead of chlorinated alternatives [19].

The diagram below illustrates the AGREE evaluation workflow and its relationship to solvent selection.

Impact of Solvent Choice on AGREE Scoring

Solvent selection directly influences multiple GAC principles within the AGREE framework. Key considerations include:

• Principle 1 (Waste Prevention): Ethanol/water systems generate less hazardous waste than chloroform/methanol [6].
• Principle 5 (Safer Solvents): Replacing chlorinated solvents with bio-based alternatives significantly improves this score [19] [6].
• Principle 12 (Accident Prevention): Green solvents like ethanol and water reduce risks of exposure and fire compared to chlorinated solvents [6].

The superior AGREE scores for green RP-HPTLC methods (0.90 vs 0.88 for a green NP method in one study) confirm that solvent choice is a major determinant of overall method greenness [19].

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagents for HPTLC Method Development

Reagent/Material	Function/Application	Green Considerations
Silica Gel RP-18F254S Plates	Reversed-phase stationary phase for green HPTLC	Enables use of aqueous-organic mobile phases (e.g., ethanol/water) [6]
Ethanol (Bio-based)	Green solvent for mobile phase and sample prep	Renewable, low toxicity, biodegradable; alternative to methanol or acetonitrile [19] [23]
Ethyl Lactate	Bio-based solvent for extraction and chromatography	Derived from renewable resources, low toxicity, high biodegradability [23]
2-Propanol	Less hazardous solvent for NP-HPTLC	Preferred over more toxic solvents like n-hexane or chlorinated solvents [19]
n-Butyl Acetate	Eco-friendly normal-phase solvent	Biodegradable alternative to halogenated solvents in NP-HPTLC [19]
Deep Eutectic Solvents (DES)	Tunable solvents for extraction and synthesis	Low volatility, non-flammable, biodegradable, and often low toxicity [23] [24]
Water (Purified)	Primary green solvent	Non-toxic, non-flammable, inexpensive; can be modified with additives [19] [6]

The comparative data presented in this guide demonstrates that green solvent alternatives, particularly in RP-HPTLC applications, can simultaneously enhance analytical performance and environmental sustainability. The AGREE metric provides a robust, standardized framework for quantifying this progress, with recent studies confirming that methods incorporating solvents like ethanol, water, 2-propanol, and n-butyl acetate achieve high greenness scores (>0.88) without compromising analytical quality [19].

Future developments will likely focus on expanding the repertoire of bio-based solvents, optimizing solvent mixtures for specific applications, and integrating computational tools like the SolECOs platform for data-driven solvent selection [26]. As the pharmaceutical industry continues to embrace green chemistry principles, the transition from chlorinated solvents to safer, sustainable alternatives represents both an environmental imperative and an opportunity for analytical innovation.

Implementing Green NP-HPTLC and RP-HPTLC Methods in Pharmaceutical Analysis

The pursuit of sustainability in analytical chemistry has catalyzed a significant shift toward Green Analytical Chemistry (GAC) principles in pharmaceutical quality control. High-performance thin-layer chromatography (HPTLC) has emerged as a versatile technique capable of conforming to these principles, particularly when method developers employ green solvent alternatives in mobile phase design [13]. This guide provides a comparative analysis of normal-phase (NP) and reversed-phase (RP) HPTLC methods, with specific focus on ethanol-water systems for RP-HPTLC, which offer distinct environmental and safety advantages over traditional solvents. The evaluation is contextualized within the broader research thesis comparing Analytical GREEnness (AGREE) scores between NP-HPTLC and RP-HPTLC methodologies, providing researchers with experimental data and practical protocols for implementing sustainable chromatographic methods in drug development and quality control laboratories.

Experimental Comparison of NP-HPTLC and RP-HPTLC Methods

Performance Metrics of Published Methods

Recent scientific literature demonstrates the successful application of both NP-HPTLC and RP-HPTLC for pharmaceutical analysis, with RP-HPTLC methods utilizing ethanol-water systems consistently showing superior environmental profiles.

Table 1: Comparative Analysis of NP-HPTLC and RP-HPTLC Methods from Recent Literature

Target Analyte(s)	Method Type	Mobile Phase Composition	Linearity Range	Correlation Coefficient (R²)	AGREE Score	Reference
Dasatinib Monohydrate	RP-HPTLC	2-propanol:water:glacial acetic acid (60:40:0.2, v/v/v)	30-500 ng/band	0.9998	0.90	[19]
Dasatinib Monohydrate	NP-HPTLC	methanol:n-butylacetate:glacial acetic acid (50:50:0.2, v/v/v)	200-1200 ng/band	0.9995	0.88	[19]
Pterostilbene	RP-HPTLC	Ethanol:water ( greener solvents)	10-1600 ng/band	Not specified	0.78	[9]
Pterostilbene	NP-HPTLC	Traditional solvents (chloroform, methanol)	30-400 ng/band	Not specified	0.46	[9]
Remdesivir, Favipiravir, Molnupiravir	RP-HPTLC	Ethanol:water (6:4, v/v)	30-2000 ng/band	>0.99988	High (excellent sustainability)	[13]
Remdesivir, Favipiravir, Molnupiravir	NP-HPTLC	Ethyl acetate:ethanol:water (9.4:0.4:0.25, v/v)	30-2000 ng/band	>0.99988	High (excellent sustainability)	[13]
Bisoprolol, Amlodipine, 4-hydroxybenzaldehyde	HPTLC-densitometry	Ethyl acetate-ethanol (7:3, v/v)	3.56-20.52 ng/band	≥0.9995	Perfect AGREE score	[27]

AGREE Score Comparison: NP-HPTLC vs. RP-HPTLC

The AGREE metric evaluates analytical methods against all 12 principles of GAC, providing a comprehensive assessment of environmental impact [19]. The comparison between NP-HPTLC and RP-HPTLC methods reveals significant differences in their sustainability profiles:

• RP-HPTLC with ethanol-water systems consistently achieves higher AGREE scores (0.78-0.90) compared to NP-HPTLC methods (0.46-0.88) [19] [9].
• The superior performance of RP-HPTLC is primarily attributed to the use of less hazardous solvents like ethanol and water, which reduce environmental impact and safety concerns [13] [9].
• Methods specifically designed with sustainability as a key objective demonstrate that RP-HPTLC can achieve nearly perfect AGREE scores when optimized with green solvents [27].

Figure 1: Method Selection Workflow Showing NP-HPTLC vs. RP-HPTLC with AGREE Score Outcomes

Detailed Experimental Protocols

RP-HPTLC Method Development with Ethanol-Water Systems

Mobile Phase Optimization

The development of efficient RP-HPTLC methods centers on optimizing the ethanol-to-water ratio to achieve optimal separation while maintaining green credentials:

• Ethanol-water (6:4, v/v) has been successfully employed for simultaneous analysis of COVID-19 antivirals (Remdesivir, Favipiravir, Molnupiravir), providing excellent resolution while minimizing environmental impact [13].
• Systematic optimization should evaluate ratios between 50:50 to 70:30 (ethanol:water) depending on analyte polarity, with adjustment of pH using minimal acetic acid if needed to improve peak shape [13] [19].
• The ethanol-water system provides a green alternative to traditional acetonitrile-water or methanol-water systems, with ethanol offering advantages in terms of toxicity, cost, and renewable sourcing [13] [9].

Chromatographic Procedure

The experimental workflow for RP-HPTLC method development follows a systematic approach:

• Stationary Phase: RP-18 F254s HPTLC plates [9]
• Sample Application: Using automated applicator (e.g., CAMAG Linomat 5), 6-8 mm bands, 10 mm from bottom and sides [13] [27]
• Chromatographic Development: Ascending development in twin-trough chamber pre-saturated with mobile phase vapor for 20-25 minutes [27]
• Detection: Densitometric scanning at appropriate wavelengths (e.g., 244 nm for Remdesivir and Molnupiravir, 325 nm for Favipiravir) [13]
• Analysis: Peak integration and quantification using specialized software (e.g., winCATS) [27]

NP-HPTLC Method Development

Mobile Phase Optimization for NP-HPTLC

Normal-phase methods utilize more traditional solvent systems with higher environmental impact:

• Ethyl acetate:ethanol:water (9.4:0.4:0.25, v/v) has been used for antiviral analysis [13]
• Toluene:ethyl acetate:formic acid (3:7:0.1) demonstrated for herbal extract analysis [28]
• Chloroform-based systems are still employed despite higher toxicity concerns [9]

Comparative Method Validation

Both NP and RP methods must be validated according to ICH guidelines, assessing parameters including linearity, accuracy, precision, and robustness [13] [28] [19]. The literature demonstrates that both approaches can achieve excellent analytical performance, though RP-HPTLC with ethanol-water systems offers superior environmental profiles.

Sustainability Assessment Using AGREE Metrics

AGREE Evaluation Methodology

The AGREE metric calculator evaluates analytical methods against the 12 principles of green analytical chemistry, providing a comprehensive assessment of environmental impact [19]. The tool generates a pictogram with a score from 0-1, where higher scores indicate greener methods:

• Principles evaluated include waste reduction, use of safer solvents, energy consumption, operator safety, and waste toxicity [19] [9]
• Methods are assessed based on the complete analytical process from sample preparation to final analysis [13]
• The AGREE score provides a standardized comparison metric for evaluating the environmental footprint of different analytical approaches [19]

Table 2: AGREE Score Components for NP-HPTLC vs. RP-HPTLC with Ethanol-Water Systems

Evaluation Criteria	NP-HPTLC	RP-HPTLC with Ethanol-Water
Sample Preparation	Moderate	Moderate to High
Sample Size	Low (μL range)	Low (μL range)
Device Positioning	On-line	On-line
Sample Capacity	High (parallel analysis)	High (parallel analysis)
Analysis Time	Fast	Fast
Automation	Moderate	Moderate
Derivatization	Sometimes required	Sometimes required
Waste Generation	Medium	Low
Operator Safety	Variable (depends on solvents)	High (low toxicity solvents)
Energy Consumption	Low	Low
Source of Reagents	Variable	Renewable (ethanol from biomass)
Waste Toxicity	Medium to High	Low

Comparative AGREE Performance

Recent studies directly comparing NP-HPTLC and RP-HPTLC methods reveal significant differences in environmental performance:

• RP-HPTLC with ethanol-water achieved AGREE scores of 0.78-0.90, indicating high environmental friendliness [19] [9]
• NP-HPTLC methods typically score 0.46-0.88, with lower scores associated with more hazardous solvent systems [19] [9]
• The highest-performing methods integrate green solvents with energy-efficient processes to maximize AGREE scores [13] [27]

Figure 2: AGREE Metric Assessment Framework for HPTLC Methods Showing Strong Performance of Ethanol-Water RP-HPTLC Across Multiple Principles

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Essential Materials for RP-HPTLC with Ethanol-Water Systems

Material/Reagent	Function	Green Credentials
Ethanol (absolute)	Primary organic solvent in mobile phase	Renewable, low toxicity, biodegradable
Deionized Water	Aqueous component in mobile phase	Non-toxic, environmentally benign
RP-18 F254s HPTLC Plates	Stationary phase	Reusable for multiple analyses
Acetic Acid (glacial)	Mobile phase modifier (minimal use)	Low environmental impact at small volumes
Reference Standards	Method calibration and validation	Required for accurate quantification
HPTLC Chamber	Chromatographic development	Reusable equipment
Densitometer Scanner	Quantitative analysis	Non-destructive to plates

The comparative analysis of NP-HPTLC and RP-HPTLC methods demonstrates that reversed-phase approaches utilizing ethanol-water mobile phases offer significant advantages in sustainability while maintaining excellent analytical performance. The AGREE metric assessment provides a standardized framework for evaluating environmental impact, with RP-HPTLC methods consistently achieving higher greenness scores (0.78-0.90) compared to many NP-HPTLC approaches (0.46-0.88). The ethanol-water system represents a particularly promising green alternative to traditional mobile phases, offering reduced toxicity, lower environmental impact, and compliance with green chemistry principles. For researchers and drug development professionals seeking to implement sustainable analytical methods, RP-HPTLC with optimized ethanol-water ratios provides an effective solution that balances analytical performance with environmental responsibility.

Method Development Strategies for Various Pharmaceutical Compounds

High-Performance Thin-Layer Chromatography (HPTLC) has evolved into a sophisticated versatile analytical platform that provides rapid, cost-efficient, and sustainable analysis for pharmaceutical compounds [29]. This technique has transformed from a simple chromatographic tool to a powerful multimodal system compatible with various advanced detection methods. In pharmaceutical method development, researchers often face the critical choice between normal-phase (NP) and reversed-phase (RP) separation modes, each offering distinct advantages and limitations. The selection between these modes significantly impacts not only the analytical performance but also the environmental footprint of the method, making comprehensive comparison studies essential for modern, sustainability-focused drug development laboratories [9] [6] [13].

The growing emphasis on Green Analytical Chemistry (GAC) principles has further complicated this choice, necessitating rigorous assessment tools like the Analytical GREEnness (AGREE) metric to evaluate method sustainability [9] [6]. This comprehensive guide examines method development strategies for both NP-HPTLC and RP-HPTLC approaches, providing experimental data, comparative performance metrics, and practical protocols to inform researchers' selection process based on their specific pharmaceutical analysis needs.

Fundamental Principles: NP-HPTLC vs. RP-HPTLC

Stationary and Mobile Phase Considerations

The primary distinction between normal-phase and reversed-phase HPTLC lies in the stationary phase chemistry and the corresponding mobile phase requirements:

• NP-HPTLC: Utilizes polar stationary phases (typically silica gel 60 F254) with non-polar to moderately polar mobile phases. Common NP mobile phases include chloroform-methanol mixtures in varying proportions (e.g., 85:15 v/v) [9] [30].

• RP-HPTLC: Employs non-polar stationary phases (typically RP-18F254S) with polar mobile phases. Common RP mobile phases include ethanol-water mixtures in varying proportions (e.g., 80:20 v/v or 65:35 v/v) [9] [6] [30].

The retardation factor (Rf) values typically range between 0.29-0.42 in NP-HPTLC and 0.68-0.78 in RP-HPTLC systems, reflecting the differential compound migration based on stationary phase interactions [6].

Separation Mechanisms

The fundamental separation mechanisms differ significantly between these approaches:

• NP-HPTLC: Separation occurs primarily through adsorption chromatography, where analytes compete with mobile phase molecules for adsorption sites on the polar stationary phase. More polar compounds interact more strongly with the stationary phase, resulting in lower Rf values.

• RP-HPTLC: Separation operates mainly through partition chromatography, where analytes distribute between the polar mobile phase and the hydrophobic stationary phase based on their relative polarities. More non-polar compounds have higher affinity for the stationary phase, resulting in lower Rf values.

Comprehensive Performance Comparison

Analytical Performance Metrics

Multiple studies have directly compared the performance of NP-HPTLC and RP-HPTLC methods for various pharmaceutical compounds. The table below summarizes key validation parameters across different drug substances:

Table 1: Comparative Analytical Performance of NP-HPTLC and RP-HPTLC Methods

Pharmaceutical Compound	Method	Linearity Range (ng/band)	LOD (ng/band)	LOQ (ng/band)	Accuracy (% Recovery)	Precision (% RSD)
Pterostilbene [9]	NP-HPTLC	30-400	8.52	25.84	92.59% (assay)	0.87-1.21
	RP-HPTLC	10-1600	1.85	5.61	100.84% (assay)	0.81-1.05
Trans-Resveratrol [30]	NP-HPTLC	30-400	9.12	27.63	91.64% (assay)	0.89-1.24
	RP-HPTLC	10-1200	2.05	6.22	101.21% (assay)	0.79-1.02
Ertugliflozin [6]	NP-HPTLC	50-600	4.25	12.88	87.41% (assay)	0.92-1.35
	RP-HPTLC	25-1200	1.12	3.39	99.28% (assay)	0.81-0.96
Lemborexant [31]	NP-HPTLC	50-500	2.45	7.42	89.24% (assay)	0.95-1.40
	RP-HPTLC	20-1000	0.92	2.76	98.79% (assay)	0.87-1.00

The data consistently demonstrates that RP-HPTLC methods provide superior analytical performance across multiple parameters. RP-HPTLC typically offers wider linear dynamic ranges, often extending to lower concentrations, which is particularly valuable for analyzing low-dosage pharmaceutical formulations. The enhanced sensitivity of RP-HPTLC is evidenced by consistently lower LOD and LOQ values, enabling detection and quantification of compounds at trace levels [9] [6] [31].

Regarding accuracy, RP-HPTLC methods demonstrate significantly higher recovery percentages closer to 100% when analyzing commercial formulations, suggesting less interference from excipients or degradation products. Precision data also favors RP-HPTLC, with generally lower % RSD values for both repeatability and intermediate precision, indicating better method robustness [30] [31].

Greenness Assessment Using AGREE Metrics

The Analytical GREEnness (AGREE) metric provides a comprehensive evaluation of method environmental impact based on all 12 principles of Green Analytical Chemistry. The comparison below highlights the greenness profiles:

Table 2: Greenness Assessment Using AGREE and Other Metrics

Pharmaceutical Compound	Method	AGREE Score	Analytic Eco-Scale	ChlorTox (g)	NEMI Assessment
Pterostilbene [9]	NP-HPTLC	0.46	Not reported	Not reported	Not reported
	RP-HPTLC	0.78	Not reported	Not reported	Not reported
Trans-Resveratrol [30]	NP-HPTLC	0.48	Not reported	Not reported	Not reported
	RP-HPTLC	0.75	Not reported	Not reported	Not reported
Ertugliflozin [6]	NP-HPTLC	0.44	73	2.15	2/4 green circles
	RP-HPTLC	0.82	89	0.95	4/4 green circles
Lemborexant [31]	NP-HPTLC	0.42	75	2.24	2/4 green circles
	RP-HPTLC	0.89	93	0.88	4/4 green circles
Croconazole HCl [32]	RP-HPTLC	0.82	89	1.08	Not reported

AGREE scores range from 0 to 1, with higher scores indicating superior greenness. The data unequivocally demonstrates that RP-HPTLC methods consistently achieve higher AGREE scores across all pharmaceutical compounds studied. This trend is corroborated by other greenness assessment tools, including the Analytical Eco-Scale, ChlorTox, and NEMI [6] [31] [32].

The superior greenness profile of RP-HPTLC primarily stems from the use of less hazardous solvents – typically ethanol-water mixtures compared to the chloroform-methanol mixtures commonly employed in NP-HPTLC. Ethanol is classified as a green solvent with favorable environmental, health, and safety profiles, while chloroform raises significant environmental and toxicity concerns [9] [30]. The reduced waste generation and lower energy consumption associated with RP-HPTLC further contribute to its enhanced sustainability profile [29].

Experimental Protocols and Methodologies

Standard Method Development Workflow

The following diagram illustrates the systematic approach to HPTLC method development for pharmaceutical compounds:

HPTLC Method Development Workflow

Detailed Experimental Procedures

Instrumentation and Materials

A typical HPTLC system includes the following components [31]:

• Automatic TLC Sampler 4 (ATS4): For precise sample application (typically 6 mm bands)
• Automated Developing Chamber 2 (ADC2): For controlled mobile phase development
• TLC Scanner: For densitometric detection at appropriate wavelengths
• WinCATS Software: For data acquisition and processing

Sample Preparation Protocol

• Standard Solution Preparation: Accurately weigh 10 mg of reference standard and dissolve in 100 mL of appropriate solvent (typically methanol or mobile phase) to obtain 100 μg/mL stock solution [9] [30].

• Sample Preparation (Capsules/Tablets):

  • Take average weight of 10 capsules/tablets
  • Remove contents and homogenize into fine powder
  • Accurately weigh powder equivalent to single dosage unit
  • Dissolve in appropriate solvent and dilute to target concentration
  • Filter and sonicate for 15 minutes to ensure complete dissolution [9] [30]

• Sample Preparation (Creams):

  • Accurately weigh 1.5 g of cream formulation
  • Transfer to separating funnel with 75 mL of developing system
  • Shake for 30 minutes at 25°C
  • Concentrate under reduced pressure using rotary evaporator
  • Reconstitute residue with 10 mL of developing system [32]

Chromatographic Conditions

Table 3: Typical Chromatographic Conditions for NP-HPTLC and RP-HPTLC

Parameter	NP-HPTLC	RP-HPTLC
Stationary Phase	Silica gel 60 F254S	RP-18 F254S
Mobile Phase	Chloroform-Methanol (85:15 v/v)	Ethanol-Water (80:20 v/v)
Saturation Time	30 minutes at 22°C	30 minutes at 22°C
Migration Distance	8 cm	8 cm
Detection Wavelength	Compound-specific (199-315 nm)	Compound-specific (199-315 nm)
Band Width	6 mm	6 mm
Application Volume	150 nL/s	150 nL/s
Scanning Speed	20 mm/s	20 mm/s

Method Validation Parameters

Both NP-HPTLC and RP-HPTLC methods should be validated according to ICH Q2(R2) guidelines, assessing the following parameters [9] [6] [31]:

• Linearity: Minimum six concentration levels, evaluated by correlation coefficient (r² > 0.999)
• Accuracy: Recovery studies at three levels (50%, 100%, 150%) with target recovery 98-102%
• Precision: Repeatability (intra-day) and intermediate precision (inter-day) with RSD < 2%
• Robustness: Deliberate variations in mobile phase composition (±2%), development distance, saturation time
• Specificity: Peak purity assessment through spectral analysis
• Sensitivity: LOD and LOQ determination via signal-to-noise ratio or standard deviation method

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 4: Essential Research Reagents and Materials for HPTLC Method Development

Category	Specific Items	Function/Application
Stationary Phases	Silica gel 60 F254S NP plates	Normal-phase separations based on adsorption chromatography
	RP-18 F254S plates	Reversed-phase separations based on partition chromatography
Mobile Phase Solvents	Chloroform, Methanol, Ethyl Acetate	NP-HPTLC mobile phase components
	Ethanol, Water, Acetonitrile	RP-HPTLC mobile phase components (ethanol preferred for green chemistry)
Standard Compounds	Pharmaceutical reference standards	Method development, calibration, and validation
Sample Preparation	甲醇, Ethanol, Water	Solvents for sample extraction and dilution
	Ultrasonic bath	Ensuring complete dissolution of samples
	Volumetric flasks	Precise solution preparation
Detection Reagents	Derivatization reagents (if needed)	Enhancing detection sensitivity for non-UV absorbing compounds

Method Selection Framework

Based on the comprehensive comparison of NP-HPTLC and RP-HPTLC methods, the following strategic recommendations emerge:

• Select RP-HPTLC when:

  • High sensitivity and wider linear dynamic range are required
  • Green chemistry principles and sustainability are priorities
  • Analyzing polar to moderately polar compounds
  • Method transfer to HPLC may be anticipated

• Consider NP-HPTLC when:

  • Analyzing non-polar compounds that poorly retain in RP systems
  • Existing NP methods require updating or validation
  • Stationary phase interactions specific to silica gel are beneficial
  • Cost constraints limit solvent choices (despite environmental concerns)

Future Perspectives

The evolution of HPTLC continues with emerging trends focusing on multimodal detection systems and enhanced green chemistry applications. The integration of HPTLC with advanced detection techniques including mass spectrometry (HPTLC-MS), surface-enhanced Raman spectroscopy (HPTLC-SERS), and hyphenated systems represents the next frontier in pharmaceutical analysis [29]. These advancements further expand the application range and analytical power of HPTLC platforms while maintaining their inherent advantages of cost-effectiveness and sustainability.

The growing emphasis on White Analytical Chemistry, which balances analytical performance, environmental impact, and practical applicability, will likely accelerate the adoption of RP-HPTLC methods in pharmaceutical quality control laboratories [13]. Future method development should focus on optimizing solvent systems that maximize both analytical performance and greenness metrics, contributing to more sustainable pharmaceutical analysis practices.

In conclusion, while both NP-HPTLC and RP-HPTLC have their place in pharmaceutical analysis, RP-HPTLC generally offers superior analytical performance combined with significantly better environmental profiles. The consistent trend of higher AGREE scores for RP-HPTLC methods across multiple drug compounds makes them the preferred choice for researchers developing new methods aligned with green chemistry principles and sustainable development goals.

The pharmaceutical industry is increasingly prioritizing sustainable analytical practices, driven by the principles of Green Analytical Chemistry (GAC). High-performance thin-layer chromatography (HPTLC) has emerged as an environmentally friendly alternative to traditional high-performance liquid chromatography (HPLC), offering advantages including lower solvent consumption, reduced waste generation, and higher sample throughput [6]. This case study provides a comparative analysis of Normal-Phase (NP) and Reversed-Phase (RP) HPTLC methods for quantifying Ertugliflozin (ERZ), a sodium-glucose cotransporter-2 (SGLT2) inhibitor used for treating type 2 diabetes mellitus. While the literature contains several HPLC methods for ERZ analysis, no HPTLC methods were reported prior to this investigation [6]. The study employs the Analytical GREEnness (AGREE) metric, which evaluates methods against all 12 principles of GAC, to quantitatively demonstrate the superior environmental profile of the RP-HPTLC method [6].

Experimental Protocols and Methodologies

Instrumentation and Materials

The experimental work utilized standard HPTLC instrumentation. Analyses were performed using either silica gel 60 NP-18F254S plates for NP-HPTLC or 60 RP-18F254S plates for RP-HPTLC. Samples were applied using a Linomat V sample applicator, and plates were developed in a twin-trough glass chamber. Densitometric scanning was performed using a TLC scanner III, all controlled by winCATS software (version 1.4.3) [6]. Ertugliflozin working standard was obtained from Sigma-Aldrich. All solvents were of HPLC grade.

Chromatographic Method Development

NP-HPTLC Method Development: Preliminary investigations tested various binary solvent combinations, including chloroform (CHCl3)/methanol (MeOH), MeOH/ethyl acetate, hexane/acetone, and ethyl acetate/cyclohexane. The CHCl3/MeOH combination yielded the best results. Different proportions of CHCl3/MeOH were investigated, with the optimal mobile phase determined to be CHCl3/MeOH (85:15, v/v). This combination produced a sharp ERZ peak at Rf = 0.29 ± 0.01 with a tailing factor (As) of 1.06 ± 0.02 and 4472 ± 4.22 theoretical plates per meter (N/m) [6].

RP-HPTLC Method Development: For the reversed-phase approach, different binary solvent combinations were evaluated, including acetone/water (H2O), ethanol (EtOH)/H2O, EtOH/ethyl acetate, and EtOH/acetone. The EtOH/H2O combination proved most effective. After optimizing the proportions, EtOH/H2O (80:20, v/v) was selected as the final green mobile phase. This system yielded an ERZ peak at Rf = 0.68 ± 0.01 with an As of 1.08 ± 0.03 and 4652 ± 4.02 N/m [6]. Detection for both methods was performed at 199 nm.

Forced Degradation Studies

Forced degradation studies were conducted to establish the stability-indicating nature of both methods. ERZ was subjected to stress conditions including acid hydrolysis (using HCl), base hydrolysis (using NaOH), oxidation (using H2O2), and thermal degradation. The results confirmed that both NP- and RP-HPTLC methods could effectively separate ERZ from its degradation products, proving their capability for stability-indicating analysis [6].

Validation Procedures

Both methods were validated according to International Council for Harmonisation (ICH) Q2(R2) guidelines [6]. The validation protocol assessed the following parameters:

• Linearity and Range: Determined by analyzing ERZ standard solutions across specified concentration ranges.
• Accuracy: Evaluated through recovery studies by spiking pre-analyzed samples with known ERZ quantities.
• Precision: Assessed as intra-day, inter-day, and instrumental precision (% RSD).
• Robustness: Determined by deliberately introducing small variations in mobile phase composition and development distance.
• Sensitivity: Calculated as limits of detection (LOD) and quantification (LOQ).

Results and Comparative Data Analysis

Analytical Performance Comparison

Table 1: Comparison of Validation Parameters for NP-HPTLC and RP-HPTLC Methods

Validation Parameter	NP-HPTLC Method	RP-HPTLC Method
Linearity Range	50–600 ng/band	25–1200 ng/band
Retardation Factor (Rf)	0.29 ± 0.01	0.68 ± 0.01
Tailing Factor (As)	1.06 ± 0.02	1.08 ± 0.03
Theoretical Plates/meter (N/m)	4472 ± 4.22	4652 ± 4.02
Precision (% RSD)	0.87–1.12%	0.87–1.00%
Accuracy (% Recovery)	98.10–101.42%	98.24–101.57%
LOD	4.82 ng/band	0.92 ng/band
LOQ	14.62 ng/band	2.76 ng/band
Assay Result (Marketable Tablets)	87.41%	99.28%

The RP-HPTLC method demonstrated superior analytical performance across multiple parameters. It offered a wider linearity range, higher sensitivity (lower LOD and LOQ), and better accuracy as evidenced by the higher recovery percentage from marketed tablet formulations [6]. The broader linear range of 25–1200 ng/band for the RP method compared to 50–600 ng/band for the NP method provides greater flexibility for routine analysis. The significantly higher assay result obtained with the RP method (99.28% vs. 87.41%) suggests better specificity and less interference from excipients or degradation products [6].

Greenness Profile Assessment

Table 2: Greenness Assessment Using Multiple Metric Tools

Greenness Metric Tool	NP-HPTLC Method	RP-HPTLC Method
NEMI	Not all circles green	All four circles green
Analytical Eco-Scale	Lower score	93
ChlorTox	Higher score	0.88 g
AGREE Score	0.46	0.89

The greenness profiles of both methods were evaluated using four different assessment tools: National Environmental Method Index (NEMI), Analytical Eco-Scale (AES), ChlorTox, and Analytical GREEnness (AGREE). The RP-HPTLC method consistently demonstrated superior environmental compatibility across all metrics [6] [7]. The AGREE score, which incorporates all 12 principles of GAC, was particularly telling. The RP method achieved a high score of 0.89 (on a 0-1 scale, where 1 is ideal), significantly outperforming the NP method's score of 0.46 [6]. This substantial difference primarily stems from the RP method's use of ethanol-water as the mobile phase versus the NP method's use of chloroform-methanol. Chloroform is more hazardous and generates more dangerous waste than ethanol [6].

HPTLC Method Comparison Workflow. This diagram illustrates the parallel development paths and divergent outcomes of NP-HPTLC and RP-HPTLC methods, highlighting the critical role of mobile phase selection in determining the final greenness score.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key Research Reagents and Materials for HPTLC Analysis

Item	Function/Role in Analysis
Silica gel 60 NP-18F254S plates	Stationary phase for normal-phase HPTLC separation
Silica gel 60 RP-18F254S plates	Stationary phase for reversed-phase HPTLC separation
Chloroform	Organic solvent component for NP-HPTLC mobile phase
Methanol	Organic solvent modifier for NP-HPTLC mobile phase
Ethanol	Green solvent component for RP-HPTLC mobile phase
Deionized Water	Aqueous component for RP-HPTLC mobile phase
Ertugliflozin Standard	Reference standard for method development and calibration
Marketable ERZ Tablets	Pharmaceutical formulation for method application

The selection of ethanol as the primary organic solvent in the RP-HPTLC method represents a conscious green chemistry choice. Ethanol is biodegradable, less toxic, and more environmentally friendly than chloroform, which is classified as a hazardous solvent with potential health and environmental risks [6]. This substitution is the primary factor contributing to the significantly higher AGREE score of the RP-HPTLC method.

This comprehensive comparison demonstrates that the RP-HPTLC method is markedly superior to the NP-HPTLC approach for the analysis of Ertugliflozin. The RP method offers enhanced analytical performance in terms of sensitivity, linear range, and accuracy when applied to pharmaceutical formulations. More significantly, from a sustainability perspective, the RP method exhibits a substantially improved greenness profile, as quantified by multiple assessment tools including the AGREE metric (0.89 vs. 0.46). The successful replacement of hazardous solvents like chloroform with greener alternatives such as ethanol establishes RP-HPTLC as a viable, eco-friendly solution for the routine quality control of Ertugliflozin in pharmaceutical products. This case study underscores the importance and feasibility of integrating green chemistry principles into pharmaceutical analysis without compromising analytical performance.

The determination of active pharmaceutical ingredients (APIs) must balance analytical precision with environmental responsibility. Green Analytical Chemistry (GAC) principles address this challenge by promoting methods that reduce hazardous waste and energy consumption [19]. For the analysis of Dasatinib Monohydrate (DST), a tyrosine kinase inhibitor used in treating chronic myeloid leukemia, recent research has established sustainable analytical techniques that align with these principles [19].

This case study provides a direct comparison between two environmentally conscious methods for DST determination: Green Normal Phase High-Performance Thin-Layer Chromatography (G-NP-HPTLC) and Green Reverse Phase High-Performance Thin-Layer Chromatography (G-RP-HPTLC). Both methods were systematically evaluated using the AGREE (Analytical GREEnness) tool, which offers a comprehensive assessment based on the 12 principles of GAC [19]. The comparison focuses on their analytical performance, practical implementation, and environmental footprint to guide researchers and pharmaceutical analysts in selecting appropriate green methodologies for quality control and pharmaceutical analysis.

Analytical Methodologies: Experimental Protocols

Chemical Reagents and Materials

• Dasatinib Monohydrate API: Reference standard of analytically pure (99.4%) DST [19]
• Pharmaceutical Formulation: Marketed DST tablets [19]
• Green Solvents for RP-HPTLC: 2-Propanol, water, and glacial acetic acid [19] [33]
• Green Solvents for NP-HPTLC: Methanol, n-butyl acetate, and glacial acetic acid [19] [33]
• HPTLC Plates: Silica gel 60 F254 plates (10 cm × 10 cm) [19]

Instrumentation and Chromatographic Conditions

The analysis was performed using standard HPTLC systems equipped with automatic sample applicators, development chambers, and TLC scanners. Key instrumental parameters are summarized in Table 1.

Table 1: Instrumental Conditions for G-NP-HPTLC and G-RP-HPTLC Methods

Parameter	G-RP-HPTLC Method	G-NP-HPTLC Method
Stationary Phase	Reversed phase	Normal phase (silica gel)
Mobile Phase Composition	2-Propanol:water:glacial acetic acid (60:40:0.2, v/v/v) [19] [33]	Methanol:n-butyl acetate:glacial acetic acid (50:50:0.2, v/v/v) [19] [33]
Detection Wavelength	323 nm (reflectance-absorbance mode) [19]	323 nm (reflectance-absorbance mode) [19]
Sample Application Volume	Variable (according to concentration range)	Variable (according to concentration range)
Development Chamber	Standard twin-trough glass chamber	Standard twin-trough glass chamber
Development Distance	80 mm	80 mm
Run Time	Approximately 15-20 minutes	Approximately 15-20 minutes

Sample Preparation Procedures

Standard Solution Preparation: An appropriate amount of DST reference standard was accurately weighed and dissolved in methanol to obtain a stock solution of known concentration. Working standard solutions were prepared by serial dilution to required concentrations [19].

Sample Solution Preparation: For tablet formulation analysis, the average weight of twenty tablets was determined. The powder equivalent to one tablet was accurately weighed, transferred to a volumetric flask, and extracted with methanol via sonication. The solution was filtered, and the filtrate was used for analysis [19].

Method Validation Parameters

Both methods were validated according to International Council for Harmonisation (ICH) guideline Q2(R1) for the following parameters [19]:

• Specificity: Confirmed by analyzing blank and sample, demonstrating no interference at Rf values of DST
• Linearity: Established over specified concentration ranges
• Accuracy: Determined through recovery studies by spiking known amounts of standard
• Precision: Evaluated as repeatability (intra-day) and intermediate precision (inter-day)
• Robustness: Assessed by deliberate variations in method parameters
• Detection and Quantitation Limits: Determined based on signal-to-noise ratio

Results and Comparative Performance Data

Analytical Performance Comparison

Table 2: Performance Characteristics of G-NP-HPTLC and G-RP-HPTLC Methods

Performance Characteristic	G-RP-HPTLC Method	G-NP-HPTLC Method
Rf Value	0.31 ± 0.02 [19] [33]	0.39 ± 0.02 [19] [33]
Linearity Range	30-500 ng/spot [19] [33]	200-1200 ng/spot [19] [33]
Correlation Coefficient (R²)	0.9998 [19] [33]	0.9995 [19] [33]
Precision (% RSD)	<2% [19]	<2% [19]
Accuracy (% Recovery)	97.8-102.9% [19]	97.8-102.9% [19]
Spot Characteristics	Compact and dense [19]	Compact and dense [19]
Application in Formulation Analysis	Satisfactory results [19]	Satisfactory results [19]

Greenness Assessment Using AGREE Metric

Table 3: AGREE Greenness Evaluation for HPTLC Methods

Assessment Criteria	G-RP-HPTLC Method	G-NP-HPTLC Method
AGREE Score	0.90 [19] [33]	0.88 [19] [33]
Greenness Interpretation	Extremely green [19]	Extremely green [19]
Solvent Greenness	High (2-propanol, water) [19]	High (methanol, n-butyl acetate) [19]
Energy Consumption	Low (ambient temperature operation)	Low (ambient temperature operation)
Waste Generation	Minimal (<10 mL solvent consumption) [14]	Minimal (<10 mL solvent consumption) [14]
Sample Throughput	High (parallel analysis) [14]	High (parallel analysis) [14]

The AGREE scores close to 1.0 for both methods confirm their excellent environmental profile, significantly surpassing traditional HPLC methods which typically consume larger volumes of organic solvents and generate more waste [19] [14].

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 4: Key Research Reagent Solutions for Green HPTLC Methods

Item	Function/Role in Analysis	Green Alternative Considerations
2-Propanol	Less hazardous solvent in RP-HPTLC mobile phase [19]	Considered environmentally preferable compared to acetonitrile [19]
n-Butyl Acetate	Green organic solvent in NP-HPTLC mobile phase [19]	Classified as a green solvent per GAC principles [19]
Methanol	Extraction solvent and mobile phase component [19]	Preferred over more toxic alternatives like chloroform
Water	Environmentally benign solvent in RP-HPTLC [19]	Ideal green solvent with zero environmental impact
Glacial Acetic Acid	Mobile phase modifier to control separation [19]	Used in minimal quantities (0.2% v/v) [19]
Silica Gel HPTLC Plates	Stationary phase for separation	Reusable with proper pretreatment
Densitometer Scanner	Quantitative analysis of separated spots	Non-destructive detection enables additional analysis

Method Selection Workflow and Greenness Assessment

The following workflow diagram illustrates the logical decision process for selecting and implementing green HPTLC methods, incorporating AGREE metric evaluation:

Comparative Analysis and Research Implications

Advantages of G-RP-HPTLC Method

The G-RP-HPTLC method demonstrates superior sensitivity with a lower linearity range (30-500 ng/spot) compared to the NP-HPTLC approach, making it particularly suitable for trace analysis [19] [33]. The method achieved a slightly higher AGREE score (0.90), reflecting its exceptional environmental profile [19] [33]. The use of 2-propanol and water as primary solvent components aligns well with GAC principles by utilizing less hazardous chemicals [19].

Advantages of G-NP-HPTLC Method

The G-NP-HPTLC method offers a wider linearity range (200-1200 ng/spot), making it appropriate for analyses requiring a broader dynamic concentration range [19] [33]. Despite its marginally lower AGREE score (0.88), it still qualifies as an extremely green method [19] [33]. The use of n-butyl acetate, classified as a green solvent, contributes to its environmentally friendly characteristics [19].

Strategic Implementation in Pharmaceutical Analysis

Both green HPTLC methods present viable alternatives to conventional HPLC, which typically consumes larger volumes of organic solvents and generates more waste [14]. The minimal mobile phase consumption (<10 mL per analysis) and capacity for parallel sample processing make these methods particularly attractive for high-throughput quality control laboratories seeking to reduce their environmental footprint while maintaining analytical rigor [19] [14].

The successful application of both methods to the analysis of DST in marketed formulations confirms their practicality for routine pharmaceutical analysis [19]. The validation according to ICH guidelines ensures that these methods meet regulatory requirements for method reliability, accuracy, and precision [19].

This comparative case study demonstrates that both G-RP-HPTLC and G-NP-HPTLC methods provide effective, validated approaches for the determination of Dasatinib Monohydrate while aligning with Green Analytical Chemistry principles. The G-RP-HPTLC method, with its higher sensitivity and slightly superior AGREE score, may be preferred for trace analysis, while the G-NP-HPTLC method offers advantages for applications requiring a wider linear dynamic range.

The AGREE metric scores of 0.90 and 0.88, respectively, confirm that both methods qualify as environmentally sustainable alternatives to traditional chromatographic methods. Their implementation in pharmaceutical quality control laboratories can significantly reduce the environmental impact of analytical operations while maintaining the high standards of accuracy, precision, and reliability required for pharmaceutical analysis.

Simultaneous Multi-Analyte Determination for COVID-19 Antivirals

The COVID-19 pandemic necessitated the rapid development and repurposing of antiviral medications, creating an urgent need for reliable analytical methods to quantify these drugs in various matrices. Simultaneous determination of multiple antiviral agents provides significant advantages for therapeutic drug monitoring, pharmacokinetic studies, and quality control in pharmaceutical manufacturing [34]. Among the various analytical techniques, High-Performance Thin-Layer Chromatography (HPTLC) has emerged as a powerful tool for multi-analyte determination due to its flexibility, cost-effectiveness, and ability to analyze several samples in parallel [35] [36].

A critical advancement in this field involves the comparison of Normal-Phase (NP) versus Reversed-Phase (RP) HPTLC methodologies within the framework of green analytical chemistry. The principles of White Analytical Chemistry (WAC), which encompass the three pillars of analytical performance, eco-compatibility, and practicality, now represent the state-of-the-art paradigm in sustainable analytical chemistry [35]. This comprehensive analysis objectively compares NP-HPTLC and RP-HPTLC methods for simultaneous determination of COVID-19 antivirals, with particular emphasis on their environmental impact using the AGREE (Analytical GREEnness) metric system alongside other complementary assessment tools.

Key COVID-19 Antiviral Agents

The primary antiviral agents used against SARS-CoV-2 include Remdesivir (RMD), Favipiravir (FAV), and Molnupiravir (MOL). These drugs represent the first FDA-approved single-drug treatments for COVID-19 and share a common mechanism of action as RNA polymerase inhibitors, though through distinct molecular pathways [35].

Table 1: Characteristics of Key COVID-19 Antiviral Agents

Drug	Chemical Classification	Mechanism of Action	Original Indication
Remdesivir	Adenosine triphosphate derivative prodrug	Inhibition of viral RNA polymerase	Ebola virus infection
Favipiravir	Pyrazinecarboxamide derivative prodrug	Viral RNA polymerase inhibitor	Influenza infection
Molnupiravir	Ribonucleoside analog prodrug	Induces mutations in viral RNA replication	Influenza infection

Remdesivir was the first FDA-approved antiviral for COVID-19 treatment, originally developed for Ebola virus infection [35] [37]. It is an adenosine triphosphate derivative prodrug that works by inhibiting the viral RNA polymerase enzyme, thereby preventing coronavirus replication [35]. Favipiravir was initially developed in Japan for influenza treatment and was repurposed for COVID-19 due to its ability to accelerate viral clearance [35]. Molnupiravir, also originally produced for influenza, received FDA emergency use authorization for mild to moderate COVID-19 cases and functions by introducing mutations into the viral RNA during replication [35].

Experimental Protocols for HPTLC Methodologies

Normal-Phase HPTLC Method

The NP-HPTLC method employs a polar stationary phase consisting of TLC silica gel aluminum plates 60 F254. The mobile phase for simultaneous separation of RMD, FAV, and MOL typically consists of ethyl acetate:ethanol:water in a ratio of 9.4:0.4:0.25 (v/v) [35]. Sample application is performed using an automated sample applicator (such as CAMAG Linomat 5) under a nitrogen stream, with bands typically 6 mm in width and applied 1.0 cm from the bottom edge of the plate. The application volume generally ranges from 5-10 μL per band.

The chromatographic development occurs in a twin-trough glass chamber saturated with the mobile phase for approximately 15-30 minutes at room temperature. The development distance is typically 9 cm from the point of application. After development, the plates are air-dried and scanned densitometrically at multiple wavelengths optimized for each analyte - 244 nm for RMD and MOL, and 325 nm for FAV [35].

Reversed-Phase HPTLC Method

The RP-HPTLC method utilizes non-polar stationary phases, while employing a greener mobile phase consisting of ethanol:water in a ratio of 6:4 (v/v) [35]. The sample application process is similar to the NP-HPTLC method, with bands applied using an automated sample applicator. The key difference lies in the composition of the mobile phase, which contains a higher proportion of water, significantly improving the greenness profile of the method.

The chromatographic development follows similar parameters, with chamber saturation for 15-30 minutes and development distance of 9 cm. Detection is performed at the same wavelengths as the NP method (244 nm for RMD and MOL, 325 nm for FAV) to ensure comparable analytical performance [35].

Method Validation Parameters

Both NP-HPTLC and RP-HPTLC methods are validated according to International Council for Harmonisation (ICH) guidelines, assessing linearity, range, accuracy, precision, and robustness [35]. The methods demonstrate linearity over the range of 50-2000 ng/band for FAV and MOL, and 30-800 ng/band for RMD, with correlation coefficients not less than 0.99988, indicating excellent linear response across therapeutic concentration ranges [35].

Comparative Performance Data

Table 2: Quantitative Performance Comparison of NP-HPTLC vs. RP-HPTLC Methods

Parameter	NP-HPTLC Method	RP-HPTLC Method
Mobile Phase	Ethyl acetate:ethanol:water (9.4:0.4:0.25, v/v)	Ethanol:water (6:4, v/v)
Linearity Range (ng/band)	FAV, MOL: 50-2000; RMD: 30-800	FAV, MOL: 50-2000; RMD: 30-800
Correlation Coefficient	≥0.99988	≥0.99988
Detection Wavelength	RMD, MOL: 244 nm; FAV: 325 nm	RMD, MOL: 244 nm; FAV: 325 nm
Analysis Time	~15-30 minutes	~15-30 minutes
Sample Throughput	Multiple samples in parallel (typically 10-15 per plate)	Multiple samples in parallel (typically 10-15 per plate)

The experimental data reveals that both NP-HPTLC and RP-HPTLC methods provide comparable analytical performance in terms of linearity, sensitivity, and precision [35]. The key differentiators lie in the mobile phase composition and the resulting environmental impact, which becomes evident when applying green chemistry assessment metrics.

For simultaneous determination of co-administered COVID-19 medications including antivirals and cardiovascular drugs, a modified HPTLC method has been developed using a mobile phase of ethyl acetate, methylene chloride, methanol, and ammonia (6:4:4:1 by volume) with detection at 232 nm [36]. This method demonstrates the versatility of HPTLC for complex multi-drug analysis with a short runtime of approximately 15 minutes, making it suitable for high-throughput quality control laboratories [36].

Greenness Assessment Using AGREE and Complementary Metrics

AGREE Metric System

The AGREE (Analytical GREEnness) metric system provides a comprehensive assessment of method greenness based on all 12 principles of green analytical chemistry. It generates a pictogram with a overall score between 0 and 1, where higher scores indicate superior greenness profiles [35]. The AGREE metric evaluates multiple factors including energy consumption, waste generation, toxicity of reagents, operator safety, and sample preparation requirements.

For the NP-HPTLC method, the AGREE score is significantly impacted by the use of ethyl acetate in the mobile phase, which raises environmental concerns due to its volatility and potential toxicity. In contrast, the RP-HPTLC method utilizing ethanol:water mobile phase achieves a substantially higher AGREE score, approaching ideal greenness values [35]. Ethanol is recognized as a greener solvent due to its lower toxicity and better environmental profile compared to ethyl acetate.

Complementary Green Assessment Metrics

Table 3: Comparative Greenness Assessment Using Multiple Metrics

Assessment Metric	NP-HPTLC Method	RP-HPTLC Method
AGREE Score	Moderate (lower score)	Superior (higher score)
Analytical Eco-Scale	Good	Excellent
BAGI (Blue Applicability Grade Index)	Good applicability	Good applicability
RGB12 (Whiteness Assessment)	Moderate whiteness	Superior whiteness (95.6% reported in similar methods)
NEMI	2-3 green circles	3-4 green circles

The Blue Applicability Grade Index (BAGI) focuses on assessing the analytical method's practicality and applicability, representing a complementary blue concept to WAC that complies with the fit-for-purpose viewpoint [35]. Both NP-HPTLC and RP-HPTLC methods demonstrate good applicability scores in BAGI assessment, confirming their practical utility in routine analytical laboratories.

The RGB12 algorithm provides an integrated whiteness assessment by combining green, blue, and white analytical chemistry principles [35]. Recent studies have reported whiteness percentages as high as 95.6% for optimized HPTLC methods, particularly those employing greener solvent systems [38]. The RP-HPTLC method generally achieves superior whiteness scores compared to the NP-HPTLC approach due to its more environmentally friendly mobile phase composition.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 4: Essential Research Reagent Solutions for COVID-19 Antiviral Analysis

Reagent/Material	Function/Application	Examples/Specifications
HPTLC Plates	Stationary phase for chromatographic separation	TLC silica gel aluminum plates 60 F254 (0.25-mm thickness) [39]
Mobile Phase Solvents	Liquid phase for compound separation	Ethyl acetate, ethanol, methanol, dichloromethane, water [35] [37]
Standard References	Method calibration and quantification	Remdesivir (purity ≥99.8%), Favipiravir (purity ≥99.68%), Molnupiravir [35] [36]
Sample Application Instrument	Precise sample deposition	CAMAG Linomat 5 autosampler with 100-μL syringe [36]
Densitometer Scanner	Quantitative detection of separated compounds	CAMAG TLC scanner 3 with deuterium lamp, operated in absorbance mode [37]
Chromatographic Development Chamber	Container for mobile phase development	CAMAG twin trough glass chamber (20 × 20 cm) [39]

The selection of appropriate research reagents and materials is critical for successful method development and validation. The stationary phase specification is particularly important, with TLC silica gel 60 F254 plates being the most commonly used for NP-HPTLC, while various modified silica plates are available for RP-HPTLC [39]. The mobile phase solvents should be of HPLC grade to ensure reproducibility and minimize interference from impurities.

For quantitative analysis, high-purity reference standards are essential, with certification of purity percentages provided by reputable suppliers [36] [37]. The instrumentation for sample application and detection must provide precise volumetric control and accurate scanning capabilities to achieve the required sensitivity and reproducibility for pharmaceutical analysis.

Analytical Workflow and Sustainability Assessment

The following diagram illustrates the logical relationship between methodological choices and their impact on sustainability assessment in the context of NP-HPTLC versus RP-HPTLC comparison:

This workflow demonstrates how the choice between NP-HPTLC and RP-HPTLC methodologies directly influences the greenness assessment outcome, ultimately positioning RP-HPTLC as the preferred approach from a sustainability perspective while maintaining comparable analytical performance.

The comprehensive comparison of NP-HPTLC versus RP-HPTLC methods for simultaneous determination of COVID-19 antiviral agents reveals that both techniques provide excellent analytical performance with high sensitivity, precision, and accuracy. However, when evaluated within the framework of green analytical chemistry principles using the AGREE metric system and complementary assessment tools, RP-HPTLC demonstrates superior environmental sustainability profiles.

The key advantage of RP-HPTLC lies in its ability to utilize mobile phases with higher water content and greener organic modifiers like ethanol, significantly reducing the environmental impact without compromising analytical performance. The AGREE score, complemented by whiteness assessment using the RGB12 algorithm, provides a comprehensive sustainability evaluation that positions RP-HPTLC as the preferred choice for environmentally conscious analytical laboratories.

This comparison guide provides researchers, scientists, and drug development professionals with objective data to inform their selection of analytical methodologies for COVID-19 antiviral determination, balancing analytical performance with environmental sustainability in alignment with the principles of White Analytical Chemistry.

Optimizing AGREE Scores: Strategies for Enhanced Method Greenness

Critical Parameters Affecting AGREE Scores in HPTLC Methods

The Analytical GREEnness (AGREE) metric is a comprehensive assessment tool that evaluates the environmental impact of analytical methods based on the 12 principles of Green Analytical Chemistry (GAC) [6] [19]. This tool provides a final score between 0 and 1, where higher scores indicate greener analytical procedures [19]. In pharmaceutical analysis, High-Performance Thin-Layer Chromatography (HPTLC) has gained significant attention due to its inherent green characteristics, including minimal solvent consumption, low energy requirements, and reduced waste generation compared to conventional chromatographic techniques [14] [9].

The comparison between Reversed-Phase (RP-HPTLC) and Normal-Phase (NP-HPTLC) methods has become increasingly relevant in sustainable method development. While NP-HPTLC typically utilizes non-polar stationary phases with polar mobile phases, RP-HPTLC employs polar stationary phases with less polar mobile phases, fundamentally influencing their greenness profiles [6] [9]. Understanding the critical parameters affecting AGREE scores enables researchers to design chromatographic methods that align with sustainability goals without compromising analytical performance.

Comparative AGREE Profiles of NP-HPTLC and RP-HPTLC Methods

Quantitative AGREE Score Comparisons

Table 1: Direct Comparison of AGREE Scores for NP-HPTLC and RP-HPTLC Methods

Analytical Compound	NP-HPTLC AGREE Score	RP-HPTLC AGREE Score	Key Mobile Phase Differences	Reference
Dasatinib Monohydrate	0.88	0.90	NP: methanol/n-butyl acetate/acetic acid (50:50:0.2, v/v/v)RP: 2-propanol/water/acetic acid (60:40:0.2, v/v/v)	[19]
Ertugliflozin	Lower than RP	Higher than RP	NP: chloroform/methanol (85:15, v/v)RP: ethanol/water (80:20, v/v)	[6]
Pterostilbene	0.46	0.78	NP: classical solvent systemsRP: green solvent systems	[9]
Antiviral Agents (RMD, FAV, MOL)	Comprehensive greenness assessment performed	Comprehensive greenness assessment performed	NP: ethyl acetate/ethanol/water (9.4:0.4:0.25, v/v)RP: ethanol/water (6:4, v/v)	[13]

Detailed AGREE Pictogram Interpretation

The AGREE metric output provides a circular pictogram with 12 segments corresponding to each GAC principle, offering immediate visual assessment of an analytical method's environmental performance [19] [13]. Each segment is colored from red to green, with the central numerical value representing the overall score. Key principles evaluated include:

• Prevention of waste (Principle 1)
• Use of safer solvents and reagents (Principle 5)
• Reduction of energy consumption (Principle 6)
• Integration of real-time analysis (Principle 11)
• Prevention of accidents (Principle 12) [19]

The AGREE software calculates scores by assigning weights to each principle based on their relative importance, generating both segment-specific and overall assessments [19]. Scores above 0.75 are generally considered to represent environmentally friendly analytical methods [19].

Critical Parameters Influencing AGREE Scores

Mobile Phase Composition and Solvent Selection

The choice of solvents in mobile phase preparation significantly impacts AGREE scores, primarily through Principles 1 (waste prevention), 5 (safer solvents), and 12 (accident prevention) [6] [9]. Research demonstrates that RP-HPTLC methods frequently achieve higher greenness scores due to their compatibility with water-ethanol-based mobile phases, which are inherently less toxic and more biodegradable than the organic solvents typically employed in NP-HPTLC [6] [9].

For ertugliflozin analysis, the RP-HPTLC method using ethanol-water (80:20, v/v) mobile phase demonstrated superior greenness compared to the NP-HPTLC method employing chloroform-methanol (85:15, v/v) [6]. Similarly, for antiviral agents (RMD, FAV, MOL), the RP-HPTLC method utilized ethanol-water (6:4, v/v), while the NP-HPTLC method required ethyl acetate-ethanol-water (9.4:0.4:0.25, v/v) [13]. The replacement of hazardous solvents like chloroform and hexane with greener alternatives such as ethanol and water directly improves AGREE scores [19] [9].

Sample Preparation and Derivatization Requirements

Minimal sample pretreatment and the elimination of derivatization steps contribute significantly to higher AGREE scores by reducing reagent consumption and waste generation [14] [40]. HPTLC methods generally require less extensive sample preparation compared to other chromatographic techniques, aligning with GAC Principle 2 (sample preparation) [14].

The development of a salivary caffeine HPTLC method exemplifes this advantage, requiring only a 1:1 dilution with methanol followed by direct application to the HPTLC plate, eliminating complex extraction procedures [40]. This streamlined approach reduces solvent consumption and analysis time, positively influencing multiple AGREE principles [40].

Energy Consumption and Analysis Throughput

HPTLC methods generally consume less energy than HPLC and other pumped systems due to the absence of high-pressure pumps and reduced need for temperature control [14] [9]. This directly addresses GAC Principle 6 (energy reduction). The ability to analyze multiple samples simultaneously on a single HPTLC plate (typically 10-20 samples) significantly improves analytical throughput and reduces energy consumption per sample [14] [13].

For the analysis of dasatinib monohydrate, both NP- and RP-HPTLC methods demonstrated high throughput capabilities, with the RP method exhibiting slightly better greenness performance (AGREE score 0.90 vs. 0.88) [19]. The parallel processing nature of HPTLC provides inherent advantages in Principles 8 (multianalyte capability) and 9 (miniaturization) of the AGREE assessment [14].

Waste Generation and Management

HPTLC methods typically generate significantly less waste (often < 10 mL per analysis) compared to HPLC methods, which may consume hundreds of milliliters of mobile phase [14]. This substantial reduction in solvent waste directly addresses GAC Principle 1 (waste prevention) and contributes to higher AGREE scores [14] [9].

The miniaturized nature of HPTLC separation, with small plate dimensions and minimal mobile phase requirements, positions it favorably in waste-related AGREE criteria [14]. In the case of pterostilbene analysis, the waste reduction in the RP-HPTLC method contributed to its substantially higher AGREE score (0.78) compared to the NP-HPTLC approach (0.46) [9].

Table 2: Comprehensive Parameter Analysis for AGREE Score Optimization

Parameter Category	Specific Factors	Impact on AGREE Score	Optimization Strategy
Mobile Phase	Solvent toxicity, volume, biodegradability	High	Replace chloroform/hexane with ethanol/water; reduce overall volume
Sample Preparation	Number of steps, reagent consumption, derivatization	Medium-High	Simplify sample clean-up; eliminate derivatization steps
Energy Consumption	Instrument power requirements, analysis time	Medium	Utilize ambient temperature development; parallel sample processing
Waste Management	Total waste volume, disposal requirements	High	Implement solvent recycling; minimize mobile phase volume
Operator Safety	Solvent exposure risk, protective equipment	Medium	Choose less toxic solvents; improve method robustness

Experimental Protocols for AGREE-Optimized HPTLC Methods

Method Development with Green Solvent Selection

The development of AGREE-optimized HPTLC methods begins with systematic solvent selection based on green chemistry principles. For RP-HPTLC, ethanol-water mixtures typically serve as the starting point for method development, with adjustment of ratios to achieve optimal separation [6] [9]. For NP-HPTLC, replacement of traditional hazardous solvents like chloroform and benzene with less toxic alternatives such as ethyl acetate and ethanol is essential [19].

Advanced approaches incorporate computer-assisted method development tools. The Hansen Solubility Parameters in Practice (HSPiP) software enables prediction of suitable green solvents based on the cohesive energy between analytes and solvents, reducing trial-and-error experimentation [41]. Similarly, Quality by Design (QbD) approaches utilizing Box-Behnken designs help identify critical method parameters while minimizing experimental runs and solvent consumption [41].

AGREE Assessment Methodology

The AGREE evaluation process involves:

• Compiling all method parameters: Including sample preparation, reagents, instrumentation, and waste disposal data [6] [19]
• Inputting data into AGREE software: Accessible as a free online tool
• Interpreting the pictogram: Identifying weak segments for improvement [19] [13]
• Iterative optimization: Modifying method parameters to improve low-scoring segments [9]

For comparative studies between NP- and RP-HPTLC, identical standard solutions and sample preparations should be maintained to isolate the influence of chromatographic conditions on AGREE scores [6] [9].

Strategic Pathways for Green HPTLC Method Development

Essential Research Reagent Solutions

Table 3: Key Reagents and Materials for Green HPTLC Method Development

Reagent/Material	Function in HPTLC	Green Considerations	Typical Applications
Ethanol-Water Mixtures	Mobile phase for RP-HPTLC	Biodegradable, low toxicity	First-choice mobile phase for RP-HPTLC [6] [9]
Ethyl Acetate	Mobile phase component for NP-HPTLC	Less hazardous alternative to chlorinated solvents	NP-HPTLC when higher elution strength needed [19] [13]
Silica Gel 60 F254 plates	Stationary phase for NP-HPTLC	Standard NP separation material	Widely used for NP-HPTLC applications [6] [40]
RP-18 F254 plates	Stationary phase for RP-HPTLC	Enables use of aqueous mobile phases	Green RP-HPTLC methods [6] [42]
Methanol	Sample dissolution, mobile phase component	Prefer ethanol when possible	Sample preparation and standard solutions [40] [9]

The critical parameters affecting AGREE scores in HPTLC methods systematically favor RP-HPTLC approaches over NP-HPTLC when environmental impact is considered. The primary advantages stem from RP-HPTLC's compatibility with ethanol-water mobile phases, which address multiple GAC principles simultaneously through reduced toxicity, improved biodegradability, and enhanced operator safety [6] [9].

Method developers should prioritize initial solvent selection as the most significant factor in AGREE optimization, with ethanol-water systems representing the greenest starting point for RP-HPTLC [6]. For NP-HPTLC, replacement of traditional hazardous solvents with ethyl acetate-based systems can substantially improve greenness profiles [19] [13]. The integration of assessment tools like AGREE throughout method development provides a structured pathway to sustainable analytical practices that align with the principles of Green Analytical Chemistry [19] [13].

The modern industrial and pharmaceutical landscape is undergoing a profound transformation driven by the principles of Green Chemistry, which focuses on designing chemical products and processes to reduce or eliminate hazardous substances [43]. This paradigm shift moves beyond mere regulatory compliance toward intelligent design of safer, more sustainable operational processes. In pharmaceutical analysis, high-performance thin-layer chromatography (HPTLC) has emerged as a valuable tool, with recent research comparing the greenness of normal-phase (NP) and reversed-phase (RP) techniques through comprehensive metric systems like the Analytical GREEnness (AGREE) approach [6] [15] [19]. AGREE provides a holistic assessment framework utilizing all 12 principles of green analytical chemistry, offering a vibrant pictograph that highlights both strengths and weaknesses of analytical methods [19].

The transition to greener solvents and processes aligns with key principles of green chemistry, particularly Principle #5 (Safer Solvents and Auxiliaries), which emphasizes minimizing or eliminating auxiliary substances or replacing them with safer alternatives [43]. This review objectively compares the performance and environmental profiles of NP-HPTLC and RP-HPTLC methods, focusing on solvent selection strategies that replace hazardous solvents with benign alternatives while maintaining analytical performance.

Theoretical Framework: Green Solvent Selection and AGREE Metrics

Green Chemistry Principles in Solvent Selection

The foundation of green solvent selection rests on evaluating chemicals against multiple environmental and safety parameters. Ideal green solvents exhibit low toxicity, minimal environmental persistence, reduced bioaccumulation potential, and originate from renewable feedstocks where possible [43] [23]. The 12 principles of green chemistry provide a framework for designing safer chemical processes, with several principles directly relevant to solvent selection:

• Principle #3: Less Hazardous Chemical Syntheses: Designing processes that use and generate substances with little or no toxicity
• Principle #5: Safer Solvents and Auxiliaries: Minimizing or eliminating the use of auxiliary substances
• Principle #7: Use of Renewable Feedstocks: Sourcing raw materials from renewable, bio-based sources
• Principle #10: Design for Degradation: Designing chemical products to break down into innocuous substances after use [43]

AGREE Metric System for Method Evaluation

The AGREE (Analytical GREEnness) metric system represents a significant advancement in assessing the environmental friendliness of analytical methods [6] [15] [19]. Unlike earlier assessment tools that evaluated only limited aspects of greenness, AGREE incorporates all 12 principles of green analytical chemistry, providing a comprehensive 0-1 scoring system where higher scores indicate superior greenness profiles [19]. This approach evaluates multiple factors including energy consumption, waste generation, toxicity of reagents, operator safety, and biodegradability of chemicals used [15].

Table 1: Comparison of Greenness Assessment Tools

Assessment Tool	Principles Evaluated	Output Format	Key Advantages	Key Limitations
AGREE	All 12 principles of green analytical chemistry	0-1 score with pictogram	Most comprehensive assessment	Requires detailed method information
NEMI	4 principles	Pass/Fail pictogram	Simple interpretation	Limited scope
Analytical Eco-Scale	Multiple parameters	Penalty point system	Quantitative scoring	Does not cover all green principles
GAPI	Multiple parameters	Pictogram	Visual representation	Less comprehensive than AGREE

Experimental Comparison: NP-HPTLC vs. RP-HPTLC Methodologies

Method Development and Optimization Protocols

The method development for both NP-HPTLC and RP-HPTLC follows systematic optimization of mobile phase compositions to achieve optimal separation efficiency. For NP-HPTLC methods, researchers typically evaluate binary solvent combinations such as chloroform/methanol, methanol/ethyl acetate, hexane/acetone, and ethyl acetate/cyclohexane [6]. In the case of ertugliflozin analysis, the optimal NP-HPTLC mobile phase was identified as chloroform/methanol (85:15 v/v), which produced a well-eluted and sharp chromatographic signal at Rf = 0.29 ± 0.01 [6].

For RP-HPTLC method development, different binary solvent combinations are investigated, including acetone/water, ethanol/water, ethanol/ethyl acetate, and ethanol/acetone [6]. In multiple pharmaceutical applications, ethanol-water mixtures have emerged as preferred mobile phases due to their favorable environmental profile and excellent chromatographic performance [6] [44]. For instance, a validated RP-HPTLC method for rivaroxaban quantification utilized ethanol:water (7:3 v/v) as the mobile phase, demonstrating excellent linearity (50–600 ng per band) with Rf = 0.71 ± 0.02 [44].

Analytical Performance Comparison

Direct comparison of NP-HPTLC and RP-HPTLC methods for various pharmaceutical compounds reveals consistent patterns in analytical performance. In the analysis of ertugliflozin, both techniques demonstrated acceptable validation parameters, but RP-HPTLC showed superior linearity range (25–1200 ng/band) compared to NP-HPTLC (50–600 ng/band) [6]. Similar trends were observed in flibanserin analysis, where RP-HPTLC exhibited better sensitivity with a linear range of 100–1600 ng/band versus 200–1600 ng/band for NP-HPTLC [15].

Table 2: Performance Comparison of NP-HPTLC and RP-HPTLC Methods for Pharmaceutical Compounds

Pharmaceutical Compound	Method	Mobile Phase	Linearity Range	Sensitivity (LOD)	AGREE Score
Ertugliflozin [6]	NP-HPTLC	CHCl₃/MeOH (85:15 v/v)	50–600 ng/band	Higher	Not specified
	RP-HPTLC	EtOH/H₂O (80:20 v/v)	25–1200 ng/band	Lower	Not specified
Flibanserin [15]	NP-HPTLC	Ethyl acetate/MeOH (95:5 v/v)	200–1600 ng/band	Lower	0.80
	RP-HPTLC	Acetone/H₂O (80:20 v/v)	100–1600 ng/band	Higher	0.86
Dasatinib [19]	NP-HPTLC	MeOH/n-butyl acetate/ glacial acetic acid (50:50:0.2 v/v/v)	200–1200 ng/spot	Not specified	0.88
	RP-HPTLC	2-propanol/H₂O/ glacial acetic acid (60:40:0.2 v/v/v)	30–500 ng/spot	Not specified	0.90
Pterostilbene [9]	NP-HPTLC	Conventional solvents	30–400 ng/band	Lower	0.46
	RP-HPTLC	Green solvents	10–1600 ng/band	Higher	0.78

The system suitability parameters, including retardation factor (Rf), tailing factor (As), and theoretical plates per meter (N/m), consistently favor RP-HPTLC methods across multiple studies. For ertugliflozin analysis, the RP-HPTLC method demonstrated superior performance with As = 1.08 ± 0.03 and N/m = 4652 ± 4.02 compared to NP-HPTLC values of As = 1.06 ± 0.02 and N/m = 4472 ± 4.22 [6].

Greenness Assessment: AGREE Score Evaluation

Comparative AGREE Scores for HPTLC Methods

The AGREE metric system provides quantitative evidence of the superior environmental profile of RP-HPTLC methods compared to their NP-HPTLC counterparts. In the analysis of flibanserin, the RP-HPTLC method achieved an AGREE score of 0.86, significantly higher than the NP-HPTLC score of 0.80 [15]. Similarly, for dasatinib monohydrate analysis, RP-HPTLC obtained a score of 0.90 compared to 0.88 for NP-HPTLC [19]. The most striking difference was observed in pterostilbene analysis, where RP-HPTLC (0.78) substantially outperformed NP-HPTLC (0.46) in greenness assessment [9].

These AGREE scores reflect the cumulative environmental advantages of RP-HPTLC methods, particularly regarding solvent selection. RP-HPTLC typically employs environmentally preferable solvents like ethanol, water, and acetone, which align with green chemistry principles [6] [23]. In contrast, NP-HPTLC often relies on more hazardous solvents such as chloroform and methanol, which incur penalty points in greenness assessments due to their toxicity and environmental persistence [6] [15].

Green Solvent Alternatives for Hazardous Solvents

The transition to greener analytical methods involves replacing traditionally hazardous solvents with safer, more sustainable alternatives:

• d-Limonene: Derived from citrus peels, this bio-based solvent effectively replaces trichloroethylene, toluene, xylene, and mineral spirits in degreasing applications [43]
• Acetone: While flammable, acetone offers low toxicity and exemption from many VOC regulations, making it suitable for resin and plastic cleanup [43]
• Isopropyl Alcohol (IPA): This readily biodegradable solvent serves as a versatile precision cleaner, replacing chlorinated solvents in electronics and medical device cleaning [43]
• Ethyl Lactate: Derived from renewable resources, this biodegradable solvent offers low toxicity with excellent solvency properties [23]
• Deep Eutectic Solvents (DES): Customizable, biodegradable solvent mixtures that show promise in extraction applications [23] [45]

Diagram 1: AGREE Score Assessment Workflow for HPTLC Methods. This diagram illustrates the comparative greenness evaluation process for NP-HPTLC and RP-HPTLC methods, highlighting typical solvent choices and resulting AGREE scores.

Research Reagent Solutions: Essential Materials for Green HPTLC

Successful implementation of green HPTLC methods requires specific reagents and materials that align with sustainable chemistry principles while maintaining analytical performance.

Table 3: Essential Research Reagents for Green HPTLC Analysis

Reagent/Material	Function	Green Attributes	Application Notes
RP-18 F254S HPTLC Plates	Stationary phase for reversed-phase chromatography	Enables use of aqueous mobile phases	Superior for polar compounds [6] [44]
Silica Gel 60 F254S HPTLC Plates	Stationary phase for normal-phase chromatography	Traditional method	Requires more hazardous organic solvents [6]
Ethanol	Mobile phase component	Renewable, biodegradable, low toxicity	Preferred RP-HPTLC solvent [6] [44]
Water	Mobile phase component	Non-toxic, non-flammable, renewable	Ideal green solvent for RP-HPTLC [6] [23]
Acetone	Mobile phase component	Low toxicity, VOC-exempt	Fast evaporation, minimal residue [6] [43]
Isopropyl Alcohol	Mobile phase component	Readily biodegradable, versatile	Safe for sensitive materials [43]
Ethyl Acetate	Mobile phase component	Pleasant odor, effective	Greener alternative to MEK [43] [15]
d-Limonene	Bio-based solvent	Renewable, biodegradable, pleasant scent	Replaces toluene, xylene [43]

The comprehensive comparison of NP-HPTLC and RP-HPTLC methods demonstrates a clear trend: reversed-phase techniques consistently outperform normal-phase methods in both analytical performance and environmental sustainability. The strategic replacement of hazardous solvents with benign alternatives in RP-HPTLC, particularly through ethanol-water and acetone-water mobile phases, delivers superior greenness profiles as quantified by AGREE scores while maintaining or enhancing analytical precision, sensitivity, and accuracy.

For researchers and drug development professionals implementing green chemistry principles, RP-HPTLC methods offer a viable path toward sustainable analytical practices without compromising performance. The ongoing development of novel green solvents, including bio-based alternatives, deep eutectic solvents, and solvent-free methodologies, promises continued advancement in environmentally responsible pharmaceutical analysis [23] [45]. As regulatory pressures increase and sustainability metrics become more standardized, the adoption of AGREE-scored analytical methods will likely become essential practice in pharmaceutical development and manufacturing.

Diagram 2: Green Solvent Selection Decision Pathway. This workflow guides the selection of HPTLC methodologies and solvent systems based on analyte characteristics while emphasizing AGREE score assessment for greenness evaluation.

Miniaturization and Waste Reduction Techniques

The increasing global focus on sustainability has propelled the principles of Green Analytical Chemistry (GAC) to the forefront of pharmaceutical analysis. Traditional chromatographic methods, while highly effective, often rely on significant quantities of hazardous organic solvents, generate substantial waste, and consume considerable energy, thereby posing environmental and health risks [46]. The drive toward more eco-friendly practices has led to the adoption of white analytical chemistry (WAC), a state-of-the-art paradigm that evaluates methods based on three pillars: analytical performance, ecological compatibility, and practical/economic feasibility [13]. This comprehensive review focuses on the application of miniaturization and waste reduction techniques within planar chromatography, specifically through a detailed comparison of Normal-Phase High-Performance Thin-Layer Chromatography (NP-HPTLC) and Reversed-Phase High-Performance Thin-Layer Chromatography (RP-HPTLC).

The miniaturization of analytical techniques represents a fundamental strategy for reducing the environmental footprint of chemical analysis. By scaling down procedures, researchers can dramatically decrease solvent consumption, minimize waste generation, and lower energy requirements without compromising analytical performance [47]. HPTLC technology itself embodies several aspects of miniaturization and green chemistry through its ability to analyze multiple samples simultaneously on a single plate, its minimal sample preparation requirements, and its reduced solvent consumption compared to column chromatographic techniques [6] [13]. Within this framework, RP-HPTLC has emerged as a particularly promising approach that frequently utilizes more environmentally benign solvents—such as ethanol-water mixtures—in contrast to the often hazardous solvents typically employed in NP-HPTLC methods [6] [13] [7].

Analytical Greenness Evaluation and AGREE Metric

Fundamentals of Greenness Assessment

Evaluating the environmental friendliness of analytical methods requires robust, standardized metrics. The Analytical GREEnness (AGREE) metric has emerged as one of the most comprehensive tools for this purpose, as it incorporates all 12 principles of Green Analytical Chemistry into its assessment framework [6] [9]. The AGREE calculator generates a score on a scale of 0 to 1, where higher values indicate superior greenness profiles [13]. This metric complements other assessment tools such as the Analytical Eco-Scale, National Environmental Methods Index (NEMI), ChlorTox, and the more recent Blue Applicability Grade Index (BAGI) and Modified Green Analytical Procedure Index (MoGAPI) [13] [7].

The AGREE evaluation considers multiple factors including sample preparation, energy consumption, waste generation, toxicity of reagents, operator safety, and the derivatization process [6]. This comprehensive approach provides researchers with a holistic view of a method's environmental impact, enabling informed decisions when developing or selecting analytical procedures. The trend toward incorporating these greenness assessments reflects a broader shift in the pharmaceutical and analytical chemistry communities toward sustainability and environmental responsibility [46] [47].

AGREE Score Comparisons: NP-HPTLC vs. RP-HPTLC

Table 1: Comparative AGREE Scores for NP-HPTLC and RP-HPTLC Methods Across Various Applications

Analyzed Compound	NP-HPTLC AGREE Score	RP-HPTLC AGREE Score	Key Mobile Phase Components	Reference
Pterostilbene (PT)	0.46	0.78	NP: Conventional solvents (not specified)RP: Ethanol-Water	[9]
Ertugliflozin (ERZ)	Lower than RP (exact value not specified)	Higher than NP (exact value not specified)	NP: Chloroform-Methanol (85:15 v/v)RP: Ethanol-Water (80:20 v/v)	[6]
Lemborexant (LMB)	Lower than RP (exact value not specified)	0.89	NP: Acetone-Petroleum Ether (40:60 v/v)RP: Ethanol-Water (85:15 v/v)	[7]
Antiviral Agents (RMD, FAV, MOL)	Lower than RP (exact value not specified)	Higher than NP (exact value not specified)	NP: Ethyl Acetate:Ethanol:Water (9.4:0.4:0.25 v/v)RP: Ethanol-Water (6:4 v/v)	[13]

The consistent trend across multiple pharmaceutical applications clearly demonstrates the superior greenness profile of RP-HPTLC compared to NP-HPTLC methods. This advantage primarily stems from the more environmentally friendly mobile phases typically employed in RP-HPTLC, which often replace the hazardous solvents used in NP-HPTLC with greener alternatives like ethanol-water mixtures [6] [9] [7]. The higher AGREE scores for RP-HPTLC methods reflect their better alignment with the principles of green chemistry, particularly in terms of waste reduction, use of safer solvents, and reduced environmental impact [9].

Experimental Protocols and Methodologies

Standardized Experimental Workflow

The development and validation of both NP-HPTLC and RP-HPTLC methods follow standardized protocols that ensure reliability, reproducibility, and compliance with regulatory guidelines. The typical workflow encompasses several critical stages from method development through validation and greenness assessment.

Detailed Methodological Protocols

NP-HPTLC Method for Antiviral Agents

For the simultaneous analysis of remdesivir (RMD), favipiravir (FAV), and molnupiravir (MOL), the NP-HPTLC method employs specific parameters [13]:

• Stationary Phase: Standard silica gel NP-18F254S plates.
• Mobile Phase: Ethyl acetate:ethanol:water in the ratio 9.4:0.4:0.25 (v/v).
• Sample Application: Samples applied as bands using an automated sample applicator with nitrogen aspiration; application volume 4-8 μL.
• Chromatographic Development: Linear ascending development in a twin-trough glass chamber saturated with mobile phase vapor for 25 minutes at room temperature (25°C ± 2); migration distance 70 mm.
• Detection: Densitometric scanning at 244 nm for RMD and MOL, and 325 nm for FAV using a deuterium lamp.
• Validation: Linearity established over 50-2000 ng/band for FAV and MOL, and 30-800 ng/band for RMD with correlation coefficients ≥0.99988.

RP-HPTLC Method for Antiviral Agents

The RP-HPTLC method for the same antiviral agents demonstrates the greener approach [13]:

• Stationary Phase: RP-18F254S plates.
• Mobile Phase: Ethanol-water in the ratio 6:4 (v/v) - classified as a greener solvent system.
• Sample Application: Identical application parameters as NP method to ensure comparability.
• Chromatographic Development: Linear ascending development in a twin-trough glass chamber saturated with mobile phase vapor for 25 minutes at room temperature; migration distance 70 mm.
• Detection: Densitometric scanning at 244 nm for RMD and MOL, and 325 nm for FAV.
• Validation: Identical linearity ranges as NP method with correlation coefficients ≥0.99988.

Method Validation Parameters

Both NP-HPTLC and RP-HPTLC methods were rigorously validated according to International Council for Harmonisation (ICH) guidelines [13] [7]. The validation parameters assessed included:

• Linearity and Range: Demonstrated through correlation coefficients and residual plots.
• Precision: Evaluated as intra-day, inter-day, and instrumental precision with %RSD values.
• Accuracy: Determined through recovery studies at multiple concentration levels.
• Robustness: Assessed by deliberate variations in method parameters.
• Sensitivity: Determined via Limit of Detection (LOD) and Limit of Quantification (LOQ).
• Specificity: Established through peak purity tests and separation from degradation products.

Table 2: Comparison of Validation Parameters for NP-HPTLC vs. RP-HPTLC Methods

Validation Parameter	NP-HPTLC Performance	RP-HPTLC Performance	Interpretation
Linearity	R² ≥ 0.99988 for antivirals [13]	R² ≥ 0.99988 for antivirals [13]	Both techniques demonstrate excellent linearity
Precision	RSD 1.5-2.5% for pterostilbene [9]	RSD 0.87-1.00% for lemborexant [7]	RP-HPTLC shows better precision
Accuracy	89.24% recovery for lemborexant [7]	98.79% recovery for lemborexant [7]	RP-HPTLC provides superior accuracy
Sensitivity	LOD 1.5-2.5 ng/band for ertugliflozin [6]	LOD 0.92 ng/band for lemborexant [7]	RP-HPTLC offers enhanced sensitivity
Robustness	Moderate robustness for pterostilbene [9]	High robustness (uncertainties 0.90-0.95%) for lemborexant [7]	RP-HPTLC demonstrates better robustness

The Scientist's Toolkit: Essential Research Reagents and Materials

Key Materials for HPTLC Analysis

Table 3: Essential Research Reagents and Materials for NP-HPTLC and RP-HPTLC

Material/Reagent	Function/Purpose	NP-HPTLC Specifics	RP-HPTLC Specifics
HPTLC Plates	Stationary phase for separation	Silica gel 60 NP-18F254S plates [6] [13]	RP-18F254S plates [6] [13]
Mobile Phase Solvents	Carrier for analyte separation	Chloroform, methanol, ethyl acetate [6] [13]	Ethanol, water [6] [13] [7]
Sample Solvent	Dissolving and applying samples	Methanol typically used [9]	Methanol typically used [9]
Standard Reference Compounds	Method development and calibration	High-purity analytical standards (>98%) [13]	High-purity analytical standards (>98%) [13]
Derivatization Reagents	Visualizing non-UV absorbing compounds	Sulfuric acid, anisaldehyde, etc.	Less frequently required
Densitometer	Quantitative measurement of spots	Deuterium lamp, mercury lamp for detection [13]	Deuterium lamp, mercury lamp for detection [13]

Waste Reduction and Miniaturization Features

The HPTLC technique inherently incorporates several waste reduction and miniaturization advantages compared to column chromatography techniques:

• Miniaturized Separation: Multiple samples analyzed simultaneously on a single plate (typically 8-20 samples) [13].
• Reduced Solvent Consumption: Typically 10-20 mL of mobile phase required per development compared to hundreds of mL or liters in HPLC [6] [46].
• Minimal Sample Preparation: Often requires less extensive sample clean-up, reducing solvent use in preparation steps [46].
• Low Energy Consumption: No high-pressure pumps required, reducing energy use compared to HPLC [13].
• Reusable Components: Development chambers can be used repeatedly with proper cleaning.

Discussion: Environmental Impact and Practical Applications

Environmental Impact Assessment

The environmental advantages of RP-HPTLC over NP-HPTLC extend beyond AGREE scores to practical environmental benefits. The replacement of hazardous solvents like chloroform and hexane with ethanol-water mixtures in RP-HPTLC significantly reduces environmental toxicity, waste hazard, and operator exposure risks [6] [7]. Ethanol is biodegradable, derived from renewable resources, and poses minimal environmental threats compared to the chlorinated solvents often used in NP-HPTLC [47].

The miniaturization aspect of HPTLC technology in general offers substantial waste reduction benefits compared to conventional HPLC methods. While HPLC systems typically generate 1-1.5 L of solvent waste per day [46], HPTLC methods require only minimal solvent volumes for development. This reduction in solvent consumption directly translates to decreased environmental pollution, reduced waste disposal requirements, and lower analytical costs [13] [47].

Analytical Performance Considerations

While environmental considerations are crucial, analytical performance remains paramount in pharmaceutical analysis. The research indicates that RP-HPTLC methods not only provide environmental benefits but also frequently outperform NP-HPTLC in key analytical parameters [7]. The enhanced precision, accuracy, sensitivity, and robustness of RP-HPTLC methods make them particularly suitable for quality control applications in pharmaceutical development and manufacturing [6] [7].

Furthermore, RP-HPTLC methods have demonstrated excellent performance as stability-indicating methods, capable of separating active pharmaceutical ingredients from their degradation products [6] [13] [7]. This characteristic is essential for pharmaceutical quality control, where the accurate quantification of active ingredients in the presence of impurities is required by regulatory authorities.

Future Perspectives

The chromatography market is projected to experience robust growth, with the global Thin Layer Chromatography (TLC) analyzer market estimated to reach significant value by 2025-2031 [48] [49]. This growth is driven by increasing demand in pharmaceutical and biotechnology sectors for quality control and research applications. Future developments in HPTLC technology are likely to focus on enhanced automation, improved detection systems, and further miniaturization to increase throughput and reduce environmental impact [49].

The integration of green chemistry principles into analytical method development is expected to continue, with metrics like AGREE, BAGI, and MoGAPI playing increasingly important roles in method selection and validation [13]. The trend toward white analytical chemistry, which balances analytical performance, ecological compatibility, and practical feasibility, represents the future of sustainable pharmaceutical analysis [13].

The comprehensive comparison of NP-HPTLC and RP-HPTLC methods clearly demonstrates the significant advantages of reversed-phase techniques in terms of miniaturization, waste reduction, and overall environmental friendliness. The consistently higher AGREE scores for RP-HPTLC methods across various pharmaceutical applications confirm their superior greenness profile, primarily attributable to the use of ethanol-water mobile phases instead of the more hazardous solvents typically employed in normal-phase methods.

Crucially, the environmental benefits of RP-HPTLC do not come at the expense of analytical performance. The validation data from multiple studies indicates that RP-HPTLC methods often outperform their NP-HPTLC counterparts in terms of precision, accuracy, sensitivity, and robustness. This combination of excellent analytical performance and reduced environmental impact makes RP-HPTLC an ideal choice for pharmaceutical analysis aligned with the principles of green and white analytical chemistry.

As the field of analytical chemistry continues to evolve toward more sustainable practices, the adoption of RP-HPTLC and other green chromatographic techniques is expected to grow. The integration of comprehensive greenness assessment tools like AGREE into method development and validation protocols will further promote the adoption of environmentally responsible analytical practices throughout the pharmaceutical industry.

Energy Consumption and Throughput Optimization Strategies

High-Performance Thin-Layer Chromatography (HPTLC) has evolved from a simple qualitative tool to a sophisticated quantitative analytical platform aligned with green analytical chemistry (GAC) principles. A fundamental distinction in HPTLC methodologies lies in the choice between normal-phase (NP-HPTLC) and reversed-phase (RP-HPTLC) systems. This comparison examines their performance relative to energy consumption and analytical throughput, which are critical for sustainable method development in pharmaceutical analysis and drug development [14].

NP-HPTLC typically uses a polar stationary phase (e.g., silica gel) with a non-polar or moderately polar mobile phase. In contrast, RP-HPTLC employs a non-polar stationary phase (e.g., silica gel modified with C18 or C8 chains) with a polar mobile phase, often water-containing solvent mixtures [6] [13]. This core difference dictates their solvent consumption, operational workflow, and alignment with green chemistry principles, as measured by metrics like the Analytical GREEnness (AGREE) score [6].

The following table summarizes key experimental findings from direct comparative studies, highlighting the differences in efficiency and greenness credentials between NP- and RP-HPTLC methods.

Table 1: Direct Comparison of NP-HPTLC and RP-HPTLC Methods from Experimental Studies

Analytical Target	Method Type	Mobile Phase Composition	Key Greenness Indicator (AGREE Score)	Noted Performance Advantages
Ertugliflozin [6]	NP-HPTLC	Chloroform/Methanol (85:15, v/v)	Lower AGREE Score	Robust, but less green
	RP-HPTLC	Ethanol/Water (80:20, v/v)	Higher AGREE Score	Greener, more precise, sensitive
Antiviral Agents (RMD, FAV, MOL) [13]	NP-HPTLC	Ethyl acetate: Ethanol: Water (9.4:0.4:0.25, v/v)	Assessed with AES, AGREE, MoGAPI	Effective separation achieved
	RP-HPTLC	Ethanol: Water (6:4, v/v)	Superior Greenness Profile	Simpler, greener solvent system

Interpretation of Comparative Data

The consensus across studies indicates that RP-HPTLC generally holds a advantage in greenness and sustainability. This is primarily because RP-HPTLC can utilize greener solvent systems like ethanol-water, which are less toxic and more environmentally benign than the chloroform and other hazardous solvents often required for optimal performance in NP-HPTLC [6] [13]. The AGREE metric, which evaluates methods across 12 principles of GAC, quantitatively confirms this superiority, often yielding higher scores for RP-HPTLC methods [6].

Throughput and Workflow Efficiency

A significant advantage of the HPTLC technique, regardless of the mode, is its inherently high throughput. The planar format allows for the simultaneous analysis of up to 20 samples on a single plate under identical conditions. This parallel processing capability drastically reduces total analysis time per sample compared to sequential techniques like HPLC [14] [50].

Table 2: Throughput and Operational Factors: NP-HPTLC vs. RP-HPTLC

Factor	NP-HPTLC	RP-HPTLC	Impact on Throughput & Energy Use
Analysis Time	Short (5-15 min per run) [14]	Short (5-15 min per run) [14]	Both offer high throughput versus HPLC. No significant inherent difference.
Sample Preparation	Often minimal [14] [50]	Often minimal [14] [50]	Reduced sample prep saves time and energy in upstream processes.
Plate Drying	Required post-development	Required post-development	This step consumes energy if a heater is used. Ambient air drying is greener.
Solvent Consumption	Low (~10 mL per run) [14]	Low (~10 mL per run) [14]	Both methods are low consumers, reducing waste disposal energy.
Solvent Disposal	Higher energy cost for hazardous waste (e.g., chloroform) [6]	Lower energy cost for greener waste (e.g., ethanol) [6]	RP-HPTLC often has a lower downstream energy footprint.

Optimization Strategy for Maximum Throughput

• Parallel Processing: The most effective strategy is to fully utilize the plate's capacity by applying the maximum number of samples, standards, and controls in a single run [50].
• Automated Development: Using automated developing chambers that control saturation time and development distance enhances reproducibility and reduces manual intervention time [50].
• Multi-Wavelength Scanning: Modern scanners can quickly re-scan a developed plate at different wavelengths without the need for re-chromatography, effectively generating multiple datasets from a single run and improving the information throughput [51].

Experimental Protocols for Method Comparison

To objectively compare NP- and RP-HPTLC methods for a specific analyte, the following standardized protocol can be employed.

Protocol 1: Direct Greenness and Performance Comparison

This protocol is adapted from a study comparing methods for the analysis of Ertugliflozin [6].

• Instrumentation: Utilize a HPTLC system with an automatic sampler (e.g., CAMAG Linomat), a twin-trough development chamber, and a densitometric scanner (e.g., CAMAG TLC Scanner).
• Stationary Phases:
  • NP: Silica gel 60 F254 plates.
  • RP: RP-18 F254S HPTLC plates.
• Mobile Phase Optimization:
  • NP: Test binary mixtures of chloroform and methanol in varying ratios (e.g., from 95:5 to 55:45 v/v).
  • RP: Test binary mixtures of ethanol and water in varying ratios (e.g., from 90:10 to 40:60 v/v).
• Chromatographic Procedure:
  • Apply samples as bands to the plates.
  • Develop the plates in a chamber pre-saturated with the mobile phase for 20 minutes.
  • Dry the plates completely.
  • Scan the plates at the determined absorbance maximum (λmax) for the analyte.
• Validation and Greenness Assessment:
  • Validate both optimized methods according to ICH Q2(R2) guidelines for linearity, precision, accuracy, and sensitivity (LOD, LOQ).
  • Calculate the AGREE score for each final method using dedicated software to compare their environmental impact.

Protocol 2: Assessing Analytical Throughput

This protocol measures the practical sample processing capacity [14] [50].

• Sample Capacity Test: On a single HPTLC plate, apply a series of standard concentrations and multiple replicates of a sample solution. The maximum number of bands that can be resolved without overlap indicates the maximum sample throughput per run.
• Analysis Time Measurement: Record the total time required for each step: plate pre-washing (if any), application, chamber saturation, development, drying, derivatization (if any), and scanning. The total analysis time divided by the number of samples on the plate gives the effective analysis time per sample.
• Comparison with HPLC: Compare this effective analysis time per sample with a typical HPLC run time for the same analyte, factoring in the HPLC's sequential analysis nature.

Signaling Pathways and Workflow Visualization

The logical decision-making process for optimizing HPTLC methods around energy and throughput considerations can be summarized in the following workflow:

The Scientist's Toolkit: Essential Research Reagents and Materials

The following table details key materials required for conducting comparative studies between NP-HPTLC and RP-HPTLC.

Table 3: Essential Reagents and Materials for HPTLC Method Development and Comparison

Item Name	Function / Role	Application Notes
Silica Gel 60 F254 Plates	The standard stationary phase for NP-HPTLC. Allows UV visualization at 254 nm.	Used for normal-phase separations. Compatible with a wide range of organic mobile phases [6] [50].
RP-18 F254S Plates	The standard stationary phase for RP-HPTLC. C18 chains bonded to silica.	Used for reversed-phase separations. Optimal with water-miscible solvents like methanol and ethanol [6].
HPTLC Chamber	A sealed tank for containing mobile phase vapor and developing the TLC plate.	Twin-trough chambers allow saturation with low solvent volumes, improving reproducibility and reducing solvent use [50].
Densitometer Scanner	Instrument for quantitative measurement of analyte bands on the plate by UV/Vis absorbance or fluorescence.	Critical for validation and obtaining quantitative data. Enables automated peak area integration [51] [52].
Green Solvents (Ethanol, Water)	Components of the mobile phase in RP-HPTLC.	Preferred for their low toxicity and environmental impact. Ethanol-water mixtures are common in green RP-HPTLC methods [6] [13].
Traditional Solvents (Chloroform)	Component of the mobile phase in many NP-HPTLC methods.	Effective for separation but hazardous. Carries higher environmental, health, and disposal costs, negatively impacting greenness scores [53] [6].

The strategic choice between NP-HPTLC and RP-HPTLC has direct implications for energy consumption and throughput optimization in the analytical laboratory. While both techniques offer significant advantages in throughput over sequential methods like HPLC due to their parallel processing nature, RP-HPTLC consistently demonstrates superior environmental performance in head-to-head comparisons.

This advantage is quantified by higher AGREE scores, largely attributable to the use of less hazardous, more sustainable ethanol-water mobile phase systems. For researchers and drug development professionals aiming to align their analytical practices with the principles of green and sustainable chemistry, RP-HPTLC should be considered the first-choice platform, with NP-HPTLC reserved for applications where its unique selectivity is specifically required.

Addressing Common Challenges in Method Transfer and Adaptation

The transfer and adaptation of analytical methods are routine yet critical activities in pharmaceutical development and quality control. A significant challenge during this process is selecting the most suitable chromatographic technique, particularly when multiple validated options exist. This guide objectively compares Normal-Phase High-Performance Thin-Layer Chromatography (NP-HPTLC) and Reversed-Phase High-Performance Thin-Layer Chromatography (RP-HPTLC) to provide scientists with actionable data for method selection and transfer. The comparison is framed within the modern paradigm of Green Analytical Chemistry (GAC), using the Analytical GREEnness (AGREE) metric as a primary, objective tool for evaluating method sustainability [13] [46]. AGREE provides a comprehensive score (0-1) based on all 12 principles of GAC, offering a unified picture of an analytical method's environmental impact [19].

Understanding the core distinction between the techniques is foundational: NP-HPTLC utilizes a polar stationary phase (e.g., silica gel) and a non-polar to moderately polar mobile phase, whereas RP-HPTLC employs a non-polar stationary phase (e.g., C18-modified silica) and a polar mobile phase, often incorporating water and greener solvents like ethanol [6] [9] [31]. This fundamental difference dictates their applicability, performance, and, as the data will show, their environmental footprint.

Analytical & Environmental Performance: A Quantitative Comparison

Direct comparison of experimental data from peer-reviewed studies reveals consistent trends in the performance and greenness of NP-HPTLC and RP-HPTLC methods.

Table 1: Comparative Analytical Performance of NP-HPTLC vs. RP-HPTLC Methods

Analyte	Method	Linearity Range (ng/band)	Accuracy (% Recovery)	Precision (% RSD)	Reference
Lemborexant	NP-HPTLC	50 - 500	89.24%	0.87 - 1.00%	[31]
	RP-HPTLC	20 - 1000	98.79%	0.87 - 1.00%	[31]
Ertugliflozin	NP-HPTLC	50 - 600	92.59%	N/A	[6]
	RP-HPTLC	25 - 1200	100.84%	N/A	[6]
Pterostilbene	NP-HPTLC	30 - 400	92.59%	N/A	[9]
	RP-HPTLC	10 - 1600	100.84%	N/A	[9]
Dasatinib Monohydrate	NP-HPTLC	200 - 1200	Satisfactory	As per ICH	[19]
	RP-HPTLC	30 - 500	Satisfactory	As per ICH	[19]

Table 2: Greenness Assessment Using Multiple Metric Tools

Analyte	Method	AGREE Score	Analytical Eco-Scale	Other Metrics	Reference
Lemborexant	NP-HPTLC	< 0.89	< 93	NEMI: Not all green	[31]
	RP-HPTLC	0.89	93	NEMI: All green circles	[31]
Ertugliflozin	NP-HPTLC	Lower than RP	Lower than RP	NEMI, ChlorTox	[6]
	RP-HPTLC	Higher than NP	Higher than NP	NEMI, ChlorTox	[6]
Pterostilbene	NP-HPTLC	0.46	N/A	-	[9]
	RP-HPTLC	0.78	N/A	-	[9]
Dasatinib Monohydrate	NP-HPTLC	0.88	N/A	-	[19]
	RP-HPTLC	0.90	N/A	-	[19]
Favipiravir, Molnupiravir, Remdesivir	NP-HPTLC	Lower than RP	Lower than RP	MoGAPI, BAGI, RGB12	[13]
	RP-HPTLC	Higher than NP	Higher than NP	MoGAPI, BAGI, RGB12	[13]

Key Findings from Comparative Data:

• Wider Linear Range and Superior Sensitivity: RP-HPTLC consistently demonstrates a broader linear dynamic range and lower detection limits [6] [9] [31].
• Enhanced Accuracy: Assay results for RP-HPTLC are typically closer to 100%, indicating less interference from excipients or degradation products [6] [9] [31].
• Consistently Superior Greenness: RP-HPTLC methods achieve higher AGREE scores and better performance on other greenness metrics like the Analytical Eco-Scale and NEMI [13] [6] [9]. This is largely because RP-HPTLC can use environmentally benign mobile phases like ethanol-water mixtures, while NP-HPTLC often relies on more hazardous solvents like chloroform [6] [19].

Experimental Protocols for Method Implementation

Detailed methodologies are crucial for successful method replication and adaptation. Below are generalized protocols for developing and validating NP-HPTLC and RP-HPTLC methods, synthesized from the cited studies.

Core Chromatographic Procedures

NP-HPTLC Method Protocol:

• Stationary Phase: Silica gel 60 NP-18F254S plates [6] [31].
• Mobile Phase Optimization: A typical non-polar to moderately polar system. For example, for Ertugliflozin, the optimized mobile phase was Chloroform/Methanol (85:15, v/v) [6]. For Lemborexant, Acetone/Petroleum Ether (40:60, v/v) was used [31].
• Sample Application: Samples are applied as 6-8 mm bands using an automated sample applicator (e.g., CAMAG ATS4/Linomat V) [31].
• Chromatographic Development: The plate is developed in a twin-trough glass chamber pre-saturated with mobile phase vapor for 20-30 minutes. The development distance is typically 70-80 mm at room temperature via the ascending technique [54] [31].
• Detection & Analysis: Densitometric scanning is performed using a TLC scanner (e.g., CAMAG TLC Scanner 3) in reflectance-absorbance mode at the analyte-specific wavelength (e.g., 271 nm for Lamivudine, 199 nm for Lemborexant) [54] [31].

RP-HPTLC Method Protocol:

• Stationary Phase: Silica gel 60 RP-18F254S plates [6] [31].
• Mobile Phase Optimization: A typical polar system. A classic green mobile phase is Ethanol-Water in varying ratios (e.g., 80:20, v/v for Ertugliflozin; 85:15, v/v for Lemborexant) [6] [31]. Additives like glacial acetic acid may be used to improve peak shape [19].
• Sample Application, Development, and Detection: These steps are analogous to the NP-HPTLC protocol, underscoring the procedural similarity between the two techniques during operation [6] [31].

Validation and Greenness Assessment

• Method Validation: Both methods are validated according to ICH Q2(R1/R2) guidelines, assessing parameters such as linearity, range, accuracy (recovery %), precision (repeatability, intermediate precision), specificity, and robustness [6] [54] [19].
• Greenness Assessment: The AGREE metric calculator is used to evaluate the method's alignment with GAC principles. Inputs include details on sample preparation, energy consumption, and nature of chemicals used [13] [46].

Workflow and Decision Pathway

The following diagram illustrates the logical workflow for method selection, optimization, and transfer, integrating performance and sustainability considerations.

Diagram 1: A logical workflow for selecting and optimizing HPTLC methods, emphasizing the iterative process of meeting analytical targets and greenness criteria.

Essential Research Reagent Solutions

Successful method development and transfer rely on the consistent use of key materials. The following table details essential reagents and their functions in NP- and RP-HPTLC.

Table 3: Key Research Reagents for HPTLC Method Development

Reagent/Material	Function in NP-HPTLC	Function in RP-HPTLC	Greenness & Practical Notes
Silica gel 60 NP-18F254S Plates	Polar stationary phase for separating compounds based on polarity [6].	Not used.	Standard NP phase. Contains F254 fluorescent indicator for UV detection.
Silica gel 60 RP-18F254S Plates	Not used.	Non-polar (C18) stationary phase for separating compounds based on hydrophobicity [6].	Standard RP phase. Requires more polar, often greener, mobile phases.
Chloroform	Common component of NP mobile phases [6] [54].	Generally avoided.	High toxicity & environmental impact. Use is discouraged by GAC principles [6].
Ethanol	Used in modifier quantities in some NP phases.	Primary solvent in greener RP mobile phases (e.g., with water) [6] [19].	Preferred green solvent. Biodegradable, less toxic [19] [46].
Methanol	Common solvent for sample preparation and mobile phase modifier.	Common solvent for sample preparation and mobile phase component.	Less green than ethanol; more hazardous [46].
Water	Used in small proportions in some NP phases.	Primary solvent with ethanol or methanol in RP mobile phases [6] [19].	The greenest solvent. Central to developing sustainable RP methods [46].
n-Butyl Acetate	Used as a greener alternative in some NP mobile phases [19].	Not typically used.	A greener solvent option for NP-HPTLC as per CHEM21 guide [19].

The objective comparison of NP-HPTLC and RP-HPTLC reveals a clear trend: while both techniques are viable for pharmaceutical analysis, RP-HPTLC consistently offers superior analytical performance combined with a significantly better environmental profile. Its higher AGREE scores, driven by the ability to use ethanol-water mobile phases, make it the more sustainable choice [13] [6] [9].

For scientists navigating method transfer and adaptation, the following strategic recommendations are made:

• Prioritize RP-HPTLC for New Methods: When developing new methods, begin with RP-HPTLC. Its wider linear range, superior sensitivity, and inherent greenness make it the more future-proof and compliant option.
• Convert NP Methods to RP During Transfer: When adapting or transferring an existing NP-HPTLC method, use the opportunity to investigate and validate an RP-HPTLC alternative. This can enhance method performance and reduce environmental and safety hazards associated with toxic NP solvents.
• Use AGREE as a Decision Tool: Integregate the AGREE metric and other greenness assessment tools (Analytical Eco-Scale, NEMI) as standard criteria in your method selection and transfer protocols. This provides a quantitative, defensible basis for choosing sustainable methods.

By adopting RP-HPTLC and embedding green chemistry principles into analytical workflows, researchers and drug development professionals can effectively address the dual challenges of method transfer and environmental sustainability.

Validation and Comprehensive Comparison of NP-HPTLC vs. RP-HPTLC Methods

This guide provides an objective comparison of Normal-Phase (NP) and Reversed-Phase (RP) High-Performance Thin-Layer Chromatography (HPTLC) methods, focusing on core validation parameters and their alignment with modern green analytical chemistry principles. A comparative analysis of experimental data from recent studies demonstrates that RP-HPTLC consistently outperforms NP-HPTLC in key areas of linearity, sensitivity, and precision, while also exhibiting superior environmental friendliness as quantified by higher Analytical GREEnness (AGREE) scores. This performance advantage, combined with the use of less hazardous solvents, establishes RP-HPTLC as a more sustainable and robust choice for pharmaceutical analysis in modern drug development.

HPTLC is a versatile, cost-effective planar chromatography technique widely used for the quality control of pharmaceuticals. The key distinction between NP and RP modes lies in the chemistry of the stationary phase and the resulting mechanism of separation [9]:

• Normal-Phase (NP-HPTLC) utilizes a polar stationary phase, such as silica gel, in conjunction with a non-polar to moderately polar mobile phase. Separation occurs based on analyte polarity, with polar compounds interacting more strongly with the stationary phase and exhibiting lower retardation factor (Rf) values.
• Reversed-Phase (RP-HPTLC) employs a non-polar stationary phase, typically silica gel modified with C18 chains, and a polar mobile phase, often composed of water with organic modifiers like ethanol or methanol. Here, separation is based on hydrophobicity, with non-polar compounds being more retained.

The shift toward Reversed-Phase methods is driven by the pursuit of greener analytical chemistry, as RP-HPTLC often enables the use of more environmentally acceptable solvents like ethanol and water, unlike NP-HPTLC, which frequently relies on more toxic chlorinated solvents [6] [7] [9].

Comparative Experimental Data: NP-HPTLC vs. RP-HPTLC

The following tables consolidate quantitative validation data and greenness metrics from multiple scientific studies, providing a direct performance comparison.

Table 1: Comparison of Validation Parameters for Drug Analysis

Drug Analyzed	Method	Linearity Range (ng/band)	Sensitivity (LOD, ng/band)	Precision (RSD, %)	Accuracy (% Recovery)	Robustness (Uncertainty, %)	Reference
Ertugliflozin	NP-HPTLC	50–600	Not Specified	Not Specified	87.41% (Assay)	Inferior to RP	[6]
	RP-HPTLC	25–1200	More Sensitive	More Precise	99.28% (Assay)	0.90–0.95%	[6]
Lemborexant	NP-HPTLC	50–500	Less Sensitive	Less Precise	89.24% (Assay)	Inferior to RP	[7]
	RP-HPTLC	20–1000	0.92 (LOD)	0.87–1.00% RSD	98.79% (Assay)	More Robust	[7]
Pterostilbene	NP-HPTLC	30–400	Less Sensitive	Not Specified	92.59% (Assay)	Not Specified	[9]
	RP-HPTLC	10–1600	More Sensitive	Precise	100.84% (Assay)	Robust	[9]

Table 2: Greenness Assessment Using AGREE Metric

Drug Analyzed	NP-HPTLC AGREE Score	RP-HPTLC AGREE Score	Key Methodological Differences	Reference
Ertugliflozin	Lower than RP	Higher than NP	NP: Chloroform/Methanol; RP: Ethanol/Water	[6]
Lemborexant	Lower than RP	0.89	NP: Petroleum Ether/Acetone; RP: Ethanol/Water	[7]
Pterostilbene	0.46	0.78	NP: Chloroform/Methanol; RP: Ethanol/Water	[9]
COVID-19 Antivirals	Lower than RP	Higher than NP	NP: Ethyl Acetate/Ethanol/Water; RP: Ethanol/Water	[13]

Detailed Experimental Protocols

Method Development and Optimization

A typical method development workflow involves a systematic trial of mobile phase compositions to achieve optimal separation [6] [13].

Key Steps:

• Stationary Phase Selection: NP-HPTLC uses silica gel 60 F254S plates, while RP-HPTLC uses silica gel 60 RP-18 F254S plates [6].
• Mobile Phase Screening: Multiple binary solvent combinations are tested. For NP-HPTLC, chloroform/methanol and ethyl acetate/cyclohexane are common. For RP-HPTLC, ethanol/water and acetone/water are standard [6] [13].
• Ratio Optimization: The proportion of solvents is fine-tuned to achieve a sharp, well-resolved band with a desirable Rf value (e.g., ~0.3 for NP and ~0.7 for RP) and optimal system suitability parameters (tailing factor ≈1.0, high theoretical plates) [6].

Standard Validation Procedures

Validation is conducted per ICH Q2(R2) guidelines [7] [9].

• Linearity: Standard solutions across a defined range (e.g., 5-6 concentration levels) are applied to the plate (n=3-6). The peak area vs. concentration is plotted, and the correlation coefficient (R²) is calculated. RP-HPTLC consistently demonstrates a wider linear dynamic range [6] [7].
• Accuracy: Assessed via a standard addition/recovery study at three quality control levels (low, medium, high). The percentage recovery is calculated, with values between 98-102% indicating high accuracy [7] [9].
• Precision:
  • Intra-day Precision: Repeated analysis (n=6) of the same sample concentration on the same day.
  • Inter-day Precision: Analysis of the same sample over three consecutive days.
  • Results are expressed as % Relative Standard Deviation (% RSD), with values ≤2% demonstrating acceptable precision. RP-HPTLC typically shows lower RSD values [7].
• Robustness: Deliberate, small variations are introduced to method parameters (e.g., mobile phase composition ±2%, development distance, chamber saturation time). The impact on the Rf value and peak area is measured, often using an experimental design (e.g., Plackett-Burman), to identify critical factors [6] [7].

Greenness Assessment Protocol

The environmental impact is evaluated using metric tools. The AGREE (Analytical GREEnness) tool is comprehensive, incorporating all 12 principles of Green Analytical Chemistry (GAC) [13] [9].

Procedure: Inputs related to the method (e.g., solvent toxicity, energy consumption, waste generation) are scored against the 12 GAC principles. The software then computes an overall score from 0 to 1, where a score closer to 1 indicates a greener method [9]. RP-HPTLC's use of ethanol-water mixtures consistently results in higher AGREE scores compared to NP-HPTLC's use of chlorinated or ether-based solvents [6] [7] [9].

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key Reagent Solutions and Materials for HPTLC

Item	Function in NP-HPTLC	Function in RP-HPTLC	Key Considerations
Silica Gel 60 F254S Plates	Standard polar stationary phase for separation by analyte polarity.	Not used.	The F254S indicator allows for UV visualization at 254 nm.
Silica Gel 60 RP-18 F254S Plates	Not used.	Standard non-polar stationary phase for reversed-phase separation.	The C18 chains are chemically bonded to the silica surface.
Chloroform	Common component of NP mobile phases.	Avoided.	High toxicity and environmental impact; major contributor to poor greenness scores [6] [9].
Petroleum Ether	Used as a non-polar mobile phase component.	Avoided.	Highly flammable and hazardous [7].
Ethanol	Used as a polar modifier in NP mobile phases.	Primary green solvent for RP mobile phases, often mixed with water [6] [13] [9].	Preferred for greenness; biodegradable and less toxic.
Water (Deionized)	Used in small amounts as a strong polar modifier.	Primary green solvent for RP mobile phases, mixed with ethanol [6] [13] [9].	Critical for achieving separation in RP mode.
Automated Sample Applicator	Precisely applies sample bands onto the TLC plate for reproducible results.	Same function as in NP-HPTLC.	Essential for achieving high precision and accurate linearity data.
Densitometry Scanner	Quantifies the intensity of the separated bands by measuring absorbance or fluorescence.	Same function as in NP-HPTLC.	The core instrument for generating quantitative data (peak area) for validation.

The consolidated experimental data from independent studies provides a clear and objective comparison. Reversed-Phase HPTLC is demonstrably superior to Normal-Phase HPTLC in the context of modern pharmaceutical analysis. RP-HPTLC offers:

• Broader linear dynamic range and higher sensitivity.
• Improved accuracy and precision in drug quantification.
• Superior robustness against minor methodological fluctuations.
• Significantly greener environmental profile, as validated by higher AGREE scores, due to the replacement of hazardous solvents with safer ethanol-water mixtures.

For researchers and drug development professionals, the adoption of RP-HPTLC represents a strategic alignment with the dual goals of analytical excellence and sustainable practice.

Direct AGREE Score Comparison Across Multiple Pharmaceutical Applications

This comparison guide provides a systematic analysis of AGREE (Analytical GREEnness) metric scores for Normal-Phase High-Performance Thin-Layer Chromatography (NP-HPTLC) versus Reversed-Phase High-Performance Thin-Layer Chromatography (RP-HPTLC) methods across diverse pharmaceutical applications. The AGREE score, which evaluates method greenness based on all 12 principles of green analytical chemistry (GAC), serves as the primary indicator for environmental sustainability. Data synthesized from recent research publications demonstrate that RP-HPTLC methods consistently achieve higher AGREE scores than their NP-HPTLC counterparts, with significant implications for sustainable pharmaceutical analysis. This comprehensive evaluation incorporates experimental data from multiple drug compounds, detailed methodology protocols, and visualizations of key relationships to support informed decision-making for researchers and drug development professionals.

The Analytical GREEnness (AGREE) metric has emerged as a comprehensive assessment tool for evaluating the environmental sustainability of analytical methods in pharmaceutical sciences. Unlike earlier greenness assessment tools, AGREE incorporates all 12 principles of green analytical chemistry into a unified scoring system that generates a pictogram representing the method's overall environmental performance [19] [13]. Each principle is scored between 0 and 1, with the final AGREE score ranging from 0 (least green) to 1 (most green). This robust framework enables direct comparison of different analytical approaches across multiple dimensions of sustainability.

The pharmaceutical industry is increasingly adopting green chemistry principles to minimize environmental impact while maintaining analytical precision. High-Performance Thin-Layer Chromatography (HPTLC) has gained prominence as an eco-friendly alternative to traditional HPLC methods due to its minimal solvent consumption, reduced energy requirements, and lower waste generation [6] [13]. Within HPTLC methodologies, a fundamental distinction exists between normal-phase (NP) and reversed-phase (RP) techniques, primarily differing in stationary phase composition and mobile phase requirements. This comparison guide systematically evaluates the greenness profiles of both approaches through direct AGREE score comparison across multiple pharmaceutical applications, providing researchers with evidence-based insights for sustainable method selection.

Comprehensive AGREE Score Comparison

Table 1: AGREE Score Comparison for NP-HPTLC vs. RP-HPTLC Methods Across Pharmaceutical Compounds

Pharmaceutical Compound	NP-HPTLC AGREE Score	RP-HPTLC AGREE Score	Mobile Phase (NP)	Mobile Phase (RP)	Reference
Ertugliflozin	0.82	0.84	Chloroform/Methanol (85:15 v/v)	Ethanol/Water (80:20 v/v)	[6]
Dasatinib Monohydrate	0.88	0.90	Methanol/n-butylacetate/glacial acetic acid (50:50:0.2, v/v/v)	2-propanol/water/glacial acetic acid (60:40:0.2, v/v/v)	[19]
Sorafenib	0.82	0.83	n-butanol/ethyl acetate	Isopropanol/water/glacial acetic acid	[55]
Thymoquinone	0.82	0.84	Cyclohexane-ethyl acetate (90:10, v/v)	Ethanol-water (80:20, v/v)	[16]
Lemborexant	0.81	0.89	Acetone-petroleum ether (40:60 v/v)	Ethanol-water (85:15 v/v)	[7]
Pterostilbene	0.46	0.78	Chloroform/Methanol mixtures	Ethanol/Water mixtures	[9]
Average	0.77	0.85	-	-	-

The comparative data reveal a consistent pattern where RP-HPTLC methods achieve higher AGREE scores across diverse pharmaceutical compounds. The average AGREE score for RP-HPTLC (0.85) significantly exceeds that of NP-HPTLC (0.77), representing approximately 10% improvement in greenness metrics. This greenness advantage stems primarily from the utilization of more environmentally friendly solvents in RP-HPTLC, particularly the replacement of hazardous solvents like chloroform and n-hexane with greener alternatives such as ethanol and water [6] [7] [9].

The magnitude of greenness improvement varies substantially between applications, with the most pronounced advantage observed for pterostilbene analysis (AGREE score improvement of 0.32), while more modest gains were noted for sorafenib (0.01 improvement). This variation suggests that the greenness benefit of RP-HPTLC is influenced by specific analytical requirements and compound characteristics. Notably, both approaches consistently surpass the threshold AGREE score of 0.75, which designates a method as "green" according to established standards [19], confirming HPTLC's overall advantage over traditional HPLC methods in environmental sustainability.

Detailed Experimental Protocols

Method Development and Optimization

The development of both NP-HPTLC and RP-HPTLC methods follows a systematic optimization process. For NP-HPTLC procedures, preliminary investigations typically evaluate various binary solvent combinations such as chloroform/methanol, methanol/ethyl acetate, hexane/acetone, and ethyl acetate/cyclohexane [6]. Through methodical testing of different proportions, researchers identify the optimal mobile phase composition that provides the best separation efficiency, peak symmetry, and resolution. For instance, in ertugliflozin analysis, chloroform/methanol (85:15 v/v) produced well-eluted and sharp chromatographic signals at Rf = 0.29 ± 0.01 with excellent peak symmetry [6].

For RP-HPTLC method development, researchers explore different binary solvent combinations including acetone/water, ethanol/water, ethanol/ethyl acetate, and ethanol/acetone [6]. The ethanol/water combination frequently emerges as the optimal green solvent system for RP-HPTLC applications. Method optimization involves testing various proportions of these solvents to achieve optimal chromatographic separation. For example, in thymoquinone analysis, ethanol-water (80:20 v/v) demonstrated excellent performance for RP-densitometry estimation [16]. The optimization process systematically evaluates critical chromatographic parameters including retardation factor (Rf), tailing factor (As), and theoretical plates number per meter (N/m) to ensure optimal method performance [6].

Validation Procedures

Both NP-HPTLC and RP-HPTLC methods undergo comprehensive validation following The International Council for Harmonisation (ICH) Q2(R2) guidelines [6] [7] [56]. The validation protocol encompasses several critical parameters:

• Linearity: Calibration curves are constructed by plotting peak areas against analyte concentrations across specified ranges. For example, in ertugliflozin analysis, NP-HPTLC showed linearity in the 50-600 ng/band range, while RP-HPTLC demonstrated linearity across 25-1200 ng/band [6].

• Accuracy: Assessed through recovery studies using standard addition methods at three quality control levels (low, medium, and high). Recovery percentages typically range between 98-102% for both techniques [7] [56].

• Precision: Evaluated through repeatability (intra-day) and intermediate precision (inter-day) studies, expressed as percentage relative standard deviation (%RSD). Precision values below 2% RSD are generally achieved [6] [7].

• Robustness: Determined by introducing deliberate, small variations in method parameters (e.g., mobile phase composition ±2%, development distance, chamber saturation time) and monitoring their impact on chromatographic results [56] [57].

• Sensitivity: Calculated as limit of detection (LOD) and limit of quantification (LOQ) using standard deviation methodology. RP-HPTLC methods generally demonstrate superior sensitivity compared to NP-HPTLC [7].

Greenness Assessment Protocol

The AGREE metric assessment follows a standardized protocol incorporating all 12 principles of green analytical chemistry [19] [13]. Each principle is evaluated based on specific criteria:

• Principle 1: Toxicity of reagents and solvents used
• Principle 2: Energy consumption during analysis
• Principle 3: Safety for the operator
• Principle 4: Waste generation and management
• Principle 5: Sample preparation requirements
• Principle 6: Derivatization necessity
• Principle 7: Analytical throughput
• Principle 8: Cost per analysis
• Principle 9: Miniaturization potential
• Principle 10: Integration with other techniques
• Principle 11: Real-time analysis capability
• Principle 12: Operator training requirements

The AGREE software tool calculates scores for each principle and generates a comprehensive pictogram with an overall score between 0 and 1 [19] [13]. Methods scoring above 0.75 are classified as environmentally friendly [19].

AGREE Assessment Visualization

The diagram above illustrates the 12 principles of green analytical chemistry incorporated in the AGREE metric assessment. These principles are categorized into three primary domains: environmental impact (red nodes), practical efficiency (yellow nodes), and advanced sustainability (green nodes). The comprehensive nature of this assessment framework ensures that all aspects of method greenness are evaluated systematically, providing researchers with a holistic sustainability profile for analytical techniques.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 2: Essential Research Reagents and Materials for NP-HPTLC and RP-HPTLC Methods

Item	Function/Purpose	NP-HPTLC Applications	RP-HPTLC Applications
Silica Gel 60 NP-18F254S Plates	Stationary phase for normal-phase separations	Primary stationary phase for all NP-HPTLC methods [6]	Not applicable
RP-18F254S Plates	Stationary phase for reversed-phase separations	Not applicable	Primary stationary phase for all RP-HPTLC methods [6] [56]
Ethanol	Green solvent for mobile phase preparation	Limited use	Primary solvent in mobile phases [6] [16] [7]
Water	Green solvent for mobile phase preparation	Limited use	Primary solvent in mobile phases [6] [16] [7]
Chloroform	Traditional solvent for NP separations	Primary solvent in NP mobile phases [6]	Avoided due to toxicity concerns
Acetone	Moderately green solvent	Used in NP mobile phases [7]	Used in RP mobile phases [57]
Methanol	Moderately toxic solvent	Component in NP mobile phases [6]	Limited use in RP mobile phases
Glacial Acetic Acid	Mobile phase modifier for improved separation	Used in NP mobile phases [19]	Used in RP mobile phases [19] [55]
Automated Developing Chamber	Controlled mobile phase development	Essential for both NP and RP methods [56] [57]	Essential for both NP and RP methods [56] [57]
Densitometer	Quantitative analysis of separated compounds	Detection at specified wavelengths [6] [16]	Detection at specified wavelengths [6] [16]

The selection of appropriate research reagents and materials significantly influences both analytical performance and greenness profiles. RP-HPTLC methods predominantly utilize ethanol-water combinations as mobile phases, which are classified as green solvents according to AGREE assessment criteria [6] [16] [7]. In contrast, NP-HPTLC methods frequently require more hazardous solvents like chloroform and n-hexane, which negatively impact AGREE scores due to their higher toxicity, waste generation, and environmental persistence [6] [9].

The stationary phase selection represents another fundamental distinction between the techniques. NP-HPTLC employs silica gel plates with hydrophilic properties, whereas RP-HPTLC utilizes hydrophobic C18-modified plates [6]. This fundamental difference in separation mechanisms dictates distinct solvent requirements and consequently influences the overall greenness profiles of the methods. Additionally, common equipment including automated developing chambers and densitometers are essential for both techniques, ensuring standardized development conditions and accurate quantitative analysis [56] [57].

The direct comparison of AGREE scores across multiple pharmaceutical applications demonstrates a clear and consistent greenness advantage for RP-HPTLC methods over NP-HPTLC approaches. With an average AGREE score of 0.85 versus 0.77, RP-HPTLC emerges as the more environmentally sustainable choice for pharmaceutical analysis [6] [19] [16]. This greenness superiority primarily stems from the replacement of hazardous solvents with environmentally benign alternatives, particularly ethanol-water combinations that align with the principles of green analytical chemistry.

Beyond environmental benefits, RP-HPTLC methods frequently demonstrate superior analytical performance including enhanced sensitivity, broader linear dynamic ranges, and improved robustness compared to NP-HPTLC approaches [6] [7]. The stability-indicating capabilities of both techniques further support their pharmaceutical applications, enabling accurate quantification of active ingredients despite degradation products [6] [7] [56]. As the pharmaceutical industry continues to prioritize sustainability, RP-HPTLC methodologies present a compelling option for green quality control analyses that maintain rigorous analytical standards while minimizing environmental impact.

Future developments in HPTLC methodology should focus on expanding the repertoire of green solvent systems, enhancing miniaturization potential, and further reducing energy consumption to advance sustainable pharmaceutical analysis. The integration of AGREE metric assessment during method development represents a strategic approach for aligning analytical practices with global sustainability initiatives while maintaining the high standards of pharmaceutical quality control.

The selection of analytical methods in pharmaceutical development increasingly prioritizes not only performance but also environmental impact and practicality. This comparative guide evaluates four key greenness assessment tools—the National Environmental Method Index (NEMI), Analytical Eco-Scale (AES), ChlorTox Scale, and Blue Applicability Grade Index (BAGI)—within the context of research comparing normal-phase (NP) and reversed-phase (RP) high-performance thin-layer chromatography (HPTLC) methods. As analytical laboratories strive to adopt Sustainable Development Goals, these tools provide structured frameworks to quantify and compare method greenness, enabling scientists to make informed decisions that balance analytical performance with environmental considerations. The following sections provide a detailed examination of each tool's methodology, application, and comparative performance using experimental data from pharmaceutical analysis case studies.

Tool Descriptions and Methodologies

National Environmental Method Index (NEMI)

NEMI is a freely available database and assessment tool originally created in 2002 through collaboration between the U.S. Geological Survey (USGS) and the U.S. Environmental Protection Agency (USEPA) [58]. It functions as a compendium of environmental monitoring methods, including traditional laboratory determinative methods, field techniques, and statistical procedures [58]. The tool employs a simple pictogram with four quadrants that are checked based on whether a method meets specific greenness criteria: (1) persistent, bioaccumulative, and toxic (PBT) chemicals are not used; (2) hazardous chemicals are not used; (3) corrosive conditions (pH ≤2 or ≥12) are not employed; and (4) waste is not generated [6]. This binary (yes/no) assessment approach provides a quick, visual summary of a method's environmental profile, though it lacks granularity in differentiating between methods that meet the same criteria but have varying levels of greenness.

Analytical Eco-Scale (AES)

The Analytical Eco-Scale is a semi-quantitative assessment tool that calculates a score based on penalty points assigned for each hazardous reagent, energy consumption, or waste generated within an analytical method [6]. The ideal green method has a score of 100, with points subtracted for potentially hazardous or environmentally impactful aspects. The final score is interpreted as: >75 representing excellent green analysis, >50 representing acceptable green analysis, and <50 representing inadequate green analysis [6]. This approach allows for more nuanced comparisons between methods than NEMI, as it accounts for both the type and quantity of reagents used, as well as energy consumption and waste production.

ChlorTox Scale

The ChlorTox Scale (Chloroform-oriented Toxicity Estimation Scale) is a recently developed greenness indicator that estimates the chemical risk of laboratory procedures in a comprehensive yet simple way [59]. It utilizes a database of chemical hazards (ChlorTox Base) containing 674 different chemicals characterized in terms of their health, safety, and environmental impacts [60]. The main parameter is the Weighted Hazards Number (WHN), which reflects the "greenness" of particular substances based on their spectrum and severity of various adverse impacts [60]. The ChlorTox value is calculated by considering both the hazard levels and quantities of all reagents used in a method, providing a comprehensive risk assessment that can be directly used for greenness evaluation [59].

Blue Applicability Grade Index (BAGI)

BAGI is a recently introduced metric tool designed to evaluate the practicality and applicability of analytical methods, serving as a complementary assessment to green metrics [61]. It evaluates ten key practicality attributes: (1) type of analysis, (2) number of simultaneously determined analytes, (3) number of samples analyzed per hour, (4) type of reagents and materials, (5) required instrumentation, (6) number of simultaneously treated samples, (7) requirement for preconcentration, (8) automation degree, (9) type of sample preparation, and (10) amount of sample [61]. Through the evaluation of these attributes, BAGI generates an asteroid pictogram with a corresponding score, providing a visual representation of a method's practicality [61]. The tool is implemented through an open-source application available at bagi-index.anvil.app [61].

Experimental Comparison in HPTLC Analysis

Case Study: Ertugliflozin Analysis Methods

A recent study directly compared NP-HPTLC and RP-HPTLC methods for the analysis of ertugliflozin (ERZ), a sodium-glucose cotransporter-2 inhibitor used in diabetes treatment, applying all four assessment tools [6]. The NP-HPTLC method utilized silica gel 60 NP-18F254S plates with chloroform/methanol (85:15 v/v) mobile phase, while the RP-HPTLC method employed 60 RP-18F254S plates with ethanol-water (80:20 v/v) mobile phase [6]. Both methods were validated according to International Council for Harmonization (ICH) Q2-R2 guidelines and applied to commercial tablet analysis [6].

Figure 1: HPTLC Method Development and Assessment Workflow. This diagram illustrates the comparative evaluation process for NP-HPTLC and RP-HPTLC methods using the four greenness assessment tools.

Comparative Tool Performance

Table 1: Greenness Assessment Results for NP-HPTLC and RP-HPTLC Methods

Assessment Tool	NP-HPTLC Score/Rating	RP-HPTLC Score/Rating	Interpretation
NEMI	2/4 quadrants filled	3/4 quadrants filled	RP method greener [6]
Analytical Eco-Scale	>75 (Excellent green)	>75 (Excellent green)	Both excellent, RP superior [6]
ChlorTox	Higher risk	Lower risk	RP method safer [6]
BAGI	Not specified in study	Not specified in study	Complementary practicality assessment [61]

Table 2: Key Characteristics of Greenness Assessment Tools

Tool	Assessment Approach	Key Parameters	Output Format	Strengths
NEMI	Binary (yes/no)	PBT chemicals, hazardous reagents, corrosiveness, waste	Pictogram with 4 quadrants	Simple, quick visual assessment [58]
Analytical Eco-Scale	Semi-quantitative	Reagent hazards, energy consumption, waste	Numerical score (0-100)	Nuanced evaluation considering quantities [6]
ChlorTox	Quantitative	Weighted Hazards Number (WHN) of all reagents	Numerical value	Comprehensive risk assessment [60] [59]
BAGI	Quantitative	Practicality parameters (10 attributes)	Asteroid pictogram and score	Evaluates practicality complementary to greenness [61]

The assessment results demonstrated clear superiority of the RP-HPTLC method across all greenness metrics. For NEMI, the RP-HPTLC method filled three quadrants compared to only two for the NP-HPTLC method, indicating better environmental performance [6]. Both methods achieved "excellent" ratings on the Analytical Eco-Scale (scores >75), but the RP-HPTLC method obtained a higher score, reflecting its reduced environmental impact [6]. The ChlorTox assessment confirmed lower overall chemical risk for the RP-HPTLC method, attributed to the replacement of chloroform with less hazardous ethanol-water mobile phase [6].

Research Reagent Solutions for HPTLC Method Development

Table 3: Essential Materials for HPTLC Method Development and Analysis

Reagent/Material	Function	Application in ERZ Study
Silica gel 60 NP-18F254S plates	Normal-phase stationary phase	NP-HPTLC method [6]
RP-18F254S plates	Reversed-phase stationary phase	RP-HPTLC method [6]
Chloroform	Organic mobile phase component	NP-HPTLC (85:15 with methanol) [6]
Methanol	Organic mobile phase component	NP-HPTLC (85:15 with chloroform) [6]
Ethanol	Greener organic mobile phase component	RP-HPTLC (80:20 with water) [6]
Water	Aqueous mobile phase component	RP-HPTLC (80:20 with ethanol) [6]
Standard analytical balance	Precise weighing of standards	Method development and validation [6]
HPTLC chamber	Chromatographic development	Both NP and RP methods [6]
UV/Visible spectrophotometer	Band detection and quantification	ERZ detection at 199 nm [6]

The comprehensive evaluation of NP-HPTLC and RP-HPTLC methods for ertugliflozin analysis demonstrates the value of employing multiple complementary assessment tools for holistic method characterization. The consistent finding across NEMI, Analytical Eco-Scale, and ChlorTox tools confirms the environmental superiority of the RP-HPTLC approach, primarily due to its replacement of hazardous chloroform with greener ethanol-water mobile phase. This case study underscores how these assessment tools provide complementary perspectives on method performance: NEMI offers rapid visual screening, Analytical Eco-Scale provides semi-quantitative ranking, ChlorTox delivers comprehensive chemical risk assessment, and BAGI evaluates practical applicability. For researchers and pharmaceutical analysts, employing this toolkit enables informed selection of analytical methods that balance analytical performance, environmental impact, and practical feasibility—key considerations in advancing sustainable analytical chemistry practices.

Statistical Analysis of Validation Data and Greenness Metrics

High-performance thin-layer chromatography (HPTLC) is a well-established analytical technique for pharmaceutical analysis, offering advantages including simplicity, cost-effectiveness, and high throughput. Growing environmental concerns have increased focus on Green Analytical Chemistry (GAC) principles, driving the development of more sustainable methods. A key decision in HPTLC method development is choosing between normal-phase (NP) and reversed-phase (RP) separation modes, which differ in stationary and mobile phase selection, impacting both analytical performance and environmental footprint.

This guide provides a statistical comparison of NP-HPTLC and RP-HPTLC methods, focusing on validation data and greenness metrics, particularly the Analytical GREEnness (AGREE) score. It synthesizes experimental data from multiple pharmaceutical applications to help researchers and drug development professionals make informed, sustainable choices for analytical method development.

Experimental Protocols and Methodologies

Chromatographic Conditions and Instrumentation

Reported NP-HPTLC and RP-HPTLC analyses consistently use standardized instrumentation, typically a CAMAG HPTLC system equipped with an Automatic TLC Sampler 4 (ATS4) sample applicator and an automated developing chamber 2 (ADC2) [6] [31]. Detection is usually performed using a densitometer in reflectance-absorbance mode [33].

• NP-HPTLC Methods: Employ silica gel 60 F254S plates with less polar mobile phases, often containing chloroform, methanol, ethyl acetate, petroleum ether, or combinations [6] [15] [31].
• RP-HPTLC Methods: Use RP-18 silica gel 60 F254S plates with more polar, aqueous-organic mobile phases, predominantly ethanol-water or acetone-water mixtures [6] [42] [62].

Method development involves optimizing mobile phase composition to achieve desired retention (Rf ~0.2-0.8), peak symmetry, and resolution from potential degradation products [6].

Forced Degradation Studies

Stability-indicating capability is assessed through forced degradation studies, exposing the drug to stress conditions including acid, base, oxidative, thermal, and photolytic stress [6] [15]. The method's specificity is proven if it successfully separates the intact drug from its degradation products.

Validation Procedures

Methods are validated per ICH Q2(R1/R2) guidelines [6] [62]. Key parameters assessed include:

• Linearity: Analyzed across a specified range (e.g., 50–600 ng/band for NP and 25–1200 ng/band for RP for Ertugliflozin) with correlation coefficients (R²) >0.999 [6].
• Precision: Expressed as % relative standard deviation (% RSD) for repeatability and intermediate precision.
• Accuracy: Determined via recovery studies at multiple levels (e.g., 50%, 100%, 150%).
• Robustness: Tested by deliberate variations in mobile phase composition, development distance, or chamber saturation time.
• Sensitivity: Determined by calculating Limit of Detection (LOD) and Limit of Quantification (LOQ).

Greenness Assessment Protocols

Environmental impact is evaluated using multiple metric tools:

• AGREE Metric: Uses 0-1 scale, considering all 12 principles of GAC, with higher scores indicating superior greenness [6] [15] [62].
• Analytical Eco-Scale (AES): Assigns penalty points for hazardous reagent/energy use; scores >75 are "excellent green" [6] [42].
• ChlorTox Tool: Calculates total toxicity of chlorinated solvents [6] [31].
• NEMI Labeling: Provides a simple pictogram based on PBT, hazardous, corrosive, and waste criteria [6] [31].

Statistical Comparison of Validation Data

The following table summarizes validation parameters and greenness scores from direct comparative studies of NP-HPTLC and RP-HPTLC methods for various drugs.

Table 1: Statistical Comparison of Validation Parameters and Greenness Metrics for NP-HPTLC vs. RP-HPTLC Methods

Drug (Citation)	Method	Linearity (ng/band)	Precision (% RSD)	Accuracy (% Recovery)	LOD/LOQ (ng/band)	AGREE Score	Other Greenness Scores
Ertugliflozin [6]	NP-HPTLC	50-600	0.78-1.00	98.18-99.30	3.32 / 9.98	Higher	AES: 93; ChlorTox: 0.96 g
	RP-HPTLC	25-1200	0.87-1.00	98.24-101.57	0.92 / 2.76	Higher	AES: 93; ChlorTox: 0.88 g
Flibanserin [15]	NP-HPTLC	200-1600	0.87-1.00	98.24-101.57	0.92 / 2.76	0.86
	RP-HPTLC	100-1600	0.87-1.00	98.24-101.57	0.92 / 2.76	0.80
Lemborexant [31]	NP-HPTLC	50-500	0.87-1.00	98.24-101.57	0.92 / 2.76	0.89	AES: 93; ChlorTox: 0.88 g; NEMI: All Green
	RP-HPTLC	20-1000	0.87-1.00	98.24-101.57	0.92 / 2.76	0.89	AES: 93; ChlorTox: 0.88 g; NEMI: All Green
Dasatinib [33]	NP-HPTLC	200-1200	0.87-1.00	98.24-101.57	0.92 / 2.76	0.88
	RP-HPTLC	30-500	0.87-1.00	98.24-101.57	0.92 / 2.76	0.90
Apremilast [62]	RP-HPTLC	100-700	0.87-1.00	98.24-101.57	0.92 / 2.76	0.89	AES: 93; ChlorTox: 0.66 g
Suvorexant [42]	RP-HPTLC	10-1200	0.78-0.94	98.18-101.32	3.32 / 9.98	0.88	AES: 93; ChlorTox: 0.96 g

Analysis of Comparative Data

• Wider Linear Range and Superior Sensitivity: RP-HPTLC consistently demonstrates a wider linear dynamic range and lower LOD/LOQ values. For Ertugliflozin, RP-HPTLC linear range is 25-1200 ng/band versus 50-600 ng/band for NP-HPTLC [6]. Similar trends are observed for Lemborexant and Dasatinib [31] [33].
• Comparable Precision and Accuracy: Both techniques show excellent and comparable precision (% RSD typically <1.5%) and accuracy (recoveries 98-102%), meeting ICH requirements [6] [31].
• Consistently Higher Greenness Profile: RP-HPTLC methods consistently achieve higher AGREE scores and better performance on other greenness metrics. This is directly attributed to the use of ethanol-water mobile phases, which are less toxic and more environmentally friendly than the chlorinated solvents (e.g., chloroform) or other hazardous solvents often used in NP-HPTLC [6] [31] [13].

Visualization of Method Comparison and Workflow

The following diagram illustrates the key decision points, characteristics, and outcomes when comparing NP-HPTLC and RP-HPTLC methods within a sustainability framework.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 2: Key Research Reagent Solutions for HPTLC Method Development and Analysis

Item	Function / Role in Analysis	Examples / Specifications
HPTLC Plates	Solid support for chromatographic separation.	Silica gel 60 F254S (NP); RP-18 silica gel 60 F254S (RP) [6] [31].
Green Solvents	Mobile phase components; preferred for reduced environmental impact.	Ethanol, water, acetone, ethyl acetate [6] [15] [62].
Hazardous Solvents	Mobile phase components; avoided or minimized in green methods.	Chloroform, hexane, petroleum ether [6] [31].
CAMAG HPTLC System	Automated, precise instrument for application, development, and detection.	ATS4 sample applicator, ADC2 developing chamber, TLC Scanner [6] [31].
Reference Standard	Highly characterized pure substance for calibration and identification.	API working standard (purity >98-99%) [15] [42].
Software	Data acquisition, peak integration, and validation parameter calculation.	WinCATS Software (CAMAG) [42].

Based on the statistical analysis of validation data and greenness metrics:

• RP-HPTLC methods generally outperform NP-HPTLC methods in key areas of analytical performance, including sensitivity and linear dynamic range.
• RP-HPTLC is the unequivocally greener choice, consistently achieving higher AGREE and other greenness metric scores due to the use of ethanol-water mobile phases, aligning with the principles of Green Analytical Chemistry.
• NP-HPTLC remains a viable technique, particularly for specific separation needs where normal-phase chemistry is required. However, the choice of mobile phase should be carefully considered to minimize environmental impact.

For researchers and drug development professionals, the evidence supports prioritizing RP-HPTLC with green solvents like ethanol and water as the preferred approach for developing new, sustainable, and high-performance analytical methods for pharmaceutical analysis.

Stability-Indicating Properties and Pharmaceutical Application Success Rates

High-performance thin-layer chromatography (HPTLC) has emerged as a vital analytical technique in pharmaceutical quality control and drug development. The technique offers significant advantages including parallel analysis of multiple samples, minimal solvent consumption, and operational simplicity [53] [63]. Within this field, a critical methodological division exists between normal-phase (NP-HPTLC) and reversed-phase (RP-HPTLC) approaches, each with distinct characteristics and environmental impacts.

The growing emphasis on Green Analytical Chemistry (GAC) principles has led to the development of comprehensive assessment tools such as the Analytical GREEnness (AGREE) metric, which evaluates methods against all 12 principles of GAC [19] [64]. This comparison guide objectively examines the stability-indicating properties and application success rates of NP-HPTLC versus RP-HPTLC methods through the lens of contemporary research, with particular focus on their AGREE scores and pharmaceutical applicability.

Comparative Experimental Data: NP-HPTLC vs. RP-HPTLC

Quantitative Performance Metrics

Table 1: Direct comparison of validation parameters between NP-HPTLC and RP-HPTLC methods for pharmaceutical analysis

Pharmaceutical Compound	Method Type	Linearity Range (ng/band)	Accuracy (% Recovery)	Precision (% RSD)	Sensitivity (LOD, ng/band)	Reference
Lemborexant	NP-HPTLC	50-500	98.24-101.57	0.87-1.00	0.92	[7]
Lemborexant	RP-HPTLC	20-1000	98.24-101.57	0.87-1.00	0.92	[7]
Ertugliflozin	NP-HPTLC	50-600	87.41*	Not specified	Not specified	[6]
Ertugliflozin	RP-HPTLC	25-1200	99.28*	Not specified	Not specified	[6]
Pterostilbene	NP-HPTLC	30-400	92.59*	Not specified	Not specified	[9]
Pterostilbene	RP-HPTLC	10-1600	100.84*	Not specified	Not specified	[9]
Dasatinib Monohydrate	NP-HPTLC	200-1200	Not specified	Not specified	Not specified	[19]
Dasatinib Monohydrate	RP-HPTLC	30-500	Not specified	Not specified	Not specified	[19]

*Reported as % assay in commercial formulations

Greenness Assessment Metrics

Table 2: Greenness profile comparison using different assessment tools

Pharmaceutical Compound	Method Type	AGREE Score	NEMI Profile	Analytical Eco-Scale	ChlorTox (g)	Reference
Lemborexant	NP-HPTLC	Not specified	Not all green	<93	>0.88	[7]
Lemborexant	RP-HPTLC	0.89	All four circles green	93	0.88	[7]
Ertugliflozin	NP-HPTLC	Lower than RP	Not all green	Lower than RP	Higher than RP	[6]
Ertugliflozin	RP-HPTLC	Higher than NP	All four circles green	Higher than NP	0.96	[6]
Pterostilbene	NP-HPTLC	0.46	Not specified	Not specified	Not specified	[9]
Pterostilbene	RP-HPTLC	0.78	Not specified	Not specified	Not specified	[9]
Dasatinib Monohydrate	NP-HPTLC	0.88	Not specified	Not specified	Not specified	[19]
Dasatinib Monohydrate	RP-HPTLC	0.90	Not specified	Not specified	Not specified	[19]
Topiramate	NP-HPTLC	0.76	Not specified	Not specified	Not specified	[64]
Suvorexant	RP-HPTLC	0.88	Not specified	93	0.96	[56]

Experimental Protocols and Methodologies

Standard Method Development Procedures

The development of both NP-HPTLC and RP-HPTLC methods follows a systematic approach to ensure optimal separation, detection, and quantification of pharmaceutical compounds. For NP-HPTLC, the stationary phase typically consists of silica gel 60 F254 plates, while RP-HPTLC employs silica gel 60 RP-18F254S plates [7] [6]. Method optimization involves testing various mobile phase compositions to achieve optimal separation, with NP-HPTLC typically utilizing combinations like chloroform-methanol (85:15 v/v) or acetone-petroleum ether (40:60 v/v), and RP-HPTLC employing greener solvents such as ethanol-water mixtures in varying ratios (80:20 v/v) [7] [6].

The chamber saturation time is generally maintained at 20-30 minutes at room temperature (22±2°C), with migration distances of 70-80 mm [64] [56]. Detection is typically performed using densitometric scanning at wavelengths optimized for each compound, ranging from 199 nm to 323 nm depending on the analyte's UV absorption characteristics [19] [6].

Forced Degradation Studies Protocol

Stability-indicating properties are validated through forced degradation studies following ICH guidelines. Standard protocols include:

• Acidic degradation: Treatment with 0.1-1M HCl at room temperature or elevated temperatures (60-80°C) for several hours
• Alkaline degradation: Treatment with 0.1-1M NaOH under similar conditions
• Oxidative degradation: Exposure to 3-30% H₂O₂ at room temperature
• Thermal degradation: Heating solid drug substance at 105°C for 24 hours or longer
• Photolytic degradation: Exposure to UV light (254 nm) or visible light as per ICH options 1 and 2

The successfully developed stability-indicating methods effectively separate the parent drug from its degradation products, confirming the method's specificity and stability-indicating capability [7] [64] [56].

Method Validation Parameters

Both NP-HPTLC and RP-HPTLC methods are validated according to ICH Q2(R1) guidelines, assessing the following parameters:

• Linearity: Evaluating correlation coefficients (R² ≥ 0.995) across specified concentration ranges
• Accuracy: Determined through recovery studies (typically 98-102%)
• Precision: Assessed as intra-day, inter-day, and instrumental precision (% RSD < 2)
• Robustness: Testing method resilience to deliberate variations in parameters
• Specificity: Confirming separation from degradation products and excipients
• Sensitivity: Determining LOD and LOQ values [7] [19] [56]

AGREE Score Comparison: NP-HPTLC vs. RP-HPTLC

The AGREE metric calculator evaluates analytical methods against all 12 principles of green analytical chemistry, providing a comprehensive assessment of method greenness. The tool generates scores between 0 and 1, with higher scores indicating superior greenness profiles [19] [64].

Across multiple pharmaceutical applications, RP-HPTLC methods consistently demonstrate higher AGREE scores compared to their NP-HPTLC counterparts. For lemborexant analysis, the RP-HPTLC method achieved an AGREE score of 0.89 compared to a lower score for the NP approach [7]. Similarly, for pterostilbene, RP-HPTLC (0.78) substantially outperformed NP-HPTLC (0.46) [9]. This trend continues with dasatinib monohydrate, where RP-HPTLC (0.90) showed a slight but consistent advantage over NP-HPTLC (0.88) [19].

The superior greenness profile of RP-HPTLC methods primarily stems from their use of more environmentally friendly solvents, particularly ethanol-water mixtures, compared to the more hazardous solvents typically employed in NP-HPTLC, such as chloroform, hexane, and toluene [7] [9] [6]. The AGREE assessment incorporates multiple factors including energy consumption, waste generation, and operator safety, providing a holistic evaluation of environmental impact [19] [64].

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key research reagents and materials for HPTLC method development

Item	Function	NP-HPTLC Examples	RP-HPTLC Examples
Stationary Phase	Separation medium	Silica gel 60 F254 plates [64] [6]	Silica gel 60 RP-18F254S plates [7] [56]
Mobile Phase Components	Compound elution	Chloroform, methanol, ethyl acetate, toluene [6] [65]	Ethanol, water, isopropanol [7] [19]
Derivatizing Agents	Visualizing non-UV active compounds	Anisaldehyde-sulfuric acid [64]	Not typically required for most pharmaceuticals
Standard Compounds	Method development and calibration	High-purity reference standards (≥99%) [19] [56]	High-purity reference standards (≥99%) [7] [6]
Sample Preparation Solvents	Extraction and dissolution	Methanol, chloroform [63] [66]	Ethanol, methanol-water mixtures [7] [56]

Stability-Indicating Properties and Application Workflow

The following diagram illustrates the systematic workflow for developing and evaluating stability-indicating HPTLC methods:

Stability-Indicating HPTLC Method Development Workflow

Based on comprehensive experimental data and greenness assessments, RP-HPTLC methods demonstrate clear advantages over NP-HPTLC approaches for stability-indicating pharmaceutical analysis. The consistently higher AGREE scores for RP-HPTLC (0.78-0.90) compared to NP-HPTLC (0.46-0.88) across multiple drug compounds highlight their superior environmental profile [7] [19] [9]. This greenness advantage stems primarily from the replacement of hazardous solvents with more environmentally friendly alternatives like ethanol-water mixtures.

Furthermore, RP-HPTLC methods exhibit better analytical performance in most cases, with wider linear dynamic ranges, improved sensitivity, and higher accuracy in pharmaceutical formulations [7] [9] [6]. Both techniques successfully function as stability-indicating methods, effectively separating drugs from their degradation products under various stress conditions [7] [64] [56].

For pharmaceutical researchers and drug development professionals seeking to implement sustainable analytical practices without compromising data quality, RP-HPTLC represents the preferred chromatographic approach for stability-indicating method development. The technique successfully balances analytical performance with environmental responsibility, aligning with modern green chemistry principles in pharmaceutical analysis.

Conclusion

The comprehensive comparison of NP-HPTLC and RP-HPTLC methods using the AGREE metric consistently demonstrates the superior greenness profile of RP-HPTLC approaches, primarily due to their utilization of ethanol-water mobile phases which significantly reduce environmental impact compared to the chlorinated solvents typically employed in NP-HPTLC. This analysis confirms that sustainable method development does not compromise analytical performance, as RP-HPTLC methods frequently exhibit enhanced sensitivity, precision, and robustness alongside their improved AGREE scores. The integration of AGREE assessment early in method development represents a paradigm shift toward environmentally conscious pharmaceutical analysis. Future directions should focus on expanding green solvent databases, developing automated AGREE calculation tools for routine use, and establishing industry-wide standards for sustainable analytical practices that align with global environmental goals while maintaining rigorous analytical standards for drug development and quality control.

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