Method Development and Validation of RP-HPLC Method for the Estimation of Diclofenac Sodium and Benzyl Alcohol in Gel Formulation

Pharmaceutical Analytical Chemistry

The identification, characterisation, quantification, and purity assessment of pharmaceutical compounds and dosage forms are the focus of the specialist field of pharmaceutical analytical chemistry. From drug discovery and formulation development to manufacturing, quality control, stability testing, and regulatory approval, it is essential to a pharmaceutical product's lifespan. The accuracy and precision of the analytical techniques used for their production are crucial to the dependability and safety of medications.¹

Assessment.²

The rapid growth of pharmaceutical sciences has resulted in the development of complex drug molecules and sophisticated dosage forms. Consequently, conventional analytical methods are no longer sufficient to meet modern analytical requirements. Advanced analytical techniques provide improved sensitivity, selectivity, reproducibility, and automation, making them suitable for compliance with regulatory standards established by ICH, USP, BP, and IP.³

Importance of Analytical Methods in Pharmaceutical Quality Assurance

Analytical methods play an important role in pharmaceutical quality assurance by ensuring that medicinal products consistently meet safety and efficacy standards. Reliable analytical procedures provide scientific evidence regarding the quality, purity, and stability of pharmaceutical products.?

Analytical techniques are extensively used for identification testing, assay determination, impurity profiling, dissolution studies, content uniformity testing, and stability evaluation of pharmaceutical dosage forms.? Improper or poorly validated analytical procedures may produce inaccurate results, which can compromise patient safety and regulatory compliance.? Therefore, analytical methods must be scientifically sound, precise, accurate, robust, and validated according to regulatory guidelines.?

Classification of Analytical Methods

Analytical methods used in pharmaceutical sciences are broadly classified into classical methods and instrumental methods. Classical analytical methods include titrimetric, gravimetric, and colorimetric techniques. These methods are simple and economical but possess limitations such as low sensitivity and greater chances of human error.?

Instrumental analytical methods involve the use of sophisticated instruments to measure physical or chemical properties of analytes. These methods provide improved accuracy, precision, sensitivity, and reproducibility compared to conventional analytical approaches.? Common instrumental techniques include UV–Visible spectroscopy, infrared spectroscopy, nuclear magnetic resonance spectroscopy, mass spectrometry, and chromatographic techniques.¹?

Chromatography

Chromatography is one of the most important analytical techniques employed in pharmaceutical analysis for the separation, identification, and quantification of components present in a mixture. The principle of chromatography is based on the differential distribution of analytes between a stationary phase and a mobile phase.¹¹

Different components of a mixture interact differently with these phases and migrate at different rates, resulting in separation. Chromatographic techniques are highly useful in pharmaceutical analysis because formulations often contain multiple components such as active ingredients, excipients, preservatives, impurities, and degradation products.¹²

Chromatography is widely used in pharmaceutical quality control laboratories for assay determination, impurity profiling, dissolution studies, stability testing, and preservative analysis.¹³ Advancements in chromatographic techniques have resulted in improved resolution, faster analysis, and enhanced sensitivity.¹?

Classification of Chromatography

Chromatographic techniques may be classified based on the physical state of the mobile phase, separation mechanism, or polarity of phases. Based on the physical state of the mobile phase, chromatography is classified into gas chromatography, liquid chromatography, and supercritical fluid chromatography.¹?

Gas chromatography uses a gaseous mobile phase and is suitable for volatile and thermally stable compounds.¹? Liquid chromatography employs a liquid mobile phase and is widely used for non-volatile and thermally unstable compounds.¹? Supercritical fluid chromatography combines the advantages of gas and liquid chromatography and utilizes supercritical fluids such as carbon dioxide as the mobile phase.¹?

Based on the separation mechanism, chromatography is classified into adsorption chromatography, partition chromatography, ion exchange chromatography, size exclusion chromatography, and affinity chromatography.¹? Adsorption chromatography involves separation due to adsorption of analytes on a solid stationary phase.²? Partition chromatography is based on partitioning of analytes between two immiscible liquid phases.²¹ Ion exchange chromatography involves electrostatic interaction between charged analytes and ion exchange resins.²² Size exclusion chromatography separates compounds based on molecular size differences.²³ Affinity chromatography depends upon specific biological interactions between analytes and the stationary phase.²?

According to the polarity of phases, chromatography is classified into normal phase chromatography and reversed phase chromatography.²? In normal phase chromatography, the stationary phase is polar while the mobile phase is non-polar.²? In reversed phase chromatography, the stationary phase is non-polar whereas the mobile phase is polar.²? Reversed phase chromatography is the most commonly used mode in pharmaceutical analysis due to its broad applicability and compatibility with aqueous systems.²?

High Performance Liquid Chromatography

High Performance Liquid Chromatography (HPLC) is an advanced form of liquid chromatography that employs high pressure to pass the mobile phase through a column packed with finely divided stationary phase particles.²? HPLC is widely used in pharmaceutical industries because of its high sensitivity, precision, reproducibility, and rapid analytical capability.³?

The principle of HPLC is based on the differential interaction of analytes between the stationary phase and mobile phase. Components with weaker interaction elute faster, whereas those with stronger interaction are retained longer, resulting in separation.³¹ Factors such as mobile phase composition, flow rate, stationary phase characteristics, and column temperature significantly influence chromatographic separation.³²

Reversed Phase High Performance Liquid Chromatography

Reversed Phase High Performance Liquid Chromatography (RP-HPLC) is the most widely employed mode of HPLC in pharmaceutical analysis. In RP-HPLC, the stationary phase is non-polar, generally consisting of C18 or C8 bonded silica, while the mobile phase is relatively polar and consists of water mixed with organic solvents such as methanol or acetonitrile.³³

RP-HPLC offers several advantages including excellent reproducibility, compatibility with aqueous samples, high resolution, and ease of method development.³? It is extensively used for assay determination, impurity profiling, dissolution studies, and stability analysis of pharmaceutical formulations.³?

Diclofenac Sodium is a widely used non-steroidal anti-inflammatory drug possessing potent analgesic and anti-inflammatory properties. Benzyl Alcohol is commonly used as a preservative in pharmaceutical gel formulations to prevent microbial contamination and improve product stability.³? Accurate and reliable simultaneous estimation of Diclofenac Sodium and Benzyl Alcohol is therefore essential to ensure product quality, safety, efficacy, and regulatory compliance.³?

Chromatographic Parameters

Chromatographic parameters are quantitative measures used to evaluate the efficiency, performance, and reliability of a chromatographic system. These parameters are essential during method development, optimization, and validation because they directly influence the accuracy, precision, and reproducibility of analytical results.³?

Retention Time (Rt)

Retention time is the time required for an analyte to travel through the chromatographic column and reach the detector after sample injection. It is an important parameter used for qualitative identification of compounds. Retention time is influenced by factors such as mobile phase composition, flow rate, stationary phase characteristics, and column temperature.³? Consistent retention time indicates stable chromatographic conditions, whereas significant variation may suggest system instability or column deterioration.??

Capacity Factor (k′)

The capacity factor, also known as retention factor, indicates the extent of retention of an analyte on the chromatographic column relative to the mobile phase.?¹ It provides information regarding the interaction between the analyte and stationary phase. An ideal capacity factor generally ranges between 1 and 10 for efficient chromatographic separation.?²

Selectivity (α)

Selectivity represents the ability of the chromatographic system to separate two closely eluting analytes. It is expressed as the ratio of capacity factors of two compounds.?³ Higher selectivity values indicate better separation efficiency between adjacent peaks.?? Mobile phase composition, pH, and stationary phase properties significantly affect selectivity.??

Resolution (Rs)

Resolution is an important chromatographic parameter that describes the degree of separation between two adjacent peaks. Adequate resolution is necessary for accurate identification and quantification of analytes in pharmaceutical formulations.?? A resolution value greater than 2 is generally considered acceptable for complete peak separation.??

Column Efficiency

Column efficiency is expressed in terms of the number of theoretical plates (N) and indicates the separating capability of the chromatographic column.?? Higher theoretical plate values indicate better column performance and sharper chromatographic peaks.?? Efficient columns improve sensitivity, reproducibility, and peak resolution.??

Peak Asymmetry and Tailing Factor

Peak asymmetry or tailing factor describes the symmetry of chromatographic peaks. Ideally, chromatographic peaks should be symmetrical; however, peak tailing or fronting may occur due to secondary interactions, inappropriate mobile phase composition, or column overloading.?¹ For acceptable chromatographic performance, the tailing factor should generally be less than 2.?²

Factors Affecting Chromatographic Parameters

Several experimental variables influence chromatographic parameters and overall system performance. These factors include mobile phase composition, pH, flow rate, column dimensions, particle size, temperature, and sample concentration.?³ Proper optimization of these variables is essential to achieve reliable and reproducible chromatographic results.??

Importance in Method Development

Chromatographic parameters play a vital role during method development and validation. They help in assessing system suitability, improving resolution, ensuring reproducibility, and establishing robustness of the analytical method.?? Regulatory guidelines recommend routine monitoring of chromatographic parameters to ensure consistent analytical performance and reliable pharmaceutical analysis.??

DRUG PROFILE

Diclofenac Sodium

Diclofenac Sodium is a non-steroidal anti-inflammatory drug (NSAID) widely used for its analgesic, anti-inflammatory, and antipyretic properties. It is commonly prescribed for the treatment of pain, inflammation, arthritis, and musculoskeletal disorders. Diclofenac Sodium is frequently formulated as topical gels to provide localized therapeutic action with reduced systemic side effects.??

The drug possesses aromatic rings and functional groups that exhibit significant UV absorption, making it suitable for analysis by RP-HPLC with UV detection. Diclofenac Sodium is slightly soluble in water and freely soluble in methanol, which facilitates its extraction and chromatographic analysis from gel formulations.??

Benzyl Alcohol

Benzyl Alcohol is a pharmaceutical excipient commonly used as a preservative in topical and injectable formulations. It possesses antimicrobial properties that help prevent microbial contamination and improve formulation stability. In pharmaceutical gels, Benzyl Alcohol plays an important role in maintaining product safety and shelf life.

Benzyl Alcohol also shows UV absorbance and can be simultaneously estimated along with active pharmaceutical ingredients using RP-HPLC methods. Accurate estimation of Benzyl Alcohol is necessary to ensure preservative levels remain within acceptable regulatory limits.??

Simultaneous estimation of Diclofenac Sodium and Benzyl Alcohol is important for ensuring therapeutic efficacy, product quality, and microbial safety of gel formulations. RP-HPLC provides adequate selectivity, sensitivity, and reproducibility for simultaneous analysis of both components without interference from excipients.??

Table 1: Physicochemical Properties of Diclofenac Sodium and Benzyl Alcohol

Analytical Relevance of Selected Drugs

Diclofenac Sodium and Benzyl Alcohol were selected for the present study because of their widespread use in topical gel formulations and the need for reliable analytical methods for quality control and regulatory compliance. Their physicochemical and chromatographic properties make them suitable candidates for simultaneous estimation using RP-HPLC.?¹

MATERIALS AND METHODS

Selection of Drug

Diclofenac Sodium was selected as the active pharmaceutical ingredient because of its extensive use as a non-steroidal anti-inflammatory drug in topical gel formulations. Benzyl Alcohol was selected as the preservative due to its antimicrobial properties and common use in pharmaceutical gels. Simultaneous estimation of both components is essential for ensuring therapeutic efficacy and formulation safety.?²

Procurement of Drug and Chemicals

All drugs, chemicals, and reagents used in the study were procured from approved sources. Analytical reagent grade chemicals and HPLC grade solvents were used throughout the experimental work.

Table 2: List of Drugs Used

Table 3: Chemicals and Reagents Used

Instruments and Apparatus

All instruments used during the study were calibrated and maintained under standard laboratory conditions to ensure reliability and reproducibility of analytical results.

Table 4: List of Instruments Used

Physicochemical Characterization

Diclofenac Sodium was evaluated for physical appearance and solubility behavior to support analytical method development. The drug appeared as a white crystalline powder and showed good solubility in methanol, indicating its suitability for RP-HPLC analysis.?³???

Table 5: Physicochemical Properties of Diclofenac Sodium

Preparation of Buffer Solution

Potassium dihydrogen phosphate buffer was prepared in distilled water, and pH was adjusted using orthophosphoric acid. The solution was filtered and degassed before use in chromatographic analysis.??

Drug–Excipient Compatibility Study

FTIR spectroscopy was used to study compatibility between Diclofenac Sodium and Benzyl Alcohol. The absence of major changes in characteristic peaks indicated compatibility between drug and preservative.??

Development of RP-HPLC Method

The RP-HPLC method was developed to achieve effective separation of Diclofenac Sodium and Benzyl Alcohol with good resolution and peak symmetry. A C18 column (250 mm × 4.6 mm, 5 µm) was selected for analysis.??

Different mobile phase combinations were evaluated, and potassium dihydrogen phosphate buffer with acetonitrile (60:40 v/v) was found suitable for optimum separation. A flow rate of 1.0 mL/min and detection wavelength of 240 nm were selected for analysis.??

Table 6: Optimized Chromatographic Conditions

Preparation of Standard Solutions

Standard stock solutions of Diclofenac Sodium and Benzyl Alcohol were prepared separately in methanol to obtain concentrations of 1000 µg/mL. Working standard solutions were prepared by suitable dilution with mobile phase.?????

Calibration Curve Preparation

Calibration curves were prepared by injecting working standard solutions of different concentrations into the HPLC system under optimized chromatographic conditions. Peak area versus concentration plots were constructed for both analytes.?¹

System Suitability Test

System suitability testing was performed before analysis to ensure adequate chromatographic performance. Parameters such as retention time, theoretical plates, peak area, and tailing factor were evaluated according to ICH guidelines.?²

Table 7: System Suitability Parameters

Method Validation

The developed RP-HPLC method was validated according to ICH Q2(R1) guidelines for parameters including linearity, accuracy, precision, LOD, LOQ, robustness, and ruggedness.?³

Linearity was evaluated by regression analysis of calibration curves, and good correlation coefficients confirmed linear response.?? Accuracy was determined by recovery studies using the standard addition method.?? Precision studies showed low %RSD values, indicating good reproducibility of the method.??

Limit of Detection (LOD) and Limit of Quantitation (LOQ) were calculated using standard deviation and slope of the calibration curve, indicating the sensitivity of the method.?? Robustness and ruggedness studies demonstrated reliability of the developed RP-HPLC method under varied analytical conditions.??

RESULTS AND DISCUSSION

FTIR Analysis Study

Fourier Transform Infrared (FTIR) spectroscopy was performed to evaluate the compatibility between Diclofenac Sodium and Benzyl Alcohol. The FTIR spectrum of Diclofenac Sodium showed characteristic peaks corresponding to aromatic C–H stretching, carboxylate group vibrations, and secondary amine groups, confirming its chemical identity and purity.

Benzyl Alcohol exhibited characteristic absorption bands corresponding to hydroxyl (O–H) stretching and aromatic C–H vibrations, confirming its identity.

The FTIR spectrum of the physical mixture showed all major characteristic peaks of both Diclofenac Sodium and Benzyl Alcohol without significant shifting or disappearance of peaks. This indicates the absence of chemical interaction between the drug and preservative, confirming their compatibility for formulation and RP-HPLC analysis.

Figure 1: FTIR spectrum of Diclofenac Sodium

Figure 2: FTIR spectrum of Benzyl Alcohol

Figure 3: FTIR spectrum of physical mixture of Diclofenac Sodium and Benzyl Alcohol

UV–Visible Spectrophotometric Analysis

UV–Visible spectrophotometric analysis was performed for selection of detection wavelength. Standard solutions of Diclofenac Sodium and Benzyl Alcohol were scanned in the range of 200–400 nm.

Diclofenac Sodium showed maximum absorbance at 240 nm, while Benzyl Alcohol also exhibited adequate absorbance at the same wavelength. Therefore, 240 nm was selected for simultaneous RP-HPLC analysis of both analytes without spectral interference.

Figure 4: Overlay UV spectrum of Diclofenac Sodium and Benzyl Alcohol

RP-HPLC Method Development Study

Chromatographic separation of Diclofenac Sodium and Benzyl Alcohol was successfully achieved using a C18 RP-HPLC column with potassium dihydrogen phosphate buffer and acetonitrile (60:40 v/v, pH 3.0).

Well-resolved and symmetrical peaks were obtained for both analytes without interference from excipients, indicating good selectivity of the method. A flow rate of 1.0 mL/min and detection wavelength of 240 nm provided satisfactory resolution and reproducible chromatographic performance.

Figure 5: Optimized RP-HPLC chromatogram of Diclofenac Sodium and Benzyl Alcohol

System Suitability Study

System suitability testing was performed by injecting a mixed standard solution of Diclofenac Sodium and Benzyl Alcohol six times under optimized chromatographic conditions.

Parameters such as retention time, peak area, theoretical plates, and tailing factor were evaluated. The %RSD values were within acceptable limits, indicating good repeatability. Theoretical plate count and tailing factor values confirmed adequate column efficiency and symmetrical peak shape, demonstrating suitability of the RP-HPLC system for analysis.

Table 8: System Suitability Parameters

7.5 Linearity Study

Linearity was evaluated to establish the proportional relationship between analyte concentration and detector response. Diclofenac Sodium and Benzyl Alcohol standard solutions were prepared at six concentration levels within the analytical range and injected in triplicate.

Calibration curves were constructed by plotting mean peak area versus corresponding concentration. Linear regression analysis demonstrated a linear response over the selected concentration range for both analytes. The high correlation coefficient values indicate good linearity and suitability of the method for quantitative estimation.

Table 9: Linearity Data for Diclofenac Sodium

Correlation coefficient (r²) = 0.9992

Figure 6: Calibration curve of Diclofenac Sodium

Table 10: Linearity Data for Benzyl Alcohol

Correlation coefficient (r²) = 0.9989

Figure 7: Calibration curve of Benzyl Alcohol

7.6 Accuracy Study

Accuracy of the method was evaluated by recovery studies using the standard addition technique at three concentration levels: 80%, 100%, and 120%. Known quantities of Diclofenac Sodium and Benzyl Alcohol were added to the pre-analyzed sample and analyzed under optimized chromatographic conditions.

The percentage recovery values obtained were within acceptable limits, confirming the accuracy of the method and absence of interference from formulation excipients.

Table 11: Accuracy Study for Diclofenac Sodium

Table 12: Accuracy Study for Benzyl Alcohol

7.7 Precision Study

Precision of the analytical method was evaluated in terms of repeatability (intra-day precision) and intermediate precision (inter-day precision). Multiple injections of the same concentration were analyzed on the same day and on different days.

Precision was expressed as %RSD of peak area values. Low %RSD values indicate good precision and reproducibility of the method.

Table 13: Intra-day Precision Study

%RSD

• Diclofenac Sodium = 0.41%
• Benzyl Alcohol = 0.46%

Table 14: Inter-day Precision Study

%RSD

• Diclofenac Sodium = 0.53%
• Benzyl Alcohol = 0.61%

Specificity Study

Specificity of the developed RP-HPLC method was evaluated using blank, placebo, standard, and sample solutions under optimized chromatographic conditions.

No interfering peaks were observed in blank and placebo chromatograms at the retention times of Diclofenac Sodium and Benzyl Alcohol. Well-resolved peaks were obtained in standard and sample chromatograms, confirming the specificity of the method for simultaneous estimation of both analytes in gel formulation.

Figure 8: Chromatogram of blank solution

Figure 7.9: Chromatogram of placebo solution.

Figure 10: Chromatogram of sample solution