Development and validation of a high-performance thin-layer chromatography method for the quantification of adapalene and preservative phenoxyethanol in gel formulation—application to stability studies

A new high-performance thin-layer chromatography (HPTLC) method for the determination of adapalene and phenoxyethanol was developed and validated. An excellent resolution with R_F values 0.43 ± 0.03 and 0.75 ± 0.02 for adapalene and phenoxyethanol was achieved by using optimized chromatographic conditions. A mixture of toluene‒acetone (8:2, V/V) was chosen as the mobile phase and detected at 228 nm. When the marketed formulation was analysed by the developed method, the percentage of drug contents were found to be 100 ± 1.3839% w/w for adapalene and 101 ± 1.1809% w/w for phenoxyethanol. The developed method was validated for linearity range, limits of detection and quantification, accuracy, precision, robustness, solution stability, specificity and forced degradation studies. The method was found to be linear, accurate, precise, robust and sensitive. The method was found to be linear in the range of 40‒240 ng per band for adapalene with R² value of 0.99 and 100‒600 ng per band for phenoxyethanol with R² value of 0.994. The limits of detection and quantification were found to be 4.79 and 14.52 ng per band for adapalene and 6.91 and 20.93 ng per band for phenoxyethanol, respectively. The percentage of drug recovery was found to be near 100% w/w confirming the accuracy of the method. In the case of precision studies, relative standard deviation (%RSD) values were found to be less than 2, indicating that the method was precise. The %RSD values of deliberate variations like amount of mobile phase and saturation time were found to be less than 2 showing that the method was robust. Forced degradation study concluded that adapalene and phenoxyethanol were highly labile for acid and alkali hydrolysis. Both were found to be stable during photo and thermal degradation.