Quality Comparative Evaluation of Eungyosan Formulations by a Validated HPLC–PDA Method for 11 Marker Components

Eungyosan (EGS) is a traditional multi-herbal formulation widely used for the treatment of respiratory diseases; however, its quality control remains challenging due to its complex chemical composition. This study aimed to develop and validate a high-performance liquid chromatography coupled with photodiode array detection (HPLC–PDA) method for simultaneous determination of 11 representative marker compounds in EGS and to apply the method to the comparative quality evaluation of laboratory-prepared and commercial EGS formulations. Chromatographic conditions were optimized, and the marker compounds were selected based on their herbal origin, phytochemical relevance, and chromatographic detectability. The method was validated in terms of linearity, sensitivity (limits of detection and quantification), precision, accuracy, and stability. All analytes exhibited excellent linearity (coefficient of determination > 0.9999), along with satisfactory precision (relative standard deviation < 2%) and recovery (95.64–105.69%). The validated method was successfully applied to a laboratory-prepared extract and three commercial granule formulations. Considerable differences in the levels of marker compounds were observed among the samples; several marker compounds were either not detected or could not be quantified because of UV spectral mismatch in certain commercial products. These findings demonstrate variability in the chemical composition of the tested EGS formulations and highlight the usefulness of the validated HPLC–PDA method for comparative quality evaluation of multi-herbal formulations.