Ultrasound‐Assisted Extraction Coupled With Derivatization for Sensitive High‐Performance Liquid Chromatography Determination of Resorcinol in Permanent Hair Dyes

Resorcinol is widely used as an oxidative dye intermediate in commercial hair coloring products, yet its accurate quantification remains challenging due to its limited native fluorescence and the strong interferences associated with complex cosmetic matrices. In this study, we report a sensitive and selective high-performance liquid chromatography-fluorescence method based on dopamine derivatization for the trace-level determination of resorcinol in hair dye formulations. The analyte was isolated from the sample matrix using ultrasound-assisted extraction with methanol. Under mild alkaline conditions, dopamine rapidly forms a highly fluorescent adduct with resorcinol, substantially enhancing analytical sensitivity and selectivity. The method was optimized with respect to derivatization parameters, chromatographic separation, and detector response. Validation demonstrated excellent linearity over the range of 1-75 ng/mL (r > 0.999), a low limit of detection (= 0.3 ng/mL), and satisfactory recoveries (76.0%-118.0%) across spiked matrices. The robustness of the derivatization procedure was evaluated using Monte-Carlo simulation experiments. Method greenness was assessed using the multicolor tool, yielding a score of 69.9%. Application to commercial products revealed resorcinol concentrations consistent with reported regulatory limits. The proposed method demonstrates promising potential for routine analytical applications; however, further validation, including inter-laboratory studies and comparison with established reference methods, is required to support its suitability for regulatory use.