Spectrofluorimetric method for determination of the skin depigmenting substance - 4-hexylresorcinol - in cosmetic products.

This study reports a rapid and sensitive batch spectrofluorimetric method for the quantification of 4-hexylresorcinol in cosmetic formulations. The approach relies on the alkaline oxidative coupling of 4-hexylresorcinol with dopamine to form a highly fluorescent azamonardine derivative (λex/λem = 440/470 nm), enabling selective detection of 4-hexylresorcinol in complex matrices. Sample preparation was optimized for lipophilic creams using a n-hexane/isopropanol (90 : 10, v/v) extraction solvent, followed by dilution and 1 min derivatization at ambient temperature. Key derivatization parameters were optimized using a Box-Behnken design, yielding optimal conditions of 0.1 mM dopamine, 100 mM carbonate buffer, pH 11.5, and 1 min reaction time. The method was validated in accordance with ICH guidelines, demonstrating excellent linearity over 5-100 ng mL-1 (r > 0.9994), satisfactory trueness and precision (recoveries 81.1-120.0%; RSD < 8.4%), and low detection capability (LOD 1.1 ng mL-1; LOQ 3.2 ng mL-1). Robustness, assessed via Monte-Carlo simulations and process capability analysis, indicated reliable performance (Cpk = 1.4). Application to commercial depigmenting creams showed 4-hexylresorcinol levels of 3.09-3.52 mg g-1 with results statistically comparable to corroborative HPLC-FLD measurements. A unified multi-color assessment (MA) confirmed the method's balanced sustainability, practicality, analytical performance, and innovation (whiteness score 62.6%). Overall, the proposed assay offers a simple, cost-effective alternative for routine quality control of 4HR in cosmetics while aligning with green analytical chemistry principles.