Dual-Mode Detection of the Non-Traditional Preservative Caprylhydroxamic Acid in Cosmetics: A Green Chemistry Approach

Caprylhydroxamic acid (CHA) exhibits strong chelating capacity toward several metal ions and thereby inhibits microbial growth. Not being a conventional preservative, CHA is typically viewed as an emerging alternative in cosmetic formulations. Despite its growing application, CHA is not explicitly regulated under current EU, USA or Chinese cosmetic guidelines, leading to unsupervised usage and potential safety concerns. Consequently, there is an urgent need to establish reliable analytical methods for the detection of CHA in cosmetics, ensuring product safety and regulatory compliance. Herein, a chromogenic system is developed for the dual-mode detection of CHA in cosmetics, allowing both quantitative spectrophotometric and semi-quantitative colorimetric analysis. First of all, Fe3+ is introduced to catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) under acidic conditions to produce a blue color. When CHA is added, it chelates Fe3+ strongly, forming a stable complex that competes with the TMB oxidation reaction, thereby causing the blue color to fade. The change in absorbance of the system at 656 nm exhibits a linearity with the concentrations of CHA ranging from 0.1 mmol/L to 2.0 mmol/L. The proposed method enables the detection of CHA in various cosmetics, including facial mask, cream, spray, and toner. A detection limit of 0.007% was achieved, far below the 0.3% limits set by South Korea and Australia, confirming its compliance with regulatory monitoring requirements. AGREE assessment of the method gave a score of 0.76, demonstrating its greenness. The dual-mode detection approach serves as an effective, rapid on-site screening tool, working alongside HPLC-UV for confirmatory analysis in the laboratory. In summary, the method is free of complex pretreatment and expensive instruments, providing a low-cost, on-site, and green approach for rapid screening. It can serve as a complement to routine laboratory detection methods.