Ascorbic Acid and Its Hydrophilic Derivatives in Commercial Cosmetics: A Cross-Sectional Study of Their Quality Control and Stability

Ascorbic acid and its hydrophilic derivatives, ascorbyl phosphate, ascorbyl glucoside, and 3-O-ethyl ascorbic acid, are wide used active ingredients in cosmetics, with concentrations increasingly declared on product labels. Since such products are not subject to routine quality control, this study aimed to evaluate the content-related quality and stability of a comprehensive sample of commercial cosmetics. For this purpose, a selective HPLC-UV analytical method for their simultaneous determination was developed, validated, and applied to 41 products. The analysis revealed 0–31.7% (m/m) contents and inappropriate labeling in 22% of the products, including the absence and replacement of the labeled vitamin C form. Over half (57%) of the determined contents corresponded to the labels, which was claimed for 31 products, while the remaining contents were considerably lower (<80%). The stability study, performed on selected cosmetics and standard solutions under ambient and elevated temperatures, demonstrated instability, which could account for their inadequate content. Thus, the instability was found to be temperature-dependent and most pronounced for ascorbic acid, whereas the product price did not show a clear correlation with greater content or stability. Overall, these results highlight the need for proper quality control and stricter cosmetics regulation as well as formulation design to achieve sufficient stabilization, as observed in certain products.