Ascorbic acid is a popular active ingredient in cosmetic products, known for its antioxidant properties. It is most commonly found in cosmetic products for skin brightening as well as in products for mature or acne-prone skin. Due to its instability, manufacturers are often replacing it with more stable derivatives.
As the stability of ascorbic acid derivatives remains relatively underexplored, one of the main objectives of this thesis was to evaluate their stability and determine their actual content in selected cosmetic products. Thus, we focused on hydrophilic derivatives of ascorbic acid, namely sodium ascorbyl phosphate, ascorbyl glucoside, and 3-O-ethyl ascorbic acid. The basis for the stability evaluation is a suitable stability indicating HPLC method, which was optimized and then successfully validated in accordance with ICH guidelines. After reviewing the products available on the market, 14 cosmetic products containing the considered derivatives were chosen, with emphasis on those that quantitatively defined their content. Thus, the presence and content of the derivatives were determined to assess the accuracy of labelling and overall product quality. The analysis showed that all listed derivatives were present in most of the cosmetic products, except for one product in which ascorbyl glucoside was not detected, and two products where additional ascorbic acid was identified. The deviations between the determined and declared contents were within 25% in most products. However, significant deviations were also observed, with actual contents up to 93.8% lower than declared. No direct correlation was observed between the product prices and the actual derivative contents or the accuracy of their quantitative declaration.
Since stability is often a reason for inadequate content, the tested cosmetic products, along with standard solutions of the derivatives, were subjected to long-term and accelerated stability testing. Among the derivatives, ascorbyl glucoside demonstrated instability, both in some of the cosmetic products and in the standard solution under accelerated storage conditions. All derivatives remained stable in long-term stability testing within the tested timeframe (<10% degradation after 4 weeks). Thus, we can confirm that all three ascorbic acid derivatives are stable under the recommended storage conditions. Based on the results of the content analysis, it can be concluded that stricter regulatory control of cosmetic product labelling would be beneficial, in order to prevent consumer deception.
|
