Comparing HPLC and UV spectrophotometric analysis methods for determining the stability of sorbic acid in nonionic creams containing lactic acid

This paper describes a comparison between ultraviolet (UV) spectrophotometric and high-performance liquid chromatographic (HPLC) methods of analysis for the determination of sorbic acid in nonionic creams containing lactic acid. Sorbic acid is an antimycotic agent and is used as a preservative in pharmaceuticals, cosmetics, and food products. UV spectrophotometric analysis was done by calculating the concentration of remaining sorbic acid from the absorbance values and the molar extinction coefficient E²⁵⁸_M = 24,080. A decrease in absorbance at 258 nm was accompanied by a simultaneous increase in total carbonyls and monoaldehyde content and the appearance of a very weak absorption maximum between 215 and 225 nm. HPLC analysis was done with a Hypersil BDS C₈ column with detection at 254 nm and employing a mobile phase consisting of a mixture of buffer and methanol (7:3 v/v) at a pH of 2.25. The buffer consisted of 0.85% H₂SO₄ in 17.5 mM KH₂PO₄. The validation results, together with statistical treatment of the data, demonstrated the reliability of both procedures. A drawback of the UV methods was, however, its lack of adequate measurement of sorbic acid stability at higher temperatures. For these assays, the HPLC method was found to be adequate, and it should therefore be used to obtain accurate stability data for sorbic acid in creams.