Determination of Lactic Acid and Poly(lactic acid)s in a Dermatological Formulation by Capillary Electrophoresis

Abstract

A stability-indicating capillary electrophoresis (CE) method was developed to determine lactic acid at a level of 0.9% m/m in a dermatological formulation. This level includes up to 34% present in the form of poly(lactic acid)s. Current methods which involve treatment of samples with sodium hydroxide to hydrolyse the various lactic acid oligomers to free lactic acid cannot strictly be regarded as true measures of stability. The harsh hydrolysis conditions used can also lead to the generation of many degradation peaks from excipients such as natural extracts which can interfere with the detection of lactic acid in HPLC and CE methods. Application of indirect UV detection using 4-methoxybenzoic acid (p-anisate) as the background electrolyte and negative voltage polarity (detector towards anode) allows the direct quantification of the total available lactic acid from the monomer and the predominant linear oligomers in the raw material, i.e., the dimer (lactoyllactate) and the trimer. Tetradecyltrimethylammonium bromide was used as the electroosmotic flow modifier, lactic acid, lithium salt, as a calibration standard and butyric acid as an internal standard. A typical RSD for standard response using migration time-corrected peak area ratios was 1.17% (n = 9). Duplicate analysis of total lactic acid from two similar product placebos spiked with raw material gave average recoveries of 101.0 and 99.6%. The method, when applied to fully and partially hydrolysed raw material, showed good agreement (99.6%) with a standard titration assay for raw materials and was successfully applied to 5 month storage samples of a product. Advantages of the method include speed, simplicity, low consumption of reagents and no organic solvents.