Stopped-flow Fourier-transform infra-red spectrometric speciation of glycolic and lactic acids in cosmetic formulations.

The dermatological activity of cosmetic formulations containing alpha-hydroxyacids depends on their different chemical forms, and it is therefore useful to determine these species in the finished products. In the present report a new procedure for studying the protonation equilibria of glycolic and lactic acids by stopped-flow Fourier-transform infra-red (FTIR) spectrometry is described. The procedure was validated for use in the speciation of glycolic and lactic acids in cosmetic formulations, with preferential attention given to glycolic acid, which is the most widely used. Species of these alpha-hydroxyacids can be approximately determined at different pHs and the total content of each alpha-hydroxyacid can be accurately determined (according to the Student t-test at 5% significance level). The recovery of the total content of glycolic acid from commercial cosmetic formulations was 101+/-4%. The RSD of the determinations of the total content and those of the species was of the order of 2-7%.