Determination of aspartame and its degradation and epimerization products by capillary electrophoresis.

Two capillary zone electrophoretic assays using run buffers of pH 9.35 and pH 2.70 have been developed for the determination of aspartame (alpha-L-Asp-L-PheOMe) and its potential degradation products including Phe, PheOMe, 5-benzyl-3,6-dioxo-2-piperazineacetic acid (DKP), the dipeptides Asp-Phe and Phe-Asp, as well as the isomeric beta-aspartame (beta-L-Asp-L-PheOMe). As an uncharged species at pH 2.7 DKP could not be determined. Between pH 2.0 and 3.5 the resolution of the diastereomers of aspartame and beta-aspartame was investigated. While the resolution of the epimeric beta-isomers exhibited a plateau between pH 2.3 and 2.7, resolution of the aspartame diastereomers peaked at pH 3.0. Using salicylic acid and Phe-Gly as internal standards at pH 9.35 and 2.70, respectively, linear calibration curves were obtained for a concentration range between 5 micrograms ml-1 and 1 mg ml-1. The R.S.D. for intraday and interday analysis ranged from 1.0 to 3.6% and 1.5% to 9.1%, respectively. The capillary electrophoresis assays were applied to analyze aspartame solutions heated to 70 degrees C. In agreement with the literature data aspartame was found to be less stable at pH 7 compared to pH 3. In contrast to aspartame itself, an approximate 20% epimerization of beta-aspartame was observed in the incubation mixtures.