Simultaneous analysis of aspartame and its hydrolysis products of Coca-Cola Zero by on-line postcolumn derivation fluorescence detection and ultraviolet detection coupled two-dimensional high-performance liquid chromatography

An innovative two-dimensional high-performance liquid chromatography system was developed for the simultaneous analysis of aspartame and its hydrolysis products of Coca-Cola Zero. A C8 reversed-phase chromatographic column with ultraviolet detection was used as the first dimension for the determination of aspartame, and a ligand-exchange chromatographic column with on-line postcolumn derivation fluorescence detection was employed as the second dimension for the analysis of amino acid enantiomers. The fluorimetric derivative reagent of amino acid enantiomers was o-phthaldialdehyde. The hydrolysis of aspartame in Coca-Cola Zero was induced by electric-heating or microwave heating. Aspartame was quantified by the matrix matched external standard calibration curve with a linear concentration range of 0–50 μg mL−1 (r2 = 0.9984). The limit of detection (LOD) and the limit of quantification (LOQ) were 1.3 μg mL−1 and 4.3 μg mL−1, respectively. The amino acid enantiomers was analyzed by the matrix matched internal standard calibration method (d-leucine as the internal standard) with a linear concentration range of 0–10 μg mL−1 (r2 = 0.9988–0.9997). The LODs and LOQs for l- and d-aspartic acid and l- and d-phenylalanine were 0.16–0.17 μg mL−1 and 0.52–0.55 μg mL−1, respectively, that was 12–13 times more sensitive than ultraviolet detection. The overall analysis accuracy for aspartame and amino acid enantiomers was 90.2–99.2% and 90.4–96.2%, respectively. The overall analysis precision for aspartame and amino acid enantiomers was 0.1–1.7% and 0.5–6.7%, respectively. Generally, the extent of aspartame hydrolysis increases with the increase of electro-thermal temperature, microwave power, and the duration of hydrolysis time. d-aspartic acid and d-phenylalanine can be observed with the electro-thermal racemization at the hydrolysis temperature 120 °C for 1 day and only d-aspartic acid can be observed at the hydrolysis temperature 90 °C for 2 and 3 days. For the microwave induced hydrolysis, only l-aspartic acid was detected at the power 560 W for 1 min and 320 W for 3 min.