Continuous on‐line derivatization and selective separation of D ‐aspartic acid by a capillary electrophoresis system with a continuous sample introduction interface

A rapid and selective method is described for the separation of D ‐aspartic acid ( D ‐Asp) using a continuous on‐line derivatization system coupled to capillary electrophoresis (CE). D ‐Asp was derivatized using o ‐phthaldialdehyde/ N ‐acetyl‐ L ‐cysteine (OPA/NAC). By on‐line derivatization, amino acid enantiomers were automatically and reproducibly converted to the UV‐absorbing diastereomer derivatives which were separated by capillary zone electrophoresis (CZE) in the presence of 10 mmol/L β‐cyclodextrin ( β‐ CD). Under the investigated separation conditions, D ‐Asp is resolved from L ‐aspartic acid ( L ‐Asp) and other amino acids in a standard mixture of amino acids. The separation could be achieved within 4 min and the sample throughput rate can reach up to 16 h ‐1 . The repeatability (defined as relative standard deviation, RSD) was 3.21%, 3.58% with peak area evaluation and 3.72%, 4.03% with peak height evaluation for L ‐Asp and D ‐Asp.