Thermospray liquid chromatography/mass spectrometry study of diastereomeric isoindole derivatives of amino acids and amino acid amides

Abstract

A thermospray liquid chromatography/mass spectrometry (TSP-LC/MS) method is described for determination of the enantiomeric excess of α-amino acids and α-amino acid amides as their o- phthalaldehyde/N- acetyl- l-cysteine (OPA/NAC) derivatives. The source temperature is an important factor in optimizing the sensitivity of the TSP-LC/MS analysis, whereas the repeller voltage is of minor importance. On-column mass spectra were acquired for the OPA/NAC derivatives of several α-amino acids and α-amino acid amides. For the main fragment ions, mass spectra fragmentation pathways are proposed. The applicability of the method is demonstrated by means of the enantiomeric excess determination of valine in a sample from an enzymatic hydrolysis experiment. Using single ion monitoring, the detection limit of d-valine in the presence of excess l-valine is 10 pmol. The present TSP-LC/MS method is useful for validating the results obtained from LC/UV or LC/fluorescence methods for the enantiomeric excess determination of α-amino acids and α-amino acid amides.