Determination of amino acids by on-line capillary electrophoresis-electrospray ionization mass spectrometry.

On-line capillary electrophoresis-electrospray ionization mass spectrometry (CE-ESMS) has been used for the separation and detection of amino acid mixtures. Four natural amino acids, histidine, tryptophan, phenylalanine, aspartic acid, and a tripeptide, glutathione, were separated and determined. Protonated molecules were detected in the CE-ESMS mode with detection limits of about one pmol. Optimum CE-ESMS operating conditions for amino acid analysis were determined employing acetic acid solutions as CE electrolytes. The examined parameters included capillary diameter (50-100 microns internal diameter), applied separation voltage (20-30 kV), and concentration of electrolyte (10-60% acetic acid). Stable working conditions were maintained when the CE currents were less than 18 microA. The use of electrolyte solutions such as those described here instead of true buffer solutions may have advantages for CE-ESMS systems which employ a "sheathless" interface.