DETERMINATION OF L-LYSINE N-ACETYLCYSTEINATE AND ITS MONOMERIC AND DIMERIC RELATED-COMPOUNDS BY LIQUID-CHROMATOGRAPHY MASS-SPECTROMETRY

Methods for the simultaneous determination of N-acetylcysteine, L-lysi ne, L-cysteine, L-cystine, N,N'-diacetylcystine and N,S-diacetylcystei ne in aqueous solutions were developed. The method is based on the on- line coupling of Liquid chromatography (LC) to tandem mass spectrometr y (MS-MS) with an atmospheric pressure chemical ionisation (APCI) inte rface. The optimal LC phase system consisted of a C-18 stationary phas e and an acidic solution of formic acid in acetonitrile as the mobile phase, while D,L-phenylalanine was selected as an internal reference c ompound. The best transitions for MS-MS detection were selected after optimisation of the APCI ionisation yield in full scan. The optimisati on was performed by loop injection. By coupling LC to MS-MS in the APC I+ (positive ion mode) as well as in the multiple reaction monitoring mode, satisfactory results were obtained with a mixture of the seven c ompounds, each at a concentration level of 1 mu g/ml. For a further im provement of the detection limit of the dimeric compounds, electrospra y ionisation was then investigated. By use of this interface a quantit ation limit down to 10(-8) M (3 ng/ml) could be achieved for both dime rs. The developed method provides improved selectivity based on the co mbination of chromatography and mass spectrometry thus facilitating id entification. (C) 1998 Elsevier Science B.V. All rights reserved.