Characterization of synthetic N-acetylcysteine conjugates by positive- and negative-ion 252Cf plasma desorption mass spectrometry

N‐Acetylcysteine and nine N‐acetylcysteine conjugates of synthetic origin were characterized by positive‐ and negative‐ion plasma desorption mass Spectrometry. For sample preparation the electrospray technique and the nitrocellulose spin deposition technique were applied. The fragmentation of these compounds, which are best seen as S‐substituted desaminoglycylcysteine dipeptides, shows a similar behaviour to that of linear peptides. In the positive‐ion mass spectra intense protonated molecular ion peaks are observed. In addition, several sequence‐specific fragment ions (A+, B+, [Y + 2H]+, Z+), immonium ions (I+) and a diagnostic fragment ion for mercap‐turic acids (RM+) are detected. The negative‐ion mass spectra exhibit deprotonated molecular ions and in contrast only one fragment ion corresponding to side‐chain specific cleavage ([RXS]−) representing the xenobiotic moiety. In the case of a low alkali metal concentration on the target, cluster molecular ions of the [nM + H]+ or [nM ‐ H]− ion type (n = 1‐3) are observed. The analysis of an equimolar mixture of eight N‐acetylcysteine conjugates shows different quasi‐molecular ion yields for the positive‐ and negative‐ion spectra.