Mass spectra of several aromatic compounds containing an amino or thiol group were recorded under electrospray conditions with a Finnigan TSQ 700, as well as on a Bruker Esquire-LC spectrometer. The MS/MS spectra displayed two main peaks: the first one arising from the cleavage of the amino or thiol group; the second one appeared at 18 Da higher than the first one. It could be shown that the second peak resulted from a nucleophilic addition of one water molecule to the fragment ion.
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Horni et al. (1999) studied this question.