The determination of the amino acid sequence in the fast atom bombardment mass spectra of dipeptides

The positive ion fast atom bombardment mass spectra of dipeptides all exhibit the MH+ ion as the most abundant one. Direct identification of the N-terminal amino acid in the mass spectrum is not possible because of the absence of its acylium ion. Fortunately the identification of the C-terminus is often possible because of the occurrence of the protonated C-terminal amino acid as a result of MH+ ion fragmentation. In all cases, including those where the abundance of this C-terminal ion is too low for identification, the metastable ion spectrum of the MH+ ion provides sufficient information for an unequivocal sequence determination.