Elucidation of some fragmentations of small peptides using sequential mass spectrometry on a hybrid instrument

Sequential daughter‐ion‐scanning analyses of small peptides have been performed using a hybrid tandem instrument of BEqQ configuration. Precursor ions are selected by B and allowed or induced (by high‐energy collisional activation) to decompose in the region preceding E. Decoupling of E from the accelerating voltage permits the selection of the first‐generation daughter ion whilst retaining appropriate float voltages for the quadrupole assemblies. The daughter ion selected by E is further subjected to low‐energy collisional‐activation dissociation (CAD) in q and the fragment‐ion spectrum is obtained by scanning Q. The sequential daughter‐ion‐scanning technique has been used to establish that ‘internal’ fragments of the types, (AY′) and (BY′), are formed via initial Y‐type cleavage. Fragmentation of a protonated peptide (angiotensin III) by loss of the C‐terminal amino acid residue, yielding a (B n′ + OH) ion, is reported for the first time. This process is analogous to that previously described for metal‐cationized peptides.