Origin of product ions in the MS/MS spectra of peptides in a quadrupole ion trap

Stored waveform inverse Fourier transform and double resonance techniques have been used in conjunction with a quadrupole ion trap to study the dissociation patterns of peptide ions. These experiments provide insight into the origin of individual product ions in an MS/MS spectrum. Results show for a series of leucine enkephalin analogues with five amino acid residues that the b(4) ion is the main product ion through which many other product ions arise. It was also observed that the percentage of the a(4) product ions that are formed directly from the protonated molecule (M+H)(+) depends on the nature of the fourth amino acid residue. in addition, it was determined that in the peptides studied here lower series b ions (e.g., b(3)) arise from direct dissociation of higher series b ions (e.g., b(4)) only about 50% of the time. (C) 1998 American Society for Mass Spectrometry.