In-source and postsource decay in negative-ion matrix-assisted laser desorption/ionization time-of-flight mass...

Cross-ring cleavage ions produced by in-source decay (ISD), as well as deprotonated molecular ions [M - H]-, are invariably observed in negative-ion linear-mode matrix-assisted laser desorption/ionization time-of-flight mass spectrometry spectra of neutral oligosaccharides with 9H-pyrido[3,4-b]indole (norharman) as a matrix. The patterns of ISD ions depend on the oligosaccharide linkage type; thus, these ions are potentially useful in linkage analysis. In postsource decay (PSD) spectra from chlorinated molecular ions [M + Cl]-, all PSD ions are observed in the deprotonated form, although no deprotonated molecular ions are detected. In oligosaccharides having an alditol at the reducing end, deprotonated molecular ions [M - H]- are clearly seen in linear-mode mass spectra and survive in the PSD measurements. These results indicate that the deprotonation process drives ISD and PSD of oligosaccharides and that keto-enol tautomerization at the reducing terminal promotes ISD and PSD processes.