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

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 ketoenol tautomerization at the reducing terminal promotes ISD and PSD processes.