LIQUID SECONDARY-ION MASS-SPECTROMETRY OF METHYL GLYCOSIDES OF OLIGOSACCHARIDES USING MATRICES CONTAINING CARBOXAMIDES

Intense cluster ions corresponding to proton-bound hetero-dimers of an amide molecule and an oligosaccharide molecule are observed in the li quid secondary ion mass spectra of methyl glycosides of oligoxylans if a solution of an aliphatic carboxamide in glycerol is used as the liq uid matrix. These cluster ions are particularly abundant and persist f or a long period if urea (U) or thiourea (TU) is used as the matrix ad ditive. In these cases, cluster ions containing more than one molecule of U or TU and two oligosaccharide molecules are also observed. The i ntense signal due to the proton-bound hetero-dimer between U or TU and the oligosaccharide can be used with advantage for a molecular weight determination. The bonding interactions between a protonated sacchari de molecule and a molecule U or TU in the proton-bound hetero-dimers a re so strong that the urea molecules remain attached to the fragment i ons during the decay of metastable cluster ions and even during collis ion-induced dissociation. Thus, the mass-analysed ion kinetic energy s pectra of these proton-bound hetero-dimers are dominated by abundant c luster ions [B-n + U] and [Y-m+ U] arising from cleavage of the glycos idic bonds within the oligosaccharides. The collisionally-activated ma ss spectra of the proton-bound hetero-dimers additionally contain peak s of the free ions B-n and Y-m. Therefore, these spectra clearly refle ct the arrangement of the monosaccharide residues in the oligosacchari de and can be used conveniently for structural analysis.