Atmospheric pressure photoionization mass spectrometry of per-O-methylated oligosaccharides related to d-xylans

Three d-xylan type per-O-methylated trisaccharides with various types of linkages between the d-xylopyranose units were examined by atmospheric pressure photoionization (APPI) mass spectrometry in the positive ion mode. The most interesting feature of a thermospray mass spectrum using the APPI source with UV lamp switched off, is the exclusive production of [M+Na]+ adduct ions. [M+Na]+ cationized ions are the most abundant species in the case of APPI mass spectrometry. The second ionization process has no analogy in the case of substances studied using APPI to date. This aspect involves the addition of a water molecule to the molecular ion of a per-O-methylated saccharide, giving rise to [M+H2O]+ adduct ions. The [M+H2O]+ species are readily detected at m/z 544, and are clearly visible for all three isomers studied. The MS/MS spectrum of [M+Na]+ ions contains a base peak at m/z 375, produced by a Y-type cleavage of the trisaccharide, along with a hydrogen rearrangement on the terminal interglycosidically linkage glycosidic oxygen atom. The [M+H2O]+ species fragment largely give rise to ions at m/z 175, 143 and, as a result, the m/z 111 ion is unique to nonreducing terminal units.

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