Matrix-assisted laser desorption/ionization (MALDI) offers a considerable increase over liquid secondary ionization in mass spectral sensitivity, and is less subject to the suppression effects that may inhibit electrospray ionization production of signal from glycoconjugates. In our laboratory, both ultraviolet (337 nm) and infrared (2.94 fm) irradiation are being used for determinations of native and derivatized glycosphingolipids. With this technique, molecular weight measurements may be made at the femtomole (fmol) level. A potential disadvantage is that few structure-revealing fragment ions are observed in spectra recorded in the normal linear or reflectron modes, but metastable fragment ion information can be obtained by operation of a reflectron instrument in the post-source decay (PSD) mode. Metastable fragmentation yields abundant product ions similar to those observed previously in collision-induced decomposition (CID) tandem (MS/MS) mass spectra. We are optimizing e xperimental conditions and assembling a reference collection of the PSD spectra of ordered sets of glycolipids to establish rules for the interpretation of the fragmentation patterns. The total ceramide size and the sequence of the carbohydrate moiety can be determined at the pmol level and below, even for the components of mixtures. The approach is being applied to glycolipids obtained in small quantities from biological sources and purified by chromatographic means.
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