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Field desorption mass spectrometry has been used to analyze carbohydrate polymers with 5 to 14 hexose units without prior derivatization. In all examples, the molecular weight of the oligosaccharide could be determined by means of the abundant quasimolecular ions of the type MNa(+), MH(+), MNa(2) (2+), and MNa(3) (3+). Fragmentation at glycosidic linkages was observed in varying extents. The reduced oligosaccharide Man(8)GlcNAcH(2), obtained from IgM [Cohen, R. E. & Ballou, C. E. (1980) Biochemistry 19, 4345-4358], gave quasimolecular ion signals MNa(+) at m/z 1544, MH(+) at m/z 1522, MNa(2) (2+) at m/z 784, and MNa(3) (3+) at m/z 530, all corresponding to its assumed molecular weight of 1519.5. Mycobacterial methylmannose polysaccharides with the general structure Man(x)MeMan(y)-OCH(3) [Yamada, H., Cohen, R. E. & Ballou, C. E. (1979) J. Biol. Chem. 254, 1972-1979] were also successfully analyzed. Man(1)MeMan(13)-OCH(3), the largest homolog, gave the expected signal of the quasimolecular ion MNa(+) at m/z 2506. The larger polysaccharides were analyzed by using a KRATOS MS-50 mass spectrometer with a high-field magnet enabling full sensitivity to be maintained up to 3000 atomic mass units. Polysaccharides up to m/z 1978 were analyzed by using a KRATOS MS-9 mass spectrometer operated at 4 Kv. The signal-to-noise ratio, which becomes a serious problem in field desorption mass spectrometry at low accelerating voltages, and the low instrument sensitivity were improved considerably by our use of a method of adding scans with low total ion currents obtained over a longer desorption time. In this way, we obtained complete sequence information on methylmannose polysaccharides up to Man(1)MeMan(9)-OCH(3)(MNa(+) at m/z 1802). Analysis of a presumed Man(1)MeMan(7)-OCH(3), gave a spectrum consistent only with the structure Man(2)MeMan(6)-OCH(3), revealing the existence of a methylmannose homolog with 2 unmethylated mannoses at the nonreducing end of the chain.