A convenient method for distinguishing underivatized isomeric monosaccharides has been established using electrospray ionization ion trap mass spectrometry (ESI-ITMS). Mass spectra of hexoses (glucose, galactose, and mannose), N-acetylhexosamines (N-acetylglucosamine, N-acetylgalactosamine, and N-acetylmannosamine) and hexosamines (glucosamine, galactosamine, and mannosamine) dissolved in solvent containing 1 mM ammonium acetate were obtained in the positive ion mode. Glucose was distinguished from galactose and mannose in the MS2 spectrum of the [M+NH4]+ ion at m/z 198. The MS3 spectra generated from [MH-NH4-H 2O-NH3]+ at m/z 163 showed that galactose and mannose could be distinguished by the ratio of peak intensities at m/z 145 and 127, while the three N-acetylhexosamine and hexosamine stereochemical isomers could be identified by the relative abundance ratios of product ions observed in MS3 spectra. The investigation of MS and MS2 spectra from complexes of these monosaccharides with Na+ and Pb2+ failed to distinguish these monosaccharide isomers. Therefore, multiple stage mass analysis by ESI-ITMS using either [M+NH4]+ or [M+H]+ was useful to distinguish between the isomers of monosaccharides.
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