The distinction of underivatized monosaccharides using electrospray ionization ion trap mass spectrometry

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.