Linkage position and residue identification of disaccharides by tandem mass spectrometry and linear discriminant analysis.

The discrimination of isomeric disaccharides with different linkage types and different monosaccharide residues--glucose (Glc), galactose (Gal), and mannose (Man) at the non-reducing end--was investigated with tandem mass spectrometry (MS/MS) and linear discriminant analysis (LDA). Conventional matrix-assisted laser desorption/ionization (MALDI)-MS has strong interference peaks from matrix ions in the low mass region (<500 Da). This greatly limits the application of MALDI-MS for the analysis of small molecules such as saccharides. We solved this problem by using LDI with acidic fullerene matrix, which gives a very clean background in the low-mass region. Disaccharides with different linkage types give different tandem mass spectral profiles from various cross-ring fragmentation pathways. Disaccharides with the same linkage type but with three different kinds of monosaccharide residues bear the same fragmentation profiles. However, the relative ratios of the fragment ion intensities were found to be distinctly different among the three disaccharide isomers. By employing statistical tools such as LDA to classify the tandem mass spectra, disaccharide isomers with either different linkages or different monosaccharide residues were successfully classified.