In positive-ion fast atom bombardment (FAB) mass spectrometry, it is difficult to determine the molecular weights of oligosaccharides due to the low abundance of [M+H]+ion. However, when oligosaccharides are mixed with an appropriate amount of NaCI and LiCI, two highly abundant peaks[M+Na]+and [M+Li]+appear in FABmass spectra and their mass difference is 16. From the adduct ions, the molecular weights of oligosaccharides can be determined unambiguously. In the same way, when a mixture of oligosaccharides was mixed with an appropriate amount of Na+ and Li+, pairs of strong peaks appeared in the FAB mass spectra. These adduct ions were easily recognized due to the fact that they were very strong and their mass difference was union(16u). From these adduct ions, the molecular weight of each oligosaccharides in mixture may be determined.
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