Secondary ion mass spectrometry for sulfoglycolipids: application of negative ion detection

A series of underivatized sulfoglycolipids (SM4g, lyso-SM4g, SM4s, SM3, SM2, SB2, and SB1a) from various tissues were analyzed by both positive (POS-SI-MS) and negative (NEG-SI-MS) secondary ion mass spectrometry. By POS-SI-MS were detected the molecular ions of sulfoglycolipids in the form with sodium or potassium together with some fragment ions useful for the carbohydrate sequence determination. The analysis of monosulfogangliotriaosyl- or monosulfogangliotetraosylceramide and bis-sulfoglycolipid was difficult due to noise in the high mass region. On the other hand, NEG-SI-MS of sulfoglycolipids gave more intense signals from molecular ion of (M-H)- for monosulfoglycolipids and [M-H+Na)-H)- for bis-sulfoglycolipid. Many fragment ions useful for the elucidation of the carbohydrate sequences were also obtained with significant intensities. The fragmentation was assessed to occur at the glycosidic linkages to form ions of the oligosaccharides with or without ceramide. These ions were useful for sugar sequencing and also for distinguishing the differences in the position of the sulfate group. The intensities of saccharide ions without sulfate were lower than those with sulfates. In the case of SB2 and SB1a, containing 2 mol of sulfate ester groups, the molecular ion was detected as [M-H+Na)-H)-. Also, fragment ions with 2 mol of sulfate were detected as the sodium-additive form. It was concluded that NEG-SI-MS is a very useful technique for the structural elucidation of higher sulfoglycolipids.