Matrix‐assisted laser desorption/ionization mass spectrometry of sphingo‐ and glycosphingo‐lipids

Sphingo‐ and glycosphingo‐lipids were examined by matrix‐assisted laser desorption/ionization mass spectrometry on a time‐of‐flight (TOF) and a magnetic sector mass spectrometer with 2,5‐dihydroxybenzoic acid (2,5‐DHB), sinapinic acid, α‐cyano‐4‐hydroxycinnamic acid, 2‐(4‐hydroxyphenylazo)benzoic acid (HABA) and esculetin as the matrices. Strong signals were produced by all lipids tested, with many of the compounds producing structurally useful fragment ions. Neutral glycosphingolipids gave the best signals in the positive‐ion mode with 2,5‐DHB, α‐cyano‐4‐hydroxycinnamic acid and esculetin. When 2,5‐DHB was used as the matrix, glycosphingolipids that did not contain HexNAc residues tended to fragment between the sugar and ceramide moieties to give ions revealing the saccharide and lipid contents. Other matrices tended not to induce this fragmentation but, instead, produced ions resulting from loss of that acylamide from the ceramide moiety. These ions revealed the distribution of carbon atoms between the acyl and sphingosine chains. Fragmentation of the oligosaccharide chain of glycolipids that did contain HexNAc groups tended to be more random than that seen when HexNAc was absent and provided considerable sequence information. The acidic gangliosides produced broad, unresolved fragment ion peaks on the TOF spectrometer. Fragmentation was the result of both decarboxylation and loss of sialic acids. On the magnetic sector instrument, these fragment ions were sharp and well resolved. Additional fragment ions resulting from glycosidic cleavages provided information about the sequence of sugars in the oligosaccharide chain. Negative‐ion signals ([M − H]) − from the acidic glycosphingolipids were generally better defined than those in the positive‐ion spectra as the result of a reduction in alkali metal adduct and salt formation.