Matrix‐assisted laser desorption/ionization mass spectrometry of phospholipids

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Abstract
Phosphatidylcholine, lysophosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylglycerol, phosphatidylinositol, phosphatidic acid and cardiolipin were examined by matrix‐assisted laser desorption/ionization mass spectrometry on both a time‐of‐flight and a magnetic sector instrument using several substituted cinnamic acids, benzoic acids and substituted coumarins as matrices. All of these phospholipids gave signals, with the strongest responses being obtained from α‐cyano‐4‐hydroxycinnamic acid, esculetin (6,7‐dihydroxycoumarin) and 2,5‐dihydroxybenzoic acid (2,5‐DHB). In the positive‐ion mode, most compounds produced mixtures of [M + H]+, [M + Na]+ and [M + K]+ ions from both the free acids and from the sodium salts to give a complex mixture of ions. The spectra were considerably simpler in the negative‐ion mode, where the molecular ion region contained mainly [M − H]− ions. Spectral simplification was also achieved in the positiveion mode by methylation with diazomethane to prevent salt formation. This procedure had the additional advantage of converting phosphatidylethanolamine and phosphatidylserine into their corresponding quaternary derivatives with a consequent gain in detection sensitivity. Fragmentation involving cleavages around the phosphate group was common, with the diacyl‐containing ion produced by loss of the substituted phosphate moiety being particularly abundant. Fragmentation appeared to be strongly dependent on the nature of the matrix and, particularly, on the presence or absence of water in the matrix solvent. With spectra recorded on the magnetic sector instrument, additional fragment ions produced by losses of the acyl groups were seen. These latter ions provided some information regarding the distribution of carbon atoms between the acyl chains.