Mass spectrometric analysis of long-chain lipids

Electrospray and matrix assisted laser desorption ionization generate abundant molecular ion species from all known lipids that have long chain fatty acyl groups esterified or amidated to many different polar headgroup features. Molecular ion species include both positive ions from proton addition [M+H](+) and negative ions from proton abstraction [M-H](-) as well as positive ions from alkali metal attachment and negative ions from acetate or chloride attachment. Collisional activation of both MALDI and ESI behave very similarly in that generated molecular species yield product ions that reveal many structural features of the fatty acyl lipids that can be detected in tandem mass spectrometric experiments. For many lipid species, collision induced dissociation of the positive [M+H](+) reveals information about the polar headgroup, while collision induced dissociation of the negative [M-H](-) provides information about the fatty acyl chain. The mechanisms of formation of many of these lipid product ions have been studied in detail and many established pathways are reviewed here. Specific examples of mass spectrometric behavior of several molecular species are presented, including fatty acids, triacylglycerol, phosphatidic acid, phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, phosphatidylglycerol, ceramide, and sphingomeylin.