Characterization of Phospholipids by Electron Impact, Field Desorption and Liquid Secondary Ion Mass Spectrometry

Abstract

Dimyristoylphosphatidylglycerol (DMPG), dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE) were characterized by electron impact (EIMS), field desorption (FDMS) and liquid secondary ion (LSIMS) mass spectrometry. The results showed that the fragmentation processes of the three phospholipids are very similar. In the EI mass spectra, no molecule-ion peaks were found, the peaks of the fatty acid chains being dominant. The results of FDMS and LSIMS showed that both techniques worked particularly well with all three kinds of phospholipids. The FD mass spectra of all three phospholipid gave strong MH+ peaks, and no fragmental peaks were found except that of DMPC. With LSIMS, the fragmentations of all the phospholipids were extensive and clear, and gave dominant peaks of MH+. The cleavages around phosphate groups were the most abundant. An interesting phenomenon was found in the LSIMS measurements that all three kinds of phospholipids formed intense peaks of molecule-ion clusters, [2M+H]+, [3M+H]+ and [4M+H]+, etc. It was found that the formation of the molecule-ion clusters was related to several factors: concentration, ionic strength, pH, etc., among which concentration is the most important. The degree of cluster formation increased with increase in concentration of phospholipid. The results also showed that the nature of the matrix is critical to a successful LSIMS analysis of phospholipids, and (S)-glycerol was confirmed as a very satisfactory matrix for these phospholipids. © 1997 John Wiley & Sons, Ltd.