CHARACTERIZATION OF A COMPLEX MIXTURE OF CERAMIDES BY FAST-ATOM-BOMBARDMENT AND PRECURSOR AND FRAGMENT ANALYSIS TANDEM MASS-SPECTROMETRY

Fast atom bombardment, precursor and fragment ion analysis mass spectrometry were employed to characterize the fatty acid and long-chain base composition of a complex ceramide mixture prepared from beef brain lipids. Precursor ion analysis of a sphingenine-specific fragment allowed selective recognition of ceramides with different fatty acids, some of which escaped undetected in the source spectre owing to background contamination of the FAB matrix. The use of a triple quadrupole yielded far better results than linked scanning on a sector mass spectrometer, owing to unit mass resolution of the precursor ions, which allows discrimination of saturated from unsaturated acids with the same number of carbon atoms. The composition of the mixture of linked fatty acid was derived from the intensity of the molecular species both in the source and in the precursor spectre, and was compared with that obtained by capillary gas chromatography of the released fatty acid methyl esters. Some discrepancy between the values obtained with the two techniques can be attributed to very different surface activity of the short- and long-chain fatty acids. Analysis of the fragment ions following collision-induced decomposition of cationized species allowed localization of the position of the double bond in a ceramide containing a C-24:1 fatty acyl chain.