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Phospholipids (PL) are increasingly analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS). As in the case of polar molecules, however, the careful selection of the matrix is crucial for optimum results. 9-Aminoacridine (9-AA) was recently suggested as the matrix of choice to analyze PL mixtures because of (a) the improved sensitivity and (b) the reduction of suppression effects compared to other matrices. However, the distinction of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) in the negative ion mode is obscured as PC is also detectable as -CH3+ ion if 9-AA is used as matrix. This may result in the erroneous assignment of PC as a PE species. Using an organic extract from hen egg yolk as example it will be shown that the contribution of PC must be taken into consideration if the negative ion mass spectra are used to evaluate the fatty acyl compositions of PE mixtures. 9-AA can as well be used in hyphenated thin-layer chromatography (TLC)-MALDI-TOF MS where PC and PE are chromatographically well separated for unequivocal assignments.

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No abstract

Matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) is increasingly used to investigate the phospholipid (PL) compositions of tissues and body fluids, often without previous separation of the total mixture into the individual PL classes. Therefore, the questions of whether all PL classes are detectable in a mixture and of whether there are significant sensitivity differences are important. Using extracts from hen egg yolk, bovine liver, and avocado, it is demonstrated that phosphatidylcholine species are primarily detectable by positive ion MALDI-TOF MS. This is due to the quaternary ammonia group and its permanent positive charge. It is also shown by using selected PL fractions obtained by thin-layer chromatography (TLC) that the sum of the spectra of the individual fractions is not equal to the spectrum of the total extract. Some PL classes are not detected in the mixture. We emphasize this result because it is relevant for PL analysis but basically holds for all compounds.