| The potential of matrix-assisted laser desorption/ionization (MALDI) coupled with a time-of-flight mass spectrometry (TOF MS) for the analysis of intact glycerol-type lipids (triacylglycerols, phospholipids and glycolipids) was investigated. The key advantage of MALDI-TOF MS over routinely used methods is its capability of analyzing intact lipid molecules. Also, in comparison to the other soft ionization sources (ESI and FAB), MALDI is more sensitive, less affected by impurities and offers rapid sample preparation. The resolution was sufficient to separate lipid species differing by one double bond. Ionization and prompt fragmentation patterns of lipids were studied. In the positive mode, triacylglycerols and glycolipids were detected only as singly sodiated molecules ([M+Na]+), while phospholipids were detected as multiple signals (protonated and/or sodiated molecules, all singly charged), determined by the type of polar-head groups. Only nitrogen-containing lipids were detected as protonated molecules. Phospholipids containing acidic hydroxyls (all except phosphatidylcholine) were detected as deprotonated ions in the negative mode.; A method to eliminate multiple-signal formation, in the positive mode, was developed. This method simplified the interpretation of MALDI spectra, and allowed the quantification of phospholipid molecular species in biological matrices. Post-source decay (PSD) coupled with MALDI-TOF MS was used for the structural analysis of the six most common phospholipid classes. In all classes, the PSD-MALDI spectra of the most heavily sodiated molecular ions were the most informative for the identification of the fatty acid. Preferential loss of the fatty acid group from carbon 1 of the glycerol backbone over that from carbon 2 allowed positional assignment of the fatty acid groups. The PSD fragmentation of molecules with the least number of sodium ions yielded fragments indicative to the loss of head groups. Lastly, MALDI-TOF MS was used to determine lipid profiles in fresh and naturally aged safflower and soybean seeds. In most cases, oxidized TAGs were significantly higher in the aged seeds compared to the fresh seeds. |
