Rapid characterization of lipids by MALDI MS. Part 2: Artifacts, ion suppression, and TLC MALDI imaging

Abstract

AbstractSeveral new methods have been developed recently that allow the direct detection of lipids without resorting to derivatization or chromatographic separation. The simplest of these is direct MALDI (matrix‐assisted laser desorption/ionization) mass spectrometry. This approach is most useful for mixtures that contain minimal amounts of ion‐suppressing interfering components. However, when such components are present, their effects can often be minimized by using simple separation techniques beforehand, such as solid phase extraction or thin layer chromatography. For example, direct MALDI has been used for rapid screening of lipids and taxonomic identification of the source organisms with no sample pretreatment. Collecting fractions from solid phase extraction cartridges have also been used to avoid the most extreme effects of ion suppression from more complex lipid mixtures. More recently, direct MALDI has been applied to the analysis of TLC plates allowing the detection of TLC‐separated lipids from the complex lipidome. Herein, we briefly describe the application of rapid MALDI MS to some typical research problems involving the characterization of lipids. In Part 1 [1] we covered bacterial taxonomy by direct analysis of intact lipids and the analysis of food oil triacylglycerols . Part 2 will address ion suppression, spontaneous fragmentation, and coupling MALDI with chromatography. The spontaneous fragmentation of protonated lipids in oils by direct MALDI produces artifactual diacylglycerol‐like ions. An understanding of this process and its minimization facilitates monitoring the decomposition of lipids by direct analysis. Suppression also has an impact on direct analysis of lipids, especially when mixtures contain both polar and non‐polar lipids. We demonstrate the use of solid phase extraction and thin layer chromatography to produce fractions or substrates from complex biological samples in which lipids can be detected by direct and rapid MALDI MS analysis.