Lipids are the molecules in charge of physiological functions such as membrane structure, energy storage or bioactive lipids themselves. Molecular weight of lipid is at most 2000, which is not so larger than that of DNA or proteins. Over 40,000 lipid species are registered in LIPID MAPS (http://www.lipidmaps.org/). It means that dozens of isomeric and isobaric lipid species are condensed in 1 mass unit. Lipidomics is a technology to analyze comprehensive lipid molecules quantitatively using mass spectrometer. Accurate MS such as time of flight or orbital trap type MS coupled with database search is major platform in lipidomics. Recent study using accurate MS based comprehensive lipidomics shown that over 1000 lipids were identified in biological sample. Widely targeted lipidomics using selected reaction monitoring (SRM) with triple quadrupole MS (TQ-MS) is the other platform. Ultra-fast TQ-MS enables to set 500 SRM transition per second including polarity switching. Here we present that development of SRM based lipidomics methods for phospholipids using TQ-MS coupled with reversed phase chromatography. Over 200 phospholipids were identified using the SRM based method from the lipid extracts of mouse tissues. Also we would like to discuss how to identify lipid isomers and isobaric species in the SRM based lipidomics focused on diacylphospholipids analysis.
Isomer analysis such as epimer identification of saccharides is one of the important subjects in mass spectrometry. Since isomers have the same mass, they cannot be distinguished by simple mass spectrometry only. Generally, the collision induced decomposition method is used for isomer analysis. A saccharide and a cation form a complex ion within selectivity. Information on the saccharide epimers is obtained from observation of the complexation behaviors with the cation in mass spectrometry. In this commentary, a method of analyzing isomers of saccharide derivatives is described, based on the complexation of saccharide derivatives with cations.
We analyzed isomeric flavonoid mono-O-glycosides by MALDI-MS. Laser irradiation decomposed the O–H bonds of the phenolic groups and glycosyl bonds on the flavonoid backbone through homolytic cleavage. Hydrogen radical removal depended on the structure of the isomeric flavonoid mono-O-glycosides, therefore, we distinguished their isomers from their MALDI-TOF MS and tandem MS/MS spectra.