Structural analysis of negative ions by postsource decay in time-of-flight secondary ion mass spectrometry

Abstract

Phospholipids (PLs) are membrane lipids of living cells whose considerable role in biological membranes include protein sorting and regulation of biophysical properties and signaling pathways. PLs are classified by their head groups into phosphatidic acid, phosphatidylcholine (PC), phosphatidylglycerol, phosphatidylinositol (PI), phosphatidylserine (PS), and cardiolipin. Since PLs have varying ionization efficiencies, depending on their electron affinity, they can be detected at positive or negative ion modes so that PC and PS are generally detected as positive ions, and phosphatidylethanolamine and PI as negative ions. As a result, metabolite analyses in time-of-flight secondary ion mass spectrometry (ToF-SIMS) should be carried out by performing tandem mass spectrometry measurements at both ion modes to identify unknown PLs. For tandem mass spectrometry measurements in ToF-SIMS, a postsource decay (PSD)-like method was successfully applied to identify several lipids by using cholesterol as a model molecule at the positive ion mode. In our study, the authors adapted 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphor-rac-(1-glycerol) ammonium salt with well-known fragmentation pathways as a model molecule at the negative ion mode to identify PI lipids. By using the PSD-like method at both ion modes, the authors successfully identified PC and PI from MDA-MB-231 breast cancer cell lysates to show that our PSD-like method would be useful in the process of identifying unknown lipids from biological samples in ToF-SIMS.