This article is available online at http://www.jlr.org glycerophospholipids (GPs). Numerous accounts have examined the role of GPs in cellular biochemistries including membrane permeability, protein aggregation, and receptor activation ( 1-6 ). MS is a powerful tool for GP structure elucidation and is commonly used in contemporary lipidomics studies in complex biological extracts. MS/MS that uses collision-induced dissociation (CID) is central to most protocols in modern lipidomics and can identify headgroup class, acyl chain length, and degree of acyl chain unsaturation ( 7-11 ). However, there are numerous important structural features of GPs that are not easily discerned by CID, including identifi cation of carbon-carbon double bond position(s), the stereochemistry of carboncarbon double bonds, and the position of substitution of each acyl chain on the glycerol backbone (i.e., sn -position) ( 9, 12, 13 ). The inability to discriminate between snpositional isomers, or even to unequivocally exclude the presence of both isomers, is an impediment to our understanding of the roles of these distinct molecular structures in biological systems.Recent reports point to specifi c arrangements of acyl chains in GPs being responsible for structural interactions that induce specifi c activity. This has been noted particularly in the interactions of GPs toward nuclear receptor proteins. For example, Liu et al. ( 14 )