Revolutions in Lipid Isomer Resolution: Application of Ultrahigh-Resolution Ion Mobility to Reveal Lipid Diversity

Many families of lipid isomers remain unresolved by contemporary liquid chromatography-mass spectrometry approaches, leading to a significant underestimation of the structural diversity within the lipidome. While ion mobility coupled to mass spectrometry has provided an additional dimension of lipid isomer resolution, some isomers require a resolving power beyond the capabilities of conventional platforms. Here, we present the application of high-resolution traveling-wave ion mobility for the separation of lipid isomers that differ in (i) the location of a single carbon-carbon double bond, (ii) the stereochemistry of the double bond (cis or trans), or, for glycerolipids, (iii) the relative substitution of acyl chains on the glycerol backbone (sn-position). Collisional activation following mobility separation allowed identification of the carbon-carbon double-bond position and sn-position, enabling confident interpretation of variations in mobility peak abundance. To demonstrate the applicability of this method, double-bond and sn-position isomers of an abundant phosphatidylcholine composition were resolved in extracts from a prostate cancer cell line and identified by comparison to pure isomer reference standards, revealing the presence of up to six isomers. These findings suggest that ultrahigh-resolution ion mobility has broad potential for isomer-resolved lipidomics and is attractive to consider for future integration with other modes of ion activation, thereby bringing together advanced orthogonal separations and structure elucidation to provide a more complete picture of the lipidome.