Structural Annotation Method for Locating sn- and C═C Positions of Lipids Using Liquid Chromatography–Electron Impact Excitation of Ions from Organics (EIEIO)–Mass Spectrometry

Definitive structural elucidation of lipids is pivotal for unraveling the functions of lipids in biological systems. Despite advancements in mass spectrometry (MS) for lipid analysis, challenges in annotation scope and efficiency remain, especially in resolving isomers. Herein, we introduce an optimized method using liquid chromatography coupled with electron impact excitation of ions from organic tandem mass spectrometry (LC-EIEIO-MS/MS) for comprehensive analysis and structural annotation of lipids. This approach integrates a six-step analytical protocol for precise lipid annotation, including (1) extracting MS information, (2) classifying lipids, (3) aligning sum composition, (4) determining sn-positions, (5) locating C═C positions, and (6) ascertaining annotation levels. In analyzing 34 lipid standards spiked into serum, our method achieved 100% and 82.4% annotation accuracy at the sn- and C═C isomer levels, respectively, compared to 26.5% and 0% in the CID mode using MS-DIAL. A total of 1312 sn-positions and 1033 C═C locations of lipids were annotated in quality control plasma pooled from healthy individuals and patients with Alzheimer's disease. The isomers of lipids revealed more pronounced differences between the healthy and diseased groups compared to the sum compositions of the lipids. Overall, the LC-EIEIO-MS/MS approach provides a comprehensive profiling and efficient annotation method for lipidomics, promising to shed new light on lipid-related biological pathways and disease mechanisms.