Photochemical epoxidation of unsaturated lipids by benzil and dissolved oxygen in a flow reactor revealed new possibilities for lipid structure analysis in RPLC-MS n and shotgun MS n experiments. By epoxidizing unsaturated FAs, glycerolipids (GLs), and GPLs, DB positions are pinpointed in positive-or negative ions mode across these experimental platforms. This increases the number of lipid classes for which DB positions are accessible in RPLC-MS n of epoxides. An additional benefit of RPLC of epoxidized lipid products compared to non-reacted counterparts is the improved separation capabilities for DB position and sn-isomers (Figure ). The herein newly described photochemical reaction workflow for lipidomics leads to the isomerization of DB bonds in non-reacted and reacted unsaturated lipids. As DB E/Z isomers are readily resolved in RPLC of lipids, the benzil-mediated photochemical epoxidation allows annotation of DB geometries. Furthermore, addition of sodium ions to epoxidized unsaturated lipids yielded sn-specific fragments in MS n experiments. This opens the possibility to study snisomerism as showcased for two lipid cell extracts. Therefore, we describe a single derivatization strategy that facilities assignments of all major aspects of GL and GPL structures, i.e., head group, FA composition, DB position, DB geometry, and sn-isomerism, in eukaryotes if appropriate sample preparation or separation methods are employed.