Next, we used the developed workflow to investigate lipid isomer abundances in bovine liver and heart extracts by RPLC-MS 2 . After performing untargeted lipidomics of the unreacted extracts, we selected a target list of lipids of different classes (PE, PC, PS, LPE, LPC, DG, TG) at the sum composition level to investigate their lipid composition after photochemical benzil-epoxidation. Lipids for which full assignment of head group, FA composition, and DB positions in positive-ion mode as [M+H] + ions after photochemical epoxidation was possible are listed in Table (heart extract). Comparison between the identified lipid composition and their relative isomeric abundances for some of these lipids in liver and heart extracts is show in Figure and additional examples are provided in Figure .