After establishing the relevant aspects of the mechanism of benzil-mediated photochemical epoxidation for lipid analysis, epoxidized authentic standards were activated via CID to identify structure-selective fragments. The results for FAs are discussed in Figure and results for GPLs are shown in Figure for PE 16:0/18:1(9Z) and Figure for other PEs and other lipid classes. First, we investigated the MS 2 of protonated compounds. Consistent with previous reports for other lipid classes, CID of epoxidized PE 16:0/18:1;9Ep results in pronounced head group loss ions at m/z 593.51 as well as additional fragments at around m/z 460 (Figure ). Two of these signals, i.e., m/z 451.38 and m/z 467.37, are consistent with epoxide fragmentation enabling DB position assignment. But also fragments stemming from FA loss are observed (Figure ) revealing the FA composition. Similar results are obtained in an MS 3 assay for negative ions (Figure ) as reported before. Next, we fragmented sodiated GPLs after epoxidation and results are shown in Figure is attributed to the loss of the sodiated epoxidized FA 18:1 moiety from the five-membered ring intermediate formed upon neutral head group loss (Figure ). Because this fragment can only be formed after formation of the five-membered ring intermediate (Figure ) and multiple groups have shown that this dominantly occurs with the involvement of sn-2 FAs of the precursor ion, this fragment is sn-isomer selective. This is also consistent with its absence in the MS 3 of sodiated PE 18:1;9Ep/16:0 (Figure ). Because the unsaturated FA in PE 18:1;9Ep/16:0 is located at sn-1, the corresponding MS 3 contains a fragment at m/z 361.27 indicating the loss of a sodiated FA 18:1;9Ep moiety from the sn-1 site (Figure ). Plausible fragmentation pathways consistent with MS 2 and MS results are given in Figure . Similar results are obtained for MS experiments of sodiated GPLs with improved signal-to-noise for PCs compared to MS 3 (Figure ). Taken together, these results demonstrate that an MS n assay of benzil-mediated epoxidation of sodiated GPLs can yield sn-position, DB position, head group, and FA composition.