The sn-isomer composition of GPLs for different cell lines can differ and polyunsaturated FA moieties not necessarily only populate sn-2 positions. This highlights the tight level of control cells exerts to build individual lipidomes as recently reported by Michael et al. for four different cell lines. To demonstrate the availability of sn-isomer information from photo-epoxidized unsaturated GPLs, we used the developed methodology to chart the sn-isomer abundance of PCs in shotgun MS experiments for HeLa and H9c2 cells (Figure ). For example, Figure and 5B shows the MS 4 spectra of sodiated PC 36:1;Ep with highlighted sn-isomer diagnostic ions. Whereas the tandem mass spectrometric result for H9c2 cells, rat cardiomyocyte cells, signals for the sn-isomer pair PC 18:0/18:1 and PC 18:1/18:0 as well as PC 16:0/20:1, fragmentation of epoxidized and sodiated PC 36:1 in the immortalized cervical cancer HeLa cell line yields the same fragments as H9c2 but additionally reveals the presence of PC 20:1/18:0. In addition to the four sn-isomers for PC 36:1 in Figure photochemical epoxidation via benzil allowed to reveal 56 sn-isomers present in at least two of the three biological replicates and 34 sn-compositions that were present in all three replicates in at least one of the two cell lines. The relative sn-isomer intensity (intensity of one sn-isomer divided by the sum of all signals of the sn-isomer pair) is shown in Figure . In agreement with the recent study by Michael et al., PCs with a small difference between the lengths or DB number of the FA moieties in sn-1 and sn-2 exhibit subtle differences in sn-isomer intensities. In both cell lines for example, fragments for the sn-pairs PC 16:0_18:2 and PC 16:0_16:1 have close to the same intensity ratios. This contrasts with the pairs for PC 16:0_20:4 and PC 18:0_20:4 for which the sn-2 location of the PUFA is in large excess to the other isomer in agreement with reported selectivity of LPCATs to preferably attach PUFAs to sn-2. But also differences between the cell lines and occupation preferences of snsites exist. HeLa cells, for example, have higher PUFA intensities in sn-1 position compared to H9c2 cells. One example is the sn-pair PC 18:1_20:3 for which HeLa cells have about the same relative intensity for the two isomers but the isomer PC 18:1/20:3 is preferred for H9c2 cells (Figure ).