The absence of similar results in experiments with PC 32:0 suggests that DBs are the primary reactive sites (Figure ). Additionally, the MS 2 -CID spectrum of [PC 16:0/18:1(9Z)+O+H] + resulted in the formation of DB position specific fragments (Figure ), consistent with literature reports for epoxidized PCs. Because we did not observe addition of benzil to unsaturated lipids or any lipidbenzil-associated fragments and no other oxygen source was added to the reaction solution, we hypothesized that dissolved molecular oxygen could be involved in the reaction. To confirm the photoactivation of molecular oxygen by benzil, we purged the reaction solution with N2 for 4 h prior to initiating the photochemical reaction of PC 16:0:/20:4(5Z,8Z,11Z,14Z). This resulted in the reduction of the +15.99 Da and 31.98 Da signals by a factor of ~20 after UV irradiation in the flow reactor (Figure ) compared to purging the reaction solution with air (Figure ). These results are consistent with the involvement of dissolved O2 in the reaction. Our findings are in line with prior reports in the organic chemical literature that showed the photoactivation of O2 by benzil. These results indicate that the use of benzil in a water/ACN solution can effectively epoxidize unsaturated lipids.