Consequently, we varied the reaction time, flow rate, solvent composition, and benzil concentration to optimize the epoxidation yield and minimize byproducts (Figure -S5 and Table ). For authentic standards, the apparent reaction yield after parameter optimization based on intensities is 60-80 % (Figure ) in line with reports with other epoxidation reactions. To validate that the reaction products are primarily epoxides, LC-MS 2 of FA standards after lightinduced benzil reaction were compared to FA epoxide standards as shown in Figure . These results confirm that the major reaction product of the described reaction are epoxides. Intriguingly, the experiments with FAs (Figure ) reveal that E/Z isomerization leads to the formation of cis and trans epoxides that are separated by LC-MS (Figure ). These findings are in line with the proposed reaction mechanism of this epoxidation reaction and in analogy to recent reports of E/Z identification of lipids in PB workflows. Figure : Benzil-induced oxidation of unsaturated lipids via 254 nm light. A) Mass spectrum of reaction products after oxidation of PC 16:0/18:1(9Z) and B) MS 2 of the base peak at m/z 776.58 with DB-selective ions highlighted. Mass spectra of reaction products after oxidation of PC 16:0/20:4(5Z,8Z,11Z,14Z) were obtained following 4 h of purging the reaction solution with nitrogen C) and synthetic air D). Some signals are labelled with corresponding m/z values and highlighted in red.