Photochemical epoxidation. For the reaction of standards, a volume of 500 µL working solution was injected in flow reactor (Figure ), and the outlet was either connected to a HESI ion source for reaction monitoring or collected in a vial for further processing after observing epoxidized lipids in the mass spectrometer. The injection pump flow rate, controlled by the instrument Tune software, was set to a 40 µL/min. For extracts, the same reaction protocol was applied but the reacted solution was collected, dried, and stored at -80 °C. Before LC-MS or shotgun MS measurements of extracts the reaction solutions was reconstituted in isopropanol:methanol(1:1,v/v) without or after addition of 200 µM NaOAc, respectively. The flow reactor was cleaned with, 500 µL dichloromethane, 500 µL acetonitrile, and 500 µL water after every reaction.