For preparing working solutions for epoxidation, lipid standards (FAs, PCs, PE, PG, PS, PI, PA) were diluted in ACN to a final concentration of 0.2 mg/mL. Benzil was dissolved in ACN:water (1:1, v/v) and sonicated for 6 min to achieve a final concentration of 0.2 mg/mL. The working solution was prepared by adding 50 µL benzil solution and 425 µL acetonitrile:water (1:1, v/v) to 25 µL of solution containing lipid standards. For liver/heart and cell extracts, 50 µL benzil was added into 15 µL liver/heart extract (25 mg/mL) stock solution and dried cell extract sample respectively, and add ACN:water (1:1, v/v) to get to the final volume at 500 µL. The working solution was vortexed for 20 s and stored at 4 °C until further use.