To investigate photochemical lipid derivatization, a simple flow reactor was designed illustrated in Figure . Multiple potentially photoreactive aldehydes and ketones were tested with the aim to derivatize unsaturated lipids in a 254 nm-triggered PB reaction. Whereas most compounds reported in the literature, for instance 3-acetyl pyridine, yielded the desired oxetanes, the mass spectrum of PC 16:0/18:1(9Z) spiked with benzil (Figure ) did not contain corresponding PB products. Instead, a pronounced mass spectrometric signal shifted +15.99 Da from the precursor m/z at 776.58 was present (Figure ). An additional, signal was also observed at +31.98 Da compared to the protonated precursor m/z but in lower intensity. These results are consistent with the epoxidation, ketone formation, or hydroxylation of PC 16:0/18:1(9Z) that is initiated by 254 nm light and requires benzil.