False positives refer to those tentatively identified TPs that in fact are not related to any parent pharmaceutical. This type of false identifications can only be proved by a reference standard of the proposed compound or an explicit and elaborate structural elucidation. This can be illustrated by the only false positive identification observed in this study, the diclofenac TP candidate with an exact mass of 188.951. Its mass spectrum indicates the typi-cal isotopic pattern of two chlorine atoms and a neutral CO2-loss.Considering that diclofenac is the only parent compound in this study containing two chlorine atoms, we hypothesized it to be 2,6-dichlorobenzoic acid, which is likely formed from the amide bondcleavage of diclofenac. However, the analysis of reference standards confirmed that the TP candidate was 2,4-dichlorobenzoic acid,which structurally cannot be linked to diclofenac. Further investigation revealed that this compound had an increasing trend not only in all incubations used for this study (including sterile control and water-only control experiments), but also in single-compound incubation where only ibuprofen or naproxen was spiked (not shown). Our hypothesis was consequently rejected. To further elucidate the formation mechanism of 2,4-dichlorobenzoic acid was beyond the scope of this study.