13.95, 14.31, and 14.85 min, respectively) were observed in the positive mode extracted ion chromatogram of m/z 253.0972 within 5 ppm error (Figure ). Looking at the structure of phenytoin, the absence of chiral carbons renders the possibility of diastereomers, which could explain the presence of multiple peaks, invalid. Analysis of the phenytoin standard showed that this compound elutes at 15.53 min, thus not matching any of the three peaks being investigated. Further inspection using MetFrag and database matching suggested that the second and third peaks belong to the positional isomers 2hydroxycarbamazepine and 3-hydroxycarbamazepine, metabolites of the anticonvulsant carbamazepine. Retention time matching using a standard confirmed that the peak at 14.85 min is indeed 3-hydroxycarbamazepine while the peak at 14.31 min can be assigned as 2-hydroxycarbamzepine (Level 3), despite the lack of standards, due to the similarity of its mass spectrum with 3-hydroxycarbamazepine. However, the first and biggest peak proved to be challenging. Inspection of the MS1 spectrum at 13.95 min shows another peak with m/z 271.1075 (mass difference equivalent to the loss of water, Figure ) can be found whose MS2 spectrum is very similar to the 253.0972 peaks at One case that needs further inspection are the stereoisomers vidarabine and adenosine, which are impossible to separate using the chromatographic method employed in this study. While there are reports on the utility of ion mobility to discriminate between stereoisomers, it is still to be tested whether such resolution is practically achievable. Published collisional cross sections of vidarabine (156.4 Å² for [M+H] + ) and adenosine (156.9 Å² for [M+H] + ) measured on the same instrument are available, revealing a difference of only 0.5 Å² or 0.3%, which is too close to distinguish currently within the typical resolving power of ion mobility spectrometers. M-H2O (18.0104) The continuous analysis of surface water using HRMS as part of the routine monitoring efforts will enable retrospective screening for newly identified contaminants that may impact local surface water quality and biota, such as the effect observed by city runoff on coho salmon. This study reports primarily Level 1 and 2a identifications due to the hard filter of MoNA score >0.9 applied during the MetFrag analysis. Other tentative identifications have been communicated with AGE and these, along with more detailed trend analysis as more temporal data points are collected, can be investigated in future works as resources allow. Quantification efforts could be further improved by using the list of pharmaceuticals identified in this work as a target list, as well as investing in isotopically labeled standards (which was beyond the scope of the current works, as target analysis is performed by AGE). Finally, as experimental databases increase in size and coverage, the ability to screen for more compounds with higher confidence with these open source methods such as the one presented here will also increase, highlighting the need for the community at large to continue to contribute to publicly available databases.