For several CECs, SPE recoveries could not be calculated at 10 ng L -1 spiked river water samples (e.g., amphetamine, 1,7 dimethylxanthine, and rifamycin); and several CECs reported poor recoveries at 10 ng L -1 but good at the higher concentrations of 100 and 500 ng L -1 (table ). Several CECs demonstrated consistently poor recoveries from river water across all three spiked concentrations, these included 4-hydroxy trimethoprim (11%) and ceftiofur (22%). Oasis HLB cartridges are often used in multi-residue analytical methods due to the ability to extract a broad and diverse range of CECs. However, it is recognized that not all CECs will be recovered or perform well, and optimization for specific CECs could be better achieved. The method performance for studied CECs was also investigated (table 2). In general, both methods (for Group A and B compounds) achieved good sensitivity for the CECs in river water matrices (MDLs <0.1 ng L -1 ) and in line with previous results . For CECs from group A, the lowest MDLs were observed for benzoylecgonine at 0.0533 ng L -1 (MQL = 0.1777 ng L -1 ) and cocaine at 0.0649 ng L -1 (MQL=0.2164 ng L -1 ). The higher MDL for this group was observed for the antihistamine cimetidine, with MDLs reported at 10 ng L -1 (MQL=34 ng L -1 ).