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Recently it was demonstrated that the analysis of drugs of abuse (DOA) in wastewater can be used to track their use in communities. This paper presents the optimization, validation, and application of an analytical procedure based on solid-phase extraction (SPE) and liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) for the determination of some emerging drugs in influent wastewater. The compounds of interest were the cathinone derivatives methylenedioxypyrovalerone (MDPV) and 4-methylmethcathinone (mephedrone (MEPH)), ketamine (KET) and its metabolites norketamine (NK) and dehydronorketamine (DHNK), as well as the major metabolite of cannabis, 11-nor-9-carboxy-delta-9-tetrahydrocannabinol (THC-COOH). Except for DHNK, deuterated analogues were used as internal standards for quantification. The use of a short C-18 column (50mm x 2 mm, 3 mu m) allowed a good separation for the compounds analyzed in positive ionization mode (KET, NK, DHNK, MDPV, and MEPH) and for THC-COOH, which was analyzed in negative ionization mode, with a total run time of 13 min. Sample preparation using SPE was optimized by comparing Oasis HLB and Oasis MCX sorbents. The method was validated for each compound by assessing the following parameters: linearity, accuracy, precision, recovery, and relative process efficiency. The lowest calibration level was considered as the lower limit of quantification (LLOQ) and was 5 ng/l for KET, NK, DHNK, MDPV, and MEPH and 20 ng/l for THC-COOH. KET, and THC-COOH could be quantified in the majority of wastewater samples from three large wastewater treatment plants in Belgium. The other compounds were below the LLOQ or could not be detected. Copyright (C) 2013 John Wiley & Sons, Ltd.