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Rapid and sensitive analytical methodology based on ultra high-performance liquid chromatography-tandem mass spectrometry has been developed for the determination of widely consumed drugs of abuse (amphetamines, MDMA, cocaine, opioids, cannabis and ketamine) and their major metabolites in urban wastewaters. Sample clean-up and pre-concentration was performed by a generic off-line SPE procedure using Oasis HLB. Special effort was made to incorporate amphetamine, which was found highly problematic in the wastewater samples tested, including an additional clean-up with Oasis MCX SPE and dispersive primary secondary amine. Correction for possible SPE losses or degradation during storage was made by the use of isotope-labelled internal standards (ILIS), available for all compounds, which were added to the samples as surrogates. Although ILIS were also efficient for matrix effects correction, the strong ionization suppression observed was not eliminated; therefore, a four-fold dilution prior to SPE was applied to influent wastewaters and a low injection volume was selected (3 mu L), in order to reach a compromise between matrix effects, chromatographic performance and sensitivity. The method was validated at 25 and 200 ng L-1 (effluent), and 100 and 800 ng L-1 (influent), obtaining limits of quantification (i.e. the lowest level that the compound can be quantified and also confirmed with at least two MS/MS transitions) between 0.4-25 ng L-1 (effluent) and 2-100 ng L-1 (influent). The applicability of the method was demonstrated by analysis of 14 influent and 14 effluent wastewater samples collected over 2 weeks in Castelln (Spain) within a European collaborative study.