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Co-administration of opioids with benzodiazepines is very common around the world. A semi-automated method was developed for the determination of four opioids and two benzodiazepines as well as their metabolites (including glucuronide metabolites) in human urine, based on on-line column-switchingsolid-phase extraction (CS-SPE) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The CS-SPE was performed by loading 200 mu L of urine sample to an Oasis HLB cartridge. Detection was achieved using a LC-MS/MS system equipped with an electrospray ionization source (ESI). For unequivocal identification and confirmation, two selected reaction monitoring transitions were registered for each compound, and no co-elution of interferences was observed at the expected retention time. Significant ion suppressions were observed for most analytes during chromatographic runs, but isotope-labeled internal standards (ISs) were used and found to be useful to compensate for the determination error caused by the matrix effect. The assay’s linearity ranged from 1-20 ng/mL to 800-1000 ng/mL for 23 compounds, except for lorazepam (LOR), whose linearity was in the range of 1-100 ng/mL. This method showed to be precise and accurate. The relative standard deviation (RSD) % values of within-run precision, between-run precision and total precision were not greater than 10.4% (n=3), 12.9% (n = 5) and 15.1% (n = 15), respectively. Accuracy values were in the range of 87.5-110%. Limits of detection (LODs) ranged from 0.2 ng/mL to 5 ng/mL, and limits of quantification (LOQs) ranged from 1 ng/mL to 20 ng/mL. The method was applied to the assay of 12 samples from forensic cases, which exemplified the co-administration of benzodiazepines (BZDs) by some heroin abusers. This method was of high sensitivity, selectivity and reliability, minimum sample manipulation, semi-automation, and fairly high throughput (analysis time per sample was 20 min). The method developed will be useful for the detection of co-administrated drugs and the study of the interactions of BDZs with opioids. (C) 2015 Elsevier B.V. All rights reserved.