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Abstract: In this study, solid phase extraction (SPE) and ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) were utilized to develop a rapid, sensitive and reliable method for trace analysis of 21 antibiotics belonging to 7 classes in influent and effluent of municipal wastewater treatment plant. Detection parameters for SPE preconcentration and UPLC-MS/MS analysis were optimized, including sample pH, eluant, mobile phase (solvent and formic acid concentration), column temperature, and flow rate. Under the optimal conditions, the recoveries for different antibiotics ranged from 60.5% to 109.7% and all analytes were detected within 10.0min by UPLC-MS/MS. The validation study indicated that the method detection limits (MDLs) for the 21 antibiotics were from 0.3 to 60.0ngL−1 for effluent, while higher MDLs (0.5–84.0ngL−1) for influent. Accuracy and precision for both within-run (−11.6 to 14.3% and 0.2 to 15.6%) and between-run tests (−7.2 to 8.7% and 1.9 to 16.4%) were acceptable. The analysis of influent and effluent samples of two municipal wastewater treatment plants in Hong Kong revealed the presence of 11 antibiotics, including ampicillin, cefalexin, sulfamethoxazole, sulfadiazine, norfloxacin, ciprofloxacin, ofloxacin, tetracycline, roxithromycin, erythromycin-H2O, and trimethoprim. Their concentration ranged from 3.5 to 720.0ngL−1 in influent and from 2.1 to 556.4ngL−1 in effluent. [Copyright &y& Elsevier]