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A novel method was developed for the simultaneous determination of mesotrione and its metabolite (4-methylsulfonyl-2-nitrobenzoic acid, MNBA) in maize by ultra performance liquid chromatography coupled with tandem mass spectrometry(UPLC-MS/MS). The samples were extracted with acetonitrile-water (50 : 50, V/V) solutions and an aliquot of the extract was acidified to pH 2 with formic acid and then cleaned. up by Oasis HLB cartridge. The residues were eluted from cartridge with methanol- diethyl ether (70 : 30, V/V). The elutions were evaporated to appropriately dry under a stream of nitrogen, dissolved in 2 mL acetonitrile. 1 mmol/L ammonium acetate (75 : 25, V/V). The residues were analyzed by mass spectrometry with negative ion mode using multiple. reaction monitoring (MRM) and quantification was achieved by matrix calibration. The method had a good linear correlation (correlation coefficient was above 0. 99) in the range of 1-200 mu g/L. The average recoveries of mesotrione and MNBA were 83. 8%-101. 1% at spiked level between 5 mu g/kg and 50 mu g/kg with the relative standard deviation (RSD) between 5. 3% and 12. 8%. The limits of quantification for mesotrione and MNBA were 1. 0 and 2. 0 mu g/kg, respectively.