Simultaneous determination of pyrazosulfuron-ethyl and mefenacet residues in rice by high performance liquid chromatography-tandem mass spectrometry

A high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method was established for simultaneous determination of pyrazosulfuron-ethyl and mefenacet residues in rice. The samples were extracted with 20 mL V(acetonitrile):V(water)=70:30, then cleaned-up with 20 mg graphite carbon black (GCB) and 30 mg primary secondary amine (PSA). The average recoveries of pyrazosulfuron-ethyl spiked in paddy soil, paddy water, rice plant, rice straw, brown rice and rice husk ranged from 76% to 107% with the relative standard deviations (RSDs) of 1.5%-14%, and of mefenacet ranged from 77% to 101% with the RSDs of 2.4%-13%. The limit of quantifications(LOQs) were between 0.004 mg/kg and 0.01 mg/kg for pyrazosulfuron-ethyl and between 0.001 mg/kg and 0.01 mg/kg for mefenacet. The method was used for analyzing samples collected from the field trial of thepyrazosulfuron-ethyl and mefenacet mixture. And the results showed that the two herbicides were easily degradable and the terminal residues were below the maximum residue limit (MRL) established in China (0.1 mg/kg for pyrazosulfuron-ethyl; 0.05 mg/kg for mefenacet).