Residues of metamifop and its metabolites in paddy rice

In this work, a method for the determination of the residues of metamifop and its three metabolites in samples of brown rice, paddy hull and rice straw was validated based on ultra performance liquid chromatography-tandem mass spectrometry. The three metabolites were N-(2-fluorophenyl)-2-hydroxy-N-methylpropionamide (HFMPA), N-(2-fluorophenyl)-2-(4-hydroxyphenoxy)-N-methylpropionamide (HPFMPA) and 6-chloro-3H-benzooxazol-2-one (6-CBO). The detection of 6-CBO was performed by tandem mass spectrometry with electrospray ionization in negative mode (ESI^-), and the other three analytes were determined in positive mode (ESI⁺). The samples of paddy rice were extracted with acetonitrile by an oscillator, and cleaned up by C₁₈ SPE cartridges. The method was validated and the matrix effects were evaluated. The method was applied to detect real final residue samples in the field trials. and the matrix effects were estimated. The average recoveries of metamifop and its metabolites in brown rice, paddy hull and rice straw at three spiked levels (0.01, 0.1 and 2.0 mg/kg) were in the range of 79%-108% with good precision (RSD<10.8%). Satisfactory linear relationships of metamifop, HFMPA, HPFMPA and 6-CBO were obtained in the range of 0.0005-0.02 mg/L with the correlation coefficient (r) over 0.9941. Limits of quantifications (LOQs) of metamifop, HFMPA, HPFMPA and 6-CBO in brown rice, paddy hull and rice straw were 0.01 mg/kg. To investigate the final residues of metamifop and its metabolites in paddy rice, the field trials were conducted in paddy rice in ten Provinces in China. The final residues of metamifop, HFMPA, HPFMPA and 6-CBO in brown rice and paddy hull were all below 0.01 mg/kg , which were lower than the maximum residue limit (MRL) of metamifop in brown rice (0.05 mg/kg) established in China. The residues of HFMPA were found in rice straw in the range of < 0.01 mg/kg-0.098 mg/kg, while the final residues of metamifop, HPFMPA and 6-CBO in rice straw were all below LOQ. It is recommended that 20% metamifop + bentazone micro-emulsion (ME, 3.3% metamifop) could be applied at the highest application dosage of 720 g a.i./hm² (metamifop 118.8 g a.i./hm² ) in rice and be applied only once.