The Determination Of Thifluzamide Residue And Its Degradation Dynamics In Rice Field

Thifluzamide belongs to the broad-spectrum fungicides which can inhibit succinic dehydrogenase synthesis. It is often used for preventing and controlling all kinds of plant diseases in some field crops, lawns, meadows and so on. Thifluzamide has an obvious effect on Rhizoctonia solani particularly. Therefore, the residue analysis method of thifluzamide in rice fields was developed, dissipation rules and its final residues were also be explored. We also evaluate the safety of thifluzamide 240 g L-1 SC and provide experimental data for the Chinese government to establish the maximum residue limit (MRL) of thifluzamide in rice and the its registration.Firstly, a method for determination of thifluzamide residues in paddy soil/water, rice straw and plant, rice hull/brown rice was developed. Water in rice was separated, purified and pre-concentrated by C18 soild phase extraction (SPE); Soil, straw, rice, rice hull and plant were extracted with acetone-petroleum ether mixture (1:1, V/V) and petroleum ether, purified with a silica gel column. The all samples were analyzed by gas chromatography with electronic capture detector (GC-ECD). The correlation coefficient (r)of this method was 0.9991, and the calibration curve was linear in the range of 0.005～10 mg L-1. The detection limit in water was 0.005 mg L-1 and the detection limits in paddy soil, rice straw, plant, rice hull and brown rice were all 0.004 mg kg-1. When the spiked levels of 0.05,0.5 and 1.0 mg L-1 in water and the spiked levels of 0.04,0.4 and 4.0 mg kg-1 in paddy soil, rice straw, green plant, rice hull and brown rice, the spiked recoveries of thifluzamide in all samples ranged from 86.6% to 107.2%, with relative standard deviations (RSDs) from 0.7% to 9.9%.Secondly, the degradation dynamic and final residue of thifluzamide were studied in field experiments. The half-life of thifluzamide was determined via a dissipation experiment, in which the 240 g L-1 aqueous suspension concentration of thifluzamide was applied at high dosages (108 g a.i.ha-1). The half-lives of thifluzamide in paddy water, paddy soil, and rice plant were 28.64,18.68,16.01 days in 2011 and 23.73,17.15,13.20 days in 2012 (Nanjing, Jiangsu),13.97,21.72,9.43 days in 2011 and 17.33,19.69,8.76 days in 2012 (Huaibei, Anhui),8.47,10.30,11.87 days in 2011 and 10.41,17.54,8.29 days in 2012 (Changsha, Hunan), respectively. The result showed that thifluzamide could be degraded easily in rice of Nanjing, Huaibei and Changsha. When thifluzamide 240 g L-1 SC solution was applied at a low dosage of 72 g a.i.ha-1 and a high dosage of 108 g a.i.ha-1 for three and four times, respectively. the final residue in brown rice were ND～0.0303 mg kg-1 (Nanjing), ND (Huaibei and Changsha) during intervaltime of 20,30,40 days.At present, no maximum residue level (MRL) of thifluzamide in rice has been set by Chinese government authorities. According to our results, thifluzamide 240 g L-1 SC could be used in rice field safely for less than three times with the recommended dosage (72 g a.i.ha-1) at a safe pre-harvest interval (PHI) of 30 days before harvesting of rice.