Studies On The Enantioselective Degradation Of Isofenphos-methyl In Vegetables And Soils

As is well-known,the enantiomers of chiral pesticides possess similar physical and chemical properties but often display distinct toxicological properties,biological and metabolic activities,and degradation behavior in the chiral environment;however,this difference is usually ignored when evaluating their environmental and public health risks in the traditional studies.Thus,the traditional risk assessment will be not accurate if the enantioselective behaviors occur.At present,the information about the enantioselective degradation of isofenphos-methyl in vegetables and soils is insufficient.Thus,we finish the studies on the enantioselective degradation of isofenphos-methyl in vegetables and soils.The main results were as follows:First of all,a series of novel and sensitive single methods for enantiomeric analysis of isofenphos-methyl in cowpea,cauline leaf of sweet potato,cucumber,pepper and soil samples using chiral liquid chromatography coupled with tandem mass spectrometry combined with reversed chiral columns.The intra-day(n=5)recoveries of the isofenphos-methyl enantiomers in the four vegetables and soil at the three spiked levels(5,100 and 2000μg/kg)ranged from 87.1%to 99.6%,with RSD values below 5.4%for the two enantiomers in four vegetables and soil.The inter-day(n=15)recoveries of the isofenphos-methyl enantiomers in the four vegetables and soil at the three spiked levels ranged from 80.7%to 98.5%,with RSD values below 6.4%for the two enantiomers in four vegetables and soil.Standard curves were established for(S)-(+)-and(R)-(-)-isofenphos-methyl concentrations in the range of 0.1-500μg/kg,and a good linearity was obtained(R~2≥0.9919 in all cases).The LOD values(S/N=3)for the two enantiomers of isofenphos-methyl were estimated to be 0.02-0.07μg/kg.The LOQ values of the enantiomers were 0.5μg/kg.The results indicated that the developed methods were effective enough for detecting the residual enantiomers in the text matrixs.Secondly,based on the established chiral analytical methods,enantioselective degradation of isofenphos-methyl in cowpea,cauline leaf of sweet potato,cucumber,pepper and soil under field conditions were investigated.The degradation of isofenphos-methyl in fourvegetables and field soil fit the first-order kinetics equation.The enantioselective behavior was different in the four vegetables and soil.The degradation of(R)-(-)-isofenphos-methyl was significantly faster than that of(S)-(+)-isofenphos-methyl in cowpea,cauline leaf of sweet potato,cucumber and field soil.Butthedegradationof(R)-(-)-isofenphos-methylwasslowerthanthatof(S)-(+)-isofenphos-methyl in pepper,and the different enantioselectivity was observed in cowpea,cauline leaf of sweet potato,cucumber and field soil.Lastly,the enantioselective degradation of isofenphos-methyl in four field soils under aerobic and dark conditions were investigated.The results showed the degradation of isofenphos-methyl of four soils fit the first-order kinetics equation.The degradation of isofenphos-methyl was slowly in four field soil.The experimental results demonstrate that enantioselective degradation of isofenphos-methyl in the soil has direct relationship with the pH of the soil.There was not substantial enantioselectivity in the degradation of isofenphos-methyl in the weakly alkaline soil(Changchun),acid soil(Nanchang)and weak acid soil.However the occurrence of enantioselectivity with(R)-(-)-isofenphos-methyl preferentially degraded in basic soil(Zhengzhou).The result is same as soil under field conditions.