Studies On Stereoselective Degradation Of Chiral Pesticide Metalaxyl

At present, about 40% of pesticides used in the agricultural production of China are chiral, and they are generally used as raceme. Although chiral pesticides have identical physicochemical properties, but there are significant differences between the enantiomers in biological activity, toxicity, environmental behavior, etc. Therefore, studies on enantioselective environmental behaviors of chiral pesticides are of great importance. Metalaxyl is an important phenylamide fungicide. There is a chiral carbon in metalaxyl, consisting of two stereoisomers, namely S-(+)-metalaxyl and R-(-)-metalaxyl. For purpose of providing more accurate information for the risk assessments, a series of researches on enantioselective degradation behaviors of metalaxyl enantiomers in nine kinds of matrices were carried out by reversed-phase HPLC in this paper. The main results were listed as followed:1. The enantiomers of metalaxyl were separated on Lux Cellulose-2 chiral column by reversed-phase HPLC. After the chromatographic conditions such as wavelength, mobile phase composition and flow rate were optimized, an excellent baseline separation of meatlaxyl enantiomers was obtained. The elution orders of the two enantiomers were determined. The first eluted enantiomer was S-(+)-metalaxyl and the second eluted was R-(-)-metalaxyl. Using the optimized instrument conditions, the residue analysis methods of metalaxyl enantiomers in plants, sludge and soil were developed. The samples were extracted by acetonitrile and cleaned up by Alumina-A SPE, The method showed excellent linearity with R2=0.9998. The mean recoveries of the enantiomers ranged from 82.6% to 104.0% at levels from 0.025-2.5 mg/kg with relative standard deviations (RSD) of 1.3-8.5% (n=5). LODs of the two enantiomers were in the range of 0.0031-0.0071 mg/kg (mg/L), with the corresponding LOQs of 0.010-0.013 mg/kg (mg/L). Accuracy, precision and sensitivity of this method were conformed to the requirements of residual analysis.2. According to the guideline on pesticide residue trials (NY/T 788-2004), the experiments of metalaxyl enantiomers in spinach, pakchoi, cucumber and cabbage were conducted under the field and greenhouse. Stereoselectivity degradation of metalaxyl enantiomers in the vegetables was investigated. Also, the stereoselective degredation of metalaxyl enantiomers under greenhouse and field condition was compared. The results showed that dissipation of metalaxyl enantiomers in several kinds of vegetables followed first-order kinetics, and there were different stereoselectivities in different vegetables. The R-(-)-metalaxyl (t1/2= 2.9 d) showed a faster degradation in cucumber of greenhouse than S-(+)-metalaxyl ((t1/2= 4.3 d). The degradation rate of the two enantiomers was identical in cabbage of field with the same half-life of 3.7 d, and no stereoselective degradation was found. Conversely, a preferential degradation of S-form was observed in spinach and pakchoi under greenhouse and field conditions. These results demonstrated that the environment conditions had no effect on the stereoselectivity of metalaxyl in spinach and pakchoi. However, the half-lives of two enantiomers in spinach of the field (1.7 d for S-form and 2.4 d for R-form) were less than that of the greenhouse (4.1 d for S-form and 4.9 d for R-form), which may be due to short hours of sunlight. On the countrary, the half-lives of the two enantiomers in pakchoi of the greenhouse (4.7 d for S-form and 5.5 d for R-form) were less than that in a field (5.3 d for S-form and 6.0 d for R-form), which demonstrated that greenhouse condition was more conducive to rapid degradation of metalaxyl in pakchoi.3. The field experiments were conducted to reseach the stereoselective behavior of metalaxyl stereoisomers in grape, tomato and rice. The results indicated that S-(+)-metalaxyl was preferentially degraded in grape and tomato, but the half-lives of the two enantiomers in tomato were shorter than that in grape. The half-life of S-form and R-form in tomato was 2.2 d and 3.0 d, respectively. The half-life was 5.5 d for S-form and 6.9 d for R-form in grape. However, a faster degradation of R-(-)-metalaxyl was observed in rice plant. Also, the half-life for S-enantiomer with 2.8 d was more than that for R-form with 2.3 d.4. In the lab, stereoselective degradation experiments of rac-metalaxyl in sludge and soil were carried out. The effects of the stereoselectivity on native and sterilization conditions were compared. The studies found that no stereoselective degradation in all experiments was discovered. The half-lifves of metalaxyl enantiomers under sterilization condition were three times as much as that under native condition for sludge, and 2.7 times for soil. This result showed that microbial degradation was the main cause of metalaxyl degradation in sludge and soil.