Residue Analysis And Degradation Rule Of Sclerotium In Tomato Planting System

A liquid chromatography-tandem mass spectrometry（LC-MS/MS）method was established to analyze dimethachlon in whole tomato fruits,plants and soil matrix,and the analytical method was verified.In 2020,field experiments of 40%dimethachlon WP on tomato were carried out in Guizhou and Guangdong.In order to providing data reference for the fair use of dimethachlon on tomato,the dissipation dynamics of dimethachlon on tomato fruits,plants and soil were tested and analyzed.At the same time,the HPLC-HRMS instrument and CD software are used to identify and analyze the metabolite produced by the degradation of dimethachlon in the matrix sample.After identifing the structure of the metabolites,and the metabolite in the tomato planting system was detected and analyzed.The research results are as follows:1.A LC-MS/MS method was established for the determination of dimethachlon in field samples.In this paper,acetonitrile was used as an extractant for tomato and plant samples（1%acetic acid-acetonitrile was used as an extractant for soil samples）,and the extract was absorbed by anhydrous magnesium sulfate（Mg SO₄）,salted out by sodium chloride（Na Cl）,purified by C₁₈,filtered and determined by LC-MS/MS.The results show that dimethachlon have a good linear relationship in methanol solvent,whole tomato,plant and soil matrix（R²>0.99）.The limit of quantification of dimethachlon in whole tomato fruit,plant and soil are 0.02 mg/kg.The average recoveries of dimethachlon in whole tomato fruit,plant and soil matrix were between 76.77%and 106.59%,with relative standard deviation（RSD）less than 5.89%.In Guizhou and Guangdong field experiments in 2020,during the tomato fruiting period,40%dimethachlon wettable powder pesticide was sprayed at the recommended dose of 3000 g/ha.The whole tomato fruits,plants and soil samples were collected 2 hours after the application,then processed and determined.The results showed that the residual of dimethachlon in the three matrixs of Guizhou and Guangdong was below the limit of quantification of 0.02 mg/kg 2 h after appalication.2.The metabolites produced by dimethachlon in soil and tomato samples were further investigated,and the structure of the metabolites was confirmed.The soil and tomato samples were extracted by acetonitrile,and the extracted solution was concentrated and purified by C₁₈.The data were determined by liquid chromatography-high resolution mass spectrometry（HPLC-HRMS）,and results were put into the Compound Discoverer 2.0（CD）software for analysis.It was found that both soil and tomato samples showed a metabolite ion peak at 6.07 min,and the exact ion mass of this peak was m/z 259.98867.This metabolite ion peak contains two the isotope peak of each chlorine atom.The structure of the metabolites was deduced and the corresponding standard products were purchased.The chromatographic retention time and the full scan msaa and MS2 mass spectrometry information of the standard products were compared with the metabolites in soil and tomato samples,and the structure of the metabolites was finally determined to be 4-（（3,5-Dichlorophenyl）amino）-4-oxobutanoic acid.3.The detection method of metabolite on LC-MS/MS was established in order to quantitative analysis of the metabolites,and the pretreatment conditions were optimized.Experiments have proved that the pretreatment method of metabolites is the same as that of the parent dimethachlon in the matrix,the results show that metabolite have a good linear relationship in three matrix（R²>0.99）.The average recoveries of metabolite in whole tomato fruit,plant and soil matrix were between 79.53%-100.11%,with relative standard deviation（RSD）less than 10.41%.In Guizhou,the results of field samples showed that the residual metabolites in whole fruit,plant of tomato and soil were0.02-0.07mg/kg,1.00-4.69 mg/kg and 0.06-0.47 mg/kg,respectively,during 0-14 days.In Guangdong,the results of field samples showed that the residual metabolites in whole fruit,plant of tomato and soil were 0.03-0.10 mg/kg,0.04-1.21 mg/kg and 0.01-0.46mg/kg,respectively,during 0-14 days.The degradation of metabolites in Guizhou soil accorded with first-order kinetic equation,with half-life of 1 d and R² of 0.8717.4.The degradation of dimethachlon in the buffer solution and the generation trend of metabolites were studied.The water sample was extracted and concentrated by dichloromethane and then detected by LC-MS/MS instrument.The results indicated that dimethachlon and metabolite have a good linear relationship in three matrix（R²>0.99）.The average recoveries of dimethachlon and metabolite in whole tomato fruit,plant and soil matrix were between 63.25%and 107.99%,RSD were between 0.56%and 12.51%.Under sterilization conditions,the half-life of dimethachlon in the p H5,p H7 and p H9buffer solution is 63 d,1.63 d and less than 2 h,respectively.Dimethachlon degrades rapidly in both neutral and alkaline aqueous solutions.It was assumed that the presence of hydrolase caused the rapid degradation of dimethachlon in the samples of tomato planting system,so the residual in the sample is less than the limit of quantification.