Residual Dynamics And Setting Of The Standards For MRL Of Dimethyl Disulfide In Tomato

Tomato is a worldwide-cultivated vegetable with high nutritional and economic value.In China,with the rapid development of cultivation techniques for the protected field,tomato has become one of the vegetables sharing large scale cultivation.However,the early and fast occurrence of a variety of diseases like root knot nematodes,Sclerotinia,Fusarium wilt,and root wilt can severely affect tomato production,which are major obstacles for greenhouse tomato cultivation.To date,root knot nematodes are the most serious pests invading greenhouse tomato.Soil sterilization techniques are commonly used to treat soil for control of some soil borne diseases,and fumigants play an important role in this process.Methyl bromide is a fumigant which had been widely applied since 1940 s.Nevertheless,it was phased out on an international level nowadays due to its enormous damage to the ozone layer.Currently,alternatives to methyl bromide mainly include metam sodium,1,3-dichloropropene,chloropicrin,and methyl iodine but a number of issues arise during their application such as high toxicity and cost.Dimethyl disulfide is an active ingredient registered in US by Arkema(France)in 2009 which is a highly-efficient and environmentally friendly soil fumigant with nematocidal,fungicidal,and herbicidal activities.It is also a novel soil fumigant proposed by MBTOC.Dimethyl disulfide acts by interfering with mitochondrial function,activating the K+ channel of ATP,and inhibiting the activity of cytochrome oxidase,exerting efficient control to soil borne fungi,nematodes,underground pests,and weeds.This study consists of three parts.First,an analytical method was developed to determine dimethyl disulfide in tomato and soil by gas chromatography-mass spectrometry(GC-MS).Optimizations were made to the traditional method including reducing the extraction solvent volume and extraction time.Sample extract gone through rapid liquid-liquid partition was directly injected to GC-MS.A number of extraction conditions like the ratio of solvent and distilled water used as extraction solution,the volume and constitution of extraction solution served in liquid-liquid partition were discussed,and the results showed that the most satisfactory recoveries were obtained when samples were first extracted using 20 mL of methanol then further extracted in liquid-liquid partition constituted by 20 mL of n-hexane and 5 mL of distilled water.The linear range of dimethyl disulfide calibration curve was between 0.002-1 mg/L with a correlation coefficient of 0.9999.The mean recovery of dimethyl disulfide in tomato was in a range of 76.4-82.1%,with RSD ranging from 3.1% to 8.9%.The mean recovery of dimethyl disulfide in soil was in a range of 71.5-84.0%,with RSD ranging from 1.5% to 6.7%.Limit of detection(LOD)of the method was 1.5 x 10-3 ng and limit of quantitation(LOQ)was 5.0 x 10-3 mg.This is a rapid method providing accurate quantitation and good reproducibility which could be applied in most pesticide residue analysis laboratories.Second,dissipation of dimethyl disulfide in tomato and soil was investigated in this study.The results showed that the content of dimethyl disulfide in tomato at three experimental sites Jilin Changchun,Hebei Langfang,and Shandong Qingdao was below 0.010 mg/kg.In 2005,half-lives(t1/2)of dimethyl disulfide in soil at Jilin Changchun,Hebei Langfang,and Shandong Qingdao were 0.3 d,3.0 d,and 4.6 d,respectively.In 2006,half-lives of dimethyl disulfide in soil at Jilin Changchun,Hebei Langfang,and Shandong Qingdao were 6.5 d,0.4 d,and 1.5 d,respectively.The results indicated that dimethyl disulfide belongs to pesticides of easy degradation(t1/2 < 30 d).Finally,the study investigated the ultimate residues of dimethyl disulfide in tomato and soil.The results showed ultimate residues of dimethyl disulfide in tomato and soil at three experimental sites Jilin Changchun,Hebei Langfang,and Shandong Qingdao were all below 0.010 mg/kg.Therefore,dimethyl disulfide(99.6%)was suggested to be used by soil injection no more than once for control of root knot nematodes at 600 kg/ha(597.6 kg a.i./ha)before tomato planting,and tomato was safe at harvest.A maximum residue limit(MRL)of 0.05 mg/kg for dimethyl disulfide in tomato in China was recommended based on the values regulated in France and US.