Enantioselective Degradation Of Acephate And Its Highly Toxic Metabolite In Vegetables

Acephate is a commonly used insecticide for vegetables in China.It is commonly metabolized into highly toxic methamidophos by crops.Both acephate and methamidophos have chiral enantiomers,but the degradation behavior accompanied by stereotoxic metabolism is still unclear.In this study,the enantioselective degradation of acephate and its metabolite in vegetables was studied.A rapid,accurate and reliable method was established for the determination of chiral enantiomers of acephate and its metabolite in rape,sweet potato(fruit and leaf)and tomato.QuEChERS sample pretreatment combined with supercritical fluid chromatography-tandem mass spectrometry was used to achieve accurate qualitative and quantitative analysis of target compounds.The calibration curves of acephate and its metabolite methamidophos had good linearities(R>0.99),and the quantitative limit of the method is less than 5.7?g/kg.The average recoveries of R-acephate,S-acephate,R-methamidophos and S-methamidophos(RSD)were 55.7%-87.1%(1.4%-7.0%),57.3%-87.7%(1.2%-8.5%),63.4%-74.1%(2.6%-9.9%)and 55.6%-78.8%(1.7%)respectively at the spiked levels of 0.005,0.05 and 0.5 mg/kg.In conclusion,this method can meet the research needs.Pot experiments were conducted to evaluate the stereoselective degradation and metabolism of acephate in three vegetables.The results showed that the degradation kinetics of R-acephate,S-acephate,acephate racemes and their metabolites R-methamidophos,S-methamidophos and methamidophos racemes fitted the first-order reaction kinetics(R~2ranged from 0.701 to 0.986)in rape and sweet potato leaves.The degradation half-life was3.98-23.93 days in rape,and 3.84-11.12 days in sweet potato leaves.The half-lives of R-acephate,S-acephate and acephate racemes in tomato fruits were 7.51,12.7 and 10.19 days respectively,which fitted the first-order reaction kinetics(R~2>0.94).The concentration of R-acephate,S-acephate,R-methamidophos and S-methamidophos in sweet potato fruits reached the peak at 2 hours after spraying,then rapidly degraded,and the dissipation rate was more than 95%at 8 hours after spraying.Only the residues of acephate in sweet potato fruits and tomatoes were lower than MRL within the safe interval(7 days),which accorded with the existing regulations.Acephate residue would drop down to MRL within 13,24,<1 and 6 days after spraying for rape,sweet potato leaves,sweet potato fruits and tomatoes,respectively.In addition,the stability of chiral enantiomers of acephate was evaluated in rape seedlings by hydroponic experiments.The results showed that there was no mutual transformation between the enantiomers of acephate in rape seedlings,while R-acephate was only metabolized to R-methamidophos,and S-acephate was only metabolized to S-methamidophos.In conclusion,this study further clarified the degradation and transformation of pesticides in vegetables,especially the chiral pesticides in vegetables,and provided scientific guidance for the safe and rational use of acephate.This study provides basic data and theoretical support for the comprehensive evaluation of pesticides on the quality and safety risks of agricultural products,and helps to ensure the safety of vegetable consumption.