| Leafy vegetables are important daily foods,but need pesticides to protect against insects.The pesticide residues should be monitored strictly to bring security to food safety.There are 28 pesticides of 4 classes widely used for leafy vegetables in Anhui province.It's difficult to do quantitative analysis of the residues simultaneously since the polarity and stability of those pesticides are much different.The common used extraction and detection methods have weak points such as slow sample preparation,inaccurate recovery,and high limit of quantitation(LOQ).To meet the challenge of rapid and efficient analysis of pesticide multi-residues in leafy vegetables,as important research of the eleventh-five key subject of Anhui province,this work has established a simultaneous detection method for the 28 pesticides using LC/MS/MS and studied the mechanism of ultrasonic solvent extraction according to the physical and chemical characteristics of the pesticides.The work indicates that:1.Ultrasound can promote both dissolving and decomposition of pesticides.With well selection of ultrasonic power and frequency,the highest recovery of pesticides can be obtained. Ultrasonic cavitation on one hand can destroy vegetable cell walls and facilitate pesticides dissolving in solvent,on the other hand it will cause the arisen of·OH radicals which can cause decomposition of pesticides.With ethyl acetate:petroleum(1:1,VN),the results of average decomposition of nine typical pesticides in solvent and average recovery of the nine pesticides in spinach matrix were compared at different ultrasonic conditions.When Pmax/t≤2.1×107 atm/s,recovery raised with Pmax/t;when Pmax/t＞2.1×107 atm/s,the amount of·OH radicals was too large to induce violent decomposition of pesticides,and the recovery decreased.The experiment indicates that the average recovery reaches the peak value when Pmax/t=2.1×107 atm/s at(45 kHz,400 W).2.Based on the optimized ultrasonic power and frequency,a rapid,low cost and efficient method for ultrasonic solvent extraction of pesticide multi-residues was established with well selection of ultrasonic conditions,such as the type of solvent,volumn of solvent and time.5 g sample spiked with 1 mL pesticide standards,sonicated for 35 min with 40 mL ethyl acetate: petroleum(2:1,V/V),the recovery of pesticides is in the range of 83.4%～97.8%with R.S.D.＜2.5%(n=6)when the spiking level is ranged from 1.9×10-2to 0.2 mg/L.The result obtained through the established method was compared with those through homogenization method.It shows that the optimized ultrasonic solvent extraction method offers better precision recovery with more simple procedure and lower cost of time.3.With optimization to mobile phase,additive and gradient elution order,a rapid,accurate and sensitive LC/MS/MS method for 28 pesticides was established under well selected qualitative, quantitative mass and instrument parameters for each pesticide.In methanol,with the concentration of pesticides in 5.0×10-2～5.0×101μg/L,the result indicates r＞0.9990 and LOQs＜0.5μg/L.4.The established ultrasonic solvent extraction-LC/MS/MS determination method was used to analyze 28 pesticides in leafy vegetables.The obtained result indicates that when the pesticide residues are lower than the MRLs establish by P.R.China and Japan,the proposed method can still provide accurate quantitation result.
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