Determination Of Thiamethoxam Residues Of Tea And Residual Degradation In Tea

Recently environment and food contamination caused by pesticided was concerned by government and public. China had plenty of tea have been exported, one third of which are exported to the worldwide. And it is well known that tea leaves are processed directly after picked up without any clean and disinfecting procedures. So pesticide residue on tea which has been drawn much attention to in recent years.Thiamethoxam is a newly introduced broad-spectrum chloronicotinyl insecticide and widely used in Pollution-free tea garden. The MRL （maximum residue limit） value of thiamethoxam has not yet been established in tea. There is no recommended MRL value of thiamethoxam in tea In china and EU except Japan.The MRL in Japanese Positive List System is15mg/kg.Methods for the determination of residue thiamethoxam in tea were investigated. An investigated of thiamethoxam degradation in tea shoots, green tea, tea process and infusion was studied and the result may be provide valuable reference for government and tea enterprises. The main results contains as follows:1Analytical method of thiamethoxam residue in green tea, tea shoots and tea infusionDifferent kinds of extraction solvent were compared, the result showed that acetonitrile gave the least matrix from tea sample. The clearn-up effect of different SPE cartridges was also investigated, and ENVI-Carb-â…¡/PSA SPE cartridges showed good result. A novel method was established for determination of thiamethoxam pesticide residue in tea by High performance liquid chromatography. After ultrasonic extracted with acetonitrile, the extraction was purified with ENVI-Carb-â…¡/PSA SPE cartridges, then the samples were detected with HPLC. the limits of detection of various samples are0.05mg/kg,0.01mg/kg and0.006mg/kg in green tea, tea shoots and tea infusion. The recoveries of thiamethoxam from tea in this method varied from77.2%to107.1%, the RSD varied from3.1%to10.4%. Therefore, The accuracy, sensitivity and precision for the method good to satisfy the essential rules of pesticide residue determination.2Dynamics of thiamethoxam in green tea and tea shoots of tea gardenThe experiment was designed with three treatments, the first treated by thiamethoxam recommended application rate （effective constituent22.5g/hm2）, the other one treated by the double recommended application rate （effective constituent45g/hm2）, and the last one was treated with water. Green tea leaves （two leaves and a bud） were plucked randomly from each treatment rep-lication at different time intervals [0（1hour after last spraying）,1,3,5,7,10and14day] and the green leafs were processed to green tea following standard manufacturing methods.The thiamethoxam degradation equation of treatment （22.5g/hm²ï¼‰ in green tea was C₁=9.320e^-0.434xt, the correlation coefficient is0.9650, the half-life of thiamethoxam was1.60d The thiamethoxam degradation equation of treatment （45g/hm²ï¼‰ in tea was y=13.78e^-0.4312x, the correlation coefficient is0.99180, the half-life of thiamethoxam was1.64d. Thiamethoxam was not detected after sprayed thiamethoxam14d. The thiamethoxam degradation equation of treatment （22.5g/hm²ï¼‰ in tee shoots was y=10.598e-0.4332x, the correlation coefficient is0.9677, the half-life of thiamethoxam was1.56d The thiamethoxam degradation equation of treatment （45g/hm²ï¼‰ in tea shoots was Ct=16.123e-0.4289x, the correlation coefficient is0.9911, the half-life of thiamethoxam was1.62d. thiamethoxam was not detected after sprayed thiamethoxam14d.The results indicated that degradation of thiamethoxam in green tea and tea shoots could be described by first-order kinetics equation in natural environment. Degradation speed of thiamethoxam is fastest in the first day after sprayed, and then gradually reduce. Degradation speed of thiamethoxam is equal in recommended application rate and the double recommended application rate.3Degradation of thiamethoxam in tea processing and tea infusionThe result showed that the thiamethoxam residue was decreased1.2%-22.7%and7.1%-18.3%in the green tea processing in treatments of low and high concentration, respectively. The average of degradation rate is8.2%. Degradation speed of thiamethoxam is fastest in one hour after sprayed.The percent transfer of thiamethoxam residues from made tea to infusion were in the range of93.0%and89.4%on1and3days, respectively. The RSD was7.2%and3.6%. Thiamethoxam absorbed into the humen’s body by drinking tea is0.042μg/kg d. The numbers are less than the pesticide’s ADI values. the safety factor of thiamethoxam is619. It can make a conclusion that to drinking the tea is safety.4The degradation products of thiamethoxam on tea shootsThe metabolites of thiamethoxam were identified with LC/MS/MS, one degradation product was detected on the tea shoots surface. Its gradually increase in1,3,5and7days of the tea shoots surface. It is one of the photolytic degradation products.