Methods For Removing Tea Matrix Interferences And Applied In Pesticide Multi-residue Analysis

In recent decades, with the improvement of living standards and the enhancement of the awareness on health, people are becoming increasingly worried about the safety of food, especially the problem of pesticide residues. China is a big country of tea production and export. The safe and hygienic tea production is the key to the development of our foreign trade, especially the problem of pesticide residue. Meanwhile, with the strict MRL in tea was set by European Union, the United States, Japan and other countries, a new detection methods of pesticide residue in tea was established as soon as possible, to promote economic development and ensure people’s safety. In this paper, on the basis of reading a large number of domestic and foreign literatures, the removal methods of main chemical components in tea were explored, like caffeine and tea polyphenols, and they were also used in the detection pretreatment method of pesticide multi-residue in tea, to study the feasibility of method. Finally, a method of simultaneous determination of multiple neonicotinoid pesticide residues in tea by HPLC analysis was established. In addition, the reason of low recovery of nitepyram in tea was investigated. The main contents are as follows:1. A method for removing caffeine in tea was investigated. In this experiment, the molecularly imprinted polymer (MIP) was synthesized by precipitation polymerization using caffeine as template molecules. Compared to the non-imprinted polymer (NIP), the MIP showed a low affinity and specificity towards caffeine in acetonitrile. Moreover, sedimentation effect of specific alkaloids precipitation agent (phosphomolybdic acid) to remove caffeine was investigated, the results shown that:the amount of caffeine decrease with the increase of adding amount of molybdophosphoric acid, but recoveries of neonicotinoid insecticides were changed. Among them, the recovery of dinotefuran had little change, ranged from90.8%to104.8%; when mass ratio of phosphomolybdic acid and caffeine were1:2and1:1, the recovery of nitenpyram were339%and151.4%, and later it was decreased with increasing of amount of molybdophosphoric acid. Recoveries of six neonicotinoid insecticides (thiamethoxam, imidacloprid, clothianidin, imidaclothiz, acetamiprid, thiacloprid) were decreased with increasing of mass ratio of phosphomolybdic acid and caffeine, and the recoveries of thiamethoxam, clothianidin, imidaclothiz ranged from72.2%to80.7%when mass ratio increase to3:1, but the average recoveries of imidacloprid, acetamiprid were decreased from104.6%to70%. The removal efficiency of caffeine in tea was remarkable when mass ratio of phosphomolybdic acid and caffeine was above3:1, so the application of alkaloid precipitation agent (molybdophosphoric acid) in pretreatment method of neonicotinoid pesticides residue analysis has certain limitations.2. The method for removing tea polyphenol was explored and evaluated. In this experiment, the conditions and adsorption properties of tea polyphenol with two polyphenols precipitator (AlCl3, PbAc) and a specific absorbent (PVPP) were investigated and optimized. The results showed that the optimum condition was:200g/L AICl3used for precipitation by adding5mL of1mol/L NaOH to adjust pH; The optimum adding precipitation volume of PbAc was5mL saturated solution; The optimal addition of PVPP was1.5g. After compared the complexity and security of three different removal methods, PVPP adsorption had been chosen as the method to remove tea polyphenols.3. The removal method of polyphenols was applied in multi-residues determination of pesticides in tea. A method was developed for determining eight neonicotinoid insecticides in tea, including dinotefuran, nitenpyram, thiamethoxam, imidacloprid, clothianidin, imidaclothiz, acetamiprid and thiacloprid by precipitation pretreatment and SPE as clean up step, then high performance liquid chromatography (HPLC) as detection step. Tea samples were soaked with water in half an hour and extracted with acetonitrile, and then an appropriate amount of absorbent was added. The extract was shaken by vortex for2min, and then centrifuged at6000r/min for8min. The organic layer of extract was cleaned up with Envi-Carb/PSA cartridge. Neonicotinoid insecticides were eluted with acetonitrile and dried extract was dissolved with methanol-water (1/9, V/V), finally analyzed with HPLC coupled with ultraviolet detector. The results showed that the recoveries of seven neonicotinoid insecticides except nitenpyram in tea sample ranged from75%to110%at the spiked level of0.1～1.0mg/kg, with relative standard deviations (RSDs) less than12.3%. The established method was simple, sensitive and accurate, could be used for monitoring neonicotinoid insecticides in tea. To investigate the reason of the low recovery of nitenpyram, the recovery study based on the established sample pretreatment procedures by recovery experiment of eight neonicotinoid insecticides was examined. The major cause of low recovery may be adding water in the re-extraction procedure.4. The application of the method that removed tea polyphenols in organochlorine and pyrethroid pesticide multi-residues analysis was investigated and evaluated. Results showed that some tea matrix interference was removed by precipitating polyphenols, and the recoveries of17pesticides (beta-Endosulfan, gamma-Endosulfan, dicloran, heptachlor, aldrin, tefluthrin, heptachlor epoxide, endosulfan-1, dieldrin, endrin, endosulfan-2, o,p-DDT, p,p-DDT, methoxychlor, L-cyhalothrin, fenvalerate, deltamethrin) in tea sample ranged from73.7%-108.8%, with RSDs was3.8%-18.5%. It met the requirements of analyzing rganochlorine and pyrethroid pesticides multi-residues in tea product.