| Since 2006 EU reduced the minimum pyrethroid pesticide residues in tea, so trace residues of pesticides in tea export trade becomes an obstacle. The GC-ECD widely used method for determination has some issues: (1) some pesticides which are heat decomposition and others with higher boiling point are not detected at the same time; (2) ECD detector as a quality detector make the results more susceptible, and higher pre-treatment of the samples is needed; so it is difficult to meet the demand for rapid detection trace pyrethroid pesticide residues efficiency. High performance liquid chromatography (HPLC) needs large amounts of organic solvents, the column efficiency is not high in detection of trace substances, and the different types of material need the corresponding column. Hence we need to develop a method to develop a rapid chromatographic separation method, and wide range of substances can be separated and analyzed. Comparison of the electric of seven pyrethroid pesticides in the solution, we decided to use MECC method as capillary electrophoresis analysis. According to the linear model, we added two organic modifier—acetonitrile and isopropanol to the background buffer. Through regulating the concentration of 3(hydroxymethyl)aminomethane(tris) and SDS, pH and organic modifiers amount , seven substances are separated in optimal conditions, namely: 25mM SDS 50mM tris, 15%isopropanol, 20% acetonitrile, pH 8.2; isolation voltage 23kV, capillary temperature: 25℃. Out of the summit following order: cyhalothrin, bifenthrin, cyfluthrin, alpha-cypermethrin, deltamethrin, permethrin, fenvalerate. From seven pyrethroids molecular structure analysis, and the strength of hydrogen bonds formed with organic solvents in the buffer solution, the order of the peak inferred and the actual analysis was basically the same.We processed actual sample through SPE method. After processed leakage test, chosed of washing solvent elution and established elution curve, we got the best pre-treatment conditions: Florida silicon used as filler to the SPE column, 2g Florida added 10ml empty SPE column and pre-6ml eluate collected for analysis. Recovery of seven materials are all above 75% in pre-treatment step.DAD or UV detector is more commonly used as capillary electrophoresis. Because of a smaller diameter capillary, detection of subsidence is relatively high. In order to meet the EU regulation, we used on-column stacking technique—REPSM. The optimal operation condition of on-column stacking was described as follows: the background solution consisted of 15% isopropyl alcohol, 20% acetonitrile, 60mM sodium dodecyl sulfate (SDS) and 50mM tris. After the extract of tea was cleaned by solid phase extraction for removing some impurities, and dried by air, the sample residue was dissolved in 1mM SDS solution. Reverse voltage was imposed after the 100s injection, and then exchanged the polarity when the current return 95% of normal. After on-column stacking, the enrichment factors of seven pyrethroid pesticide residue were higher than 17, and their recoveries in on-column stacking step were more than 82%. Under the optimal operation condition, the detected concentration limits of seven kinds of pyrethroid pesticide residues in tea were 0.025mg/kg for cyhalothrin, 0.025mg/kg for bifenthrin, 0.025mg/kg for cyfluthrin, 0.025mg/kg for alpha cypermethrin, 0.2mg/kg for deltamethrin, 0.025mg/kg for permethrin and 0.05mg/kg for fenvalerate, respectively.In the process of analysis of the tea self-contained seven pyrethroids 0.05 mg/kg. For a true record of pyrethroid concentrations in sample , uncertainty factors which may be introduced in the testing process was analysed.Seven pyrethroids of measurements and uncertainty in sample are as follows : cyhalothrin, 0.048±0.0091mg/kg bifenthrin 0.411±0.0028mg/kg, cyfluthrin0.0476±0.0025mg/kg,alpha-cypermethrin0.0366±0.0011mg/kg,deltamethrin0.0758±0.0027mg/ kg, permethrin 0.0386±0.0023mg/kg, fenvalerate 0.0441±0.0090mg/kg.
|
PDF Full Text Request