AOAC-RI ERP Book - FAOM METHOD.pdf

1050 C HANG ET AL .: J OURNAL OF AOAC I NTERNATIONAL V OL . 99, N O . 4, 2016 natural degradation dynamics of bifenthrin, fenpropathrin, cypermethrin, and buprofezin on new shoots of the Oolong tea plant using GC. Chen et al. (17) developed a GC-MS method for the analysis of bifenthrin, cyhalothrin, teÀubenzuron, Àufenoxuron, and chlorÀuazuron in dried Oolong tea leaf samples, and then studied the natural degradation of these pesticides in the leaves of Oolong tea trees and the effect of processing steps on the residue. Material

(a) SPE cartridge .—Cleanert ® Triple Phase of Tea SPE (Cleanert TPT; 10 mL, 2000 mg; Agela, Tianjin, China). (b) Homogenizer .—Rotational speed higher than 13 500 rpm (report also in g -force units; T-25B; IKA-Labortechnik, Staufen, Germany), or equivalent. (c) Rotary evaporator .—Buchi EL131 (Flawil, Switzerland), or equivalent. (d) Centrifuge .—Centrifugal force higher than 2879 × g (Z320; B. HermLe AG, Gosheim, Germany), or equivalent. (e) Nitrogen evaporator .—EVAP 112 (Organomation Associates, Inc., New Berlin, MA), or equivalent. (a) GC-MS/MS system.— Model 7890A gas chromatograph connected to aModel 7000B triple quadrupolemass spectrometer with electron ionization (EI) source, and equipped with a Model 7693 autosampler with tMass Hunter data processing software system (Agilent Technologies, Wilmington, DE). GC separation was achieved on a DB-1701 capillary column (30 m × 0.25 mm × 0.25 ȝm; Agilent - W Scienti¿c, Folsom, CA). (b) Conditions— The oven temperature was programmed as follows: 40°C hold for 1 min, increase to 130°C at 30°C/min, increase to 250°C at 5°C/min, increase to 300°C at 10°C/min, and hold for 5 min. The carrier gas was helium, purity •99.999 ; the Àow rate, 1.2 mL/min; the injection port temperature, 290°C; the injection volume, 1 ȝL; the injection mode, splitless, purge on after 1.5 min; the ionization voltage, 70 eV; the ion source temperature, 230°C; the GC/MS interface temperature, 280°C; and the ion monitoring mode was multireaction monitor mode. Each compound is monitored by one quantifying precursor/ product ion transition and one qualifying precursor/product ion transition. Pass Oolong tea leaves (free from the target pesticide after testing) through 10-mesh and then 16-mesh sieves after initial blending in a blender. Spread 500 g sieved Oolong tea leaves uniformly over the bottom of a stainless steel vessel 40 cm in diameter to await spraying. Accurately transfer a certain amount of pesticide mixed standard solution into the full-glass sprayer and spray the tea leaves. Spray while stirring the tea leaves with a glass rod for uniform coverage. After spraying, continue to stir the tea leaves for 30 min to dissipate the volatile solvents from the tea leaves. Place the sprayed tea leaves in a 4 L brown bottle to avoid exposure to light. Store at room temperature and continue oscillation blending for 12 h. Spread the aged tea on the bottom of a Àat-bottomed vessel, draw an ; and weigh a total of ¿ve portions of aged tea samples collected from the symmetrical four points of the X and from the central area. Submit the samples for GC-MS/MS determination, and calculate the average value of the pesticide content of the aged tea samples and RSD. When the RSD is 4 for GC-MS/ MS, it can be judged that tea samples have been sprayed and mixed homogeneously. Preparation Procedures for Aged Tea Samples Apparatus and Conditions

Hitherto, less attention has been given to the degradation of pesticides in aged tea. To research the degradation regularity of pesticides in aged tea samples, on the basis of our previous studies (18–20), the developed GC-MS/MS method was used to determine the multiresidue of 271 pesticides, including organonitrogen, organophosphorus, organochlorine, organosulfur, carbamates, and pyrethroids, in aged Oolong tea over 3 to 4months. Meanwhile, the regularity of 271 pesticides in aged Oolong tea determined over 40 and 120 days was discussed in different aspects according to ¿tting curves. Subsequently, 20 representative pesticides from different classes were optimized for further study. At a higher spray concentration, the residues of the selected 20 pesticides in aged Oolong tea were studied over 90 days to investigate the degradation regularity at different concentrations. The degradation values of target pesticides on a speci¿c day could be predicted by the logarithmic function obtained from plotting the determination time (day) on the x -axis and the difference between each determined value and the ¿rst-time-determined value of target pesticides on the y -axis, according to the degradation results of the 20 pesticides at the higher concentration over 90 days. Lastly, the proposed procedure was validated by predicting the pesticide residue at one of the Youden pair concentrations according to the logarithmic function fromanother concentration. The predicted values were compared to the measured results, and they were evaluated by their deviation ratios. (a) Solvents .—Acetonitrile, dichloromethane, isooctane, and methanol (HPLC grade) were purchased from Dikma Co. (Beijing, China). (b) Anhydrous sodium sulfate .—Analytically pure. Baked at 650°C for 4 h and stored in a desiccator. (c) Pesticide standards and internal standard (ISTD; heptachlor epoxide) .—Purity •95 (LGC Promochem, Wesel, Germany). (d) Stock standard solutions .—Weigh 5–10 mg individual pesticide and chemical pollutant standards (accurate to 0.1 mg) into a 10 mL volumetric Àask. Dissolve and dilute to volume with methanol, toluene, acetone, acetonitrile, isooctane, etc., depending on each individual compound’s solubility. Store all standard stock solutions in the dark at 0–4°C. (e) Mixed standard solutions .—Depending on properties and retention time of each pesticide, all 271 pesticides for GC-MS/ MS analysis are divided into three groups. The concentration of each mixed standard solution depends on the sensitivity of each compound for the instrument used for analysis. Mixed standard solutions should be stored in the dark below 4°C. Experimental Reagents