Residue and decline dynamics of abamectin in Brassica campestrisand Brassica oleracea under supervised field trials

Supervised field trials were conducted in Beijing and Shandong to study the residue and decline dynamics of abamectin in Chinese cabbage(Brassica campestris L.) and head cabbage(Brassica oleracea L.var.capitata L.) from year 2006 to 2007.Abamectin was extracted by acetonitrile,partitioned by n-hexane,and then detected by high performance liquid chromatography (HPLC) with fluorescence detector (FLD) after derivatization with acetonitrile,N-methylimidazole and trifluoroacetic anhydride.At fortified level of 0.005 to 0.100 mg/kg,average recoveries of abamectin in Chinese cabbage and head cabbage were 90.1% －93.2% and 87.7% －93.0%,with relative standard deviation (RSD) of 2.3% －4.7% and 3.1% －6.3%,respectively;limit of determination (LOD) and limit of quantification (LOQ) of this method were 3 ng and 0.005 mg/kg,respectively.Results of decline study showed that the degradation of abamectin in Chinese cabbage and head cabbage fit first-order model with R2 ranged between 0.900 5 －0.973 8,and half lives were 1.3－1.7 d and 1.4－1.6 d,respectively.Terminal residue experiments showed that level of abamectin residue in Chinese cabbage and head cabbage declined below the MRL of 0.05 mg/kg 7 and 3 days after the last spray with residue ranges of not detected to 0.043 mg/kg and not detected to 0.035 mg/kg,respectively.