Residue and dissipation of fluopicolide, its metabolite and propamocarb in the leaf lettuce using a modified QuEChERS method and HPLC-MS/MS

A modified QuEChERS approach had been developed for simultaneous determination of fluopicolide, its metabolite 2,6-dichlorobenzamide and propamocarb in the leaf lettuce by HPLC-MS/MS. The samples were extracted by acetonitrile with multi-wall carbon nanotubes (MWCNTs) for purification. The quantitative analysis of the samples achieved using the external standard of the matrix solution. The validation results showed that, in the range of 0.005-1 mg/L, the response of the concentrations of the compounds and the instruments had good linear relationship (R² ≥ 0.999). Satisfied recoveries (85%-106%) and relative standard deviations (1.0%-18%) were obtained when spiked with 0.02, 0.1, 2, and 100 mg/kg in the samples. The acceptable limits of quantitation were 0.02 mg/kg for all the three compounds. In the field experiments, after sprayed by 928.13 g a.i./hm² for 3 times and with a spraying interval of 7 d, the dissipation of fluopicolide and propamocarb in the leaf lettuce were following with the first-order kinetics equation, and their half-lives were 3.9-4.2 d for fluopicolide and 4.8-4.9 d for propamocarb, respectively. When the sample intervals were 5 d and 7 d, the residues levels of fluopicolide and propamocarb in the leaf lettuce were 3.45-8.51 mg/kg and 9.05-31.1 mg/kg, respectively. In terms of 2,6-dichlorobenzamide, the residues were lower than 0.02 mg/kg. Based on the growth characteristics and consumption habits of leaf lettuce, and the MRLs of China and Codex Alimentarius Commission (CAC), 5 d was selected as the post-harvest interval.