Degradation and residues of indoxacarb enantiomers in rice plants, rice hulls and brown rice using enriched S-indoxacarb formulation and enantiopure formulation

Abstract

A chiral liquid chromatography-tandem high resolution mass spectroscopic method was developed for the analysis of indoxacarb enantiomers in rice plants, rice hulls and brown rice. Chiral separation of two enantiomers was carried out on a Superchiral S-OD column maintained at 20°C and eluted with 0.3 mL/min methanol. Samples were extracted by acetonitrile solution with ultrasound and cleansed by dispersive solid-phase extraction of 50 mg of primary secondary amine and 50 mg of C₁₈ . This method was successfully used to study the degradation and residues of two enantiomers with enriched S-indoxacarb (2.33S/1R) and pure S-indoxacarb in rice plants. The half-lives of R-indoxacarb and S-indoxacarb were 4.20-4.33 and 3.45-3.57 days in rice plants during the degradation of enriched S-indoxacarb in Guizhou and Hunan, respectively, whereas the half-lives of pure S-indoxacarb were 2.68 and 3.69 days in Guizhou and Hunan, respectively. The results indicated that preferential S-indoxacarb degradation occurred and that enantiomeric transformation was absent in the total experiment periods of pure S-indoxacarb in rice plants. The final residue concentrations of indoxacarb enantiomers in brown rice were significantly less than those in rice plants and rice hulls in the same rice field after applying indoxacarb SC and indoxacarb EC.

Keywords: enantioselective degradation; high resolution mass spectroscopy; indoxacarb enantiomers; residues; rice.