Abstract
The residues of fipronil and its metabolites, namely fipronil-desulfinyl, fipronil-sulfone, and fipronil-sulfide, have attracted increasing attention since the European egg contamination incident. In this study, by optimizing the pretreatment method and chromatographic conditions, a simple extraction method coupled with high performance liquid chromatography-tandem mass spectrometry was developed for the simultaneous determination of fipronil and its metabolites, including fipronil-desulfinyl, fipronil-sulfone, and fipronil-sulfide, in livestock and poultry liver. The optimal pretreatment method for fipronil and its metabolites was determined by comparing the recoveries obtained with different extraction solvents (methanol, acetonitrile, acetone, and ethyl acetate), and by purification with N-propyl ethylenediamine (PSA) and octadecylsilane (C18). The samples were extracted with 10 mL acetonitrile, then purified with 150 mg PSA and 100 mg C18, following which the extracted solutions were directly injected for analysis. Separation was performed on an Agilent ZORBAX SB-C18 column (150 mm×2.1 mm, 3.5 μm) with gradient elution using acetonitrile and water as the mobile phases. The target compounds were detected by electrospray ionization (ESI) in the negative mode under multiple reaction monitoring, and quantified by the external standard method with matrix-matched standard correction curves. The results indicated that the linear ranges for the four compounds ranged from 0.1 μg/L to 10.0 μg/L with correlation coefficients (r(2)) higher than 0.995. The limit of detection was 0.2 μg/kg and limit of quantitation was 0.5 μg/kg. The average recoveries were between 81.1% and 99.8% at three spiked levels of 0.5, 1.0, and 10 μg/kg, with relative standard deviations (RSDs) of 6.1%-11.7%. The matrix effect experiment showed that fipronil and its metabolites had matrix inhibition effects. Matrix-matched standard curve correction was performed to eliminate matrix inhibition effects. The proposed method was used for the analysis of 99 liver samples, where fipronil-sulfone was detected in four samples with values of 1.25-2.82 μg/kg. The method is simple, sensitive, and accurate, and is suitable for the determination of fipronil and its metabolites in livestock and poultry liver.