Abstract:A liquid chromatography -tandem mass spectrometry （LC -MS/MS）was established for the determination of fipronil and its metabolites in egg ，chicken and cake .The samples were extracted with acidacetonitrile，cleaned up by Cleanert PEP Plus（60 mg/3 mL）.The separation was performed on an Agilent Eclipse plus-C18 column（2.1 mm×100 mm，1.8 μm）with the mobile phases of 10 mmol/L ammonium acetate aqueous solution containing 0.1% formic acid and acetonitrile containing 0.1% formic acid.The identification and quantitation of fipronil and its metabolites were performed by MS/MS in negative electrospray ionization（ESI-）and multiple reaction monitoring （MRM）mode.The quantitative analysis was carried out by external standard method.Three matrixes containing egg，chicken and cake were experimented.A good linear relationship，with the correlation coefficient（R2）greater than 0.99，was found in the mass concentration range of 1 μg/kg-50 μg/kg.The limit of detection and limit of quantitation were 0.4 μg/kg and 1 μg/kg respectively.The recoveries of fipronil and its metabolites in different matrixes spiked with 1，2，5 μg/kg varied from 82.3 % to 112.5%，and the relative standard deviations were between 1.2%and 4.5%.The method is sensitive，accurate and suitable for the monitoring of fipronil and its metabolites.