Determination of Fipronil and Four Metabolites in Foodstuffs of Animal Origin Using a Modified QuEChERS Method and GC–NCI–MS/MS

A sensitive, effective, and reliable method for the accurate determination of fipronil and four metabolites (fipronil carboxamide, fipronil sulfone, fipronil sulfide, and fipronil desulfinyl) in foodstuffs of animal origin (egg, milk, beef kidney, beef liver, chicken, and chicken liver) was developed by isotope dilution–gas chromatography–negative chemical ionization–tandem mass spectrometry (GC–NCI–MS/MS). Samples were purified by a modified QuEChERS method. Four isotopically labeled internal standards were added in the sample extraction process to compensate for the matrix effect. The average recoveries were 78.2–107.1% with RSD ≤ 8.5%. The limits of quantification (LOQs) were 0.2 μg kg−1 for fipronil carboxamide and fipronil sulfone and 0.1 μg kg−1 for fipronil, fipronil sulfide, and fipronil desulfinyl. At the same time, the fragmentation mechanism of the five target compounds was analyzed via mass spectral data to help identify the compounds.