Determination of Residues of Fipronil and Its Metabolites in Tea by Gas Chromatography-Negative Chemical Ionization-Tandem Mass Spectrometry with QuEChERS

2.00 g of the tea sample was taken, then soaked in 10 mL of water, and centrifuged for extraction with 10 mL of acetonitrile and QuEChERS extraction salt (containing 6 g of magnesium sulfate and 1.5 g of sodium acetate). 8 mL of the supernatant was taken and added into the QuEChERS purification salt tube containing 1 200 mg of magnesium sulfate, 400 mg of N-propyl ethylenediamine (PSA), 400 mg of C₁₈ and 200 mg of graphite carbon black (GCB). After shaking and centrifuging, the supernatant was taken and passed through 0.45 μm filter membrane. Gas chromatography-negative chemical ionization-tandem mass spectrometry (GC-NCI-MS/MS) was used to determine fipronil and its metabolites fipronil desulfinyl, fipronil sulfone and fipronil sulfoxide in filtrate, with matrix matching method for quantification. As shown by the results, linear relationships between values of the corresponding peak area and mass fraction of fipronil and its metabolites were found in the range of 0.005-0.50 mg·kg^-1, with detection limits (3S/N) in the range of 0.25-0.5 μg·kg^-1. Test for recovery was made by the standard addition method, giving the results in the range of 96.6%-136%, with RSDs (n=6) of the determined values less than 7.0%. This method was used for the analysis of 100 batches of tea, and fipronil and its metabolites were detected in a certain batch of tea samples, with a total amount of 77.1 μg·kg^-1 (calculated by fipronil).