Determination of 30 Pesticide Residues in Savia by Ultra-High Performance Liquid Chromatography-Tandem Mass Spectrometry after Protection with Dithiothreitol and Purification with Modified QuEChERS Adsorbents

To resolve problem of low recovery caused by incomplete testing items, easy adsorption and degradation of some pesticides in the determination of pesticide residues, the title method was proposed, and the purified condition, type and amount of protective reagent were optimized. The sample was crushed, and an aliquot (3 g±0.05 g) was taken and mixed with 100 μL of 1.0 mol·L^-1 dithiothreitol solution and 15 mL of 1% (φ) acetic acid solution for soaking for 30 min. After 15 mL of acetonitrile was added, the mixture was oscillated for 5 min. Mixed powder (7.5 g) composed of anhydrous magnesium sulfate and anhydrous sodium sulfate at mass ratio of 4∶1 was added, and the mixture was oscillated for 3 min, cooled in ice bath for 10 min, and centrifuged for 5 min. An aliquot (9 mL) of the supernatant was taken and purified with a purification tube containing 900 mg of anhydrous magnesium sulfate, 300 mg of N-propylethylenediamine, 300 mg of C₁₈ and 300 mg of silica. After oscillation for 5 min and centrifugation for 5 min, 5 mL of the supernatant was concentrated to about 0.4 mL with nitrogen-blowing at 40 ℃, mixed with 150 μL of water, and diluted to 1 mL with acetonitrile. After vortexing, the above solution was passed through a 0.22 μm filtrate membrane, and the filtrate was introduced into ultra-high performance liquid chromatograph. Gradient elution was performed on ZORBAX Eclipse plus C₁₈ column with mixed solution of acetonitrile and 10 mmol·L^-1 ammonium formate solution containing 0.1% (φ) formic acid at different volume ratios, and the 30 pesticides separated were detected in positive ion mode of electrospray ion source and multiple reaction monitoring mode in tandem mass spectrometer, and quantitatively analyzed by matrix-matched method. As shown by results, the QuEChERS adsorbents without GCB could reduce the adsorption loss of chlorsulfuron, ethametsulfuron, metsulfuron-methyl and chlordimeform. Using dithiothreitol as protective reagent, the degradation loss of systox, phorate sulfone and fenamiphos was also reduced. Thirty pesticides could be separated within 20 min, and linear ranges of standard curves were found in the range of 10-150 μg·L^-1, with detection limits in the range of 0.001-0.005 mg·kg^-1. Test for recovery was made by standard addition method on the blank sample, giving recoveries in the range of 81.3%-110%, and RSDs (n=6) of the determined values ranged from 1.3% to 11%. The proposed method was applied to the analysis of the 3 batches of actual samples, and fenamiphos was found in one of these samples, with detection amount of 0.012 mg·kg^-1.