Determination of Residues of Paraquat and Diquat in Vegetable Oil by Ultra-High Performance Liquid Chromatography Tandem Mass Spectrometry with Solid Phase Extraction

A method for determination of residues of paraquat and diquat in vegetable oil was proposed by ultra-high performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) with solid phase extraction. 4.00 g of the sample was taken, 10 mL of n-hexane was added as dispersant, and the mixture was vortexed for 1 min. And then 20 mL of the mixture of 0.1 mol·L^-1 hydrochloric acid and methanol at volume ratio of 1∶1 was added. After vortex extraction for 20 min and centrifugation for 5 min, the upper liquid was discarded. 15 mL of the extract was taken and purified by ProElut PXC solid phase extraction column (activated with 3 mL of methanol and 3 mL of water), which was washed with 3 mL of water and 3 mL of methanol in turn and eluted with 3 mL of the mixture of 2 mol·L^-1 ammonium chloride solution and methanol at volume ratio of 1∶1. The effluent was filtered through a 0.22 μm nylon membrane, and paraquat and diquat in the filtrate were determined by UHPLC-MS/MS, with Dikma HILIC column as the stationary phase and mixtures of 10 mmol·L^-1 ammonium formate solution (pH 3.0) and acetonitrile at different volume ratios as the mobile phase for gradient elution. Multiple reaction monitoring (MRM) mode was adopted into MS analysis and external standard method was used for quantification. As shown by the results, linear ranges of the standard curves were 2.0-200.0 μg·L^-1 for paraquat and 1.0-100.0 μg·L^-1 for diquat, with detection limits (3S/N) of 0.6 μg·kg^-1 for paraquat and 0.3 μg·L^-1 for diquat, respectively. Test for recovery was made by the standard addition method, giving results in the range of 80.5%-93.6%, with RSDs (n=6) of the determined values less than 10%. The method was used for the analysis of 30 vegetable oil samples, and only diquat in 3 samples was detected, with the highest detection amount of 10.5 μg·kg^-1.