UPLC-MS/MS Determination of Residual Amounts of Imidacloprid and Buprofezin in Tea with Purification by QuEChERS

2.00 g of tea sample were moistened with 8.00 mL of water in a polypropylene centrifuging tube and extracted ultrasonically with 10.00 mL of acetonitrile for 20 min. After addition of 1 g of NaCl and 4 g of anhydr. Na₂SO₄, the mixture was centrifuged for 5 min. An aliquot of 3.00 mL of the acetonitrile extract was taken and purified by mixing with 150 mg of PSA and 200 mg of anhydr. CaCl₂ which were placed beforehand in the purification tube, and then the mixture was centrifuged for 5 min. An aliquot of 0.50 mL of the purified extract was taken, and mixed with 0.50 mL of a mixed solution of 10 mmol·L^-1 HCOONH₄ solution and 6 mmol·L^-1 HCOOH solution at a volume ratio of 1:1. The extract was introduced and passed through the ACQUITY UPLC BEH C₁₈ chromatographic column. Mixtures of (A) mixed solution of HCOONH₄-HCOOH (described above for its preparation) and (B) acetonitrile in various ratios were used as mobile phase in the gradient elution. Mode of ESI⁺ and of MRM were adopted in MS/MS determination. The method of matrix matching was used in the preparation of standard curves to overcome the matrix effect. The 2 pesticides, imidaclopride (ICP) and buprofezin (BPFZ) were separated and determined satisfactorily under the above conditions. Linearity ranges for the 2 pesticides were found same within 100.0 μg·L^-1. Values of lower limits of determination (10S/N) found were 0.01 mg·kg^-1 and 0.005 mg·kg^-1 respectively. Recovery was tested by addition of standard solutions to blank sample as matrix, giving results of recovery over (including) 80%. Values of RSDs (n=6) found were all less than 13%. Twenty-eight batches of tea samples were analyzed by the present method, ICP was found in samples of 4 batches of tea with mass fractions ranged from 0.012 mg·kg^-1 to 0.048 mg·kg^-1, and BPFZ was found in samples of 3 batches of tea with mass fractions in the range of 0.024-0.63 mg·kg^-1. These determined values were all lower than the specified max. limits of residual amounts of the pesticides, i.e. 0.5 mg·kg^-1 (for ICP) and 10 mg·kg^-1 (for BPFZ).