UPLC-MS/MS Determination of Residual Amounts of 4 Fungicides in Fruits and Vegetables with Quantification by Using Isotope-Labelled Internal Standards

Analytical samples of fruits and vegetables were prepared in accordance with the National Standard GB 2763-2016 and cryopreserve at -20℃. 5.000 g of the sample, after adding 500 μL of the mixed internal standard solution of the 4 isotope-labelled fungicides, was extracted ultrasonically with 10 mL of acetonitrile for 30 min, and then the mixture was centrifuged for 5 min at high speed. The supernatant was collected and frozen at -40℃ for at least 30 min, until phase separation between CH₃CN and H₂O occured. An aliquot of 1.0 mL of the supernatant was taken rapidly, mixed with an adsorbent mixture containing 50 mg of PSA, 50 mg of C₁₈, 7.5 mg of GCB and 150 mg of anly. MgSO₄ for 1 min and centrifuged at high speed for 5 min for purification. The supernatant was taken and filtered through 0.22 μm filtering membrane, the filtrate was used as testing solution for UPLC-MS/MS analysis. Waters Acquity^TM UPLC BEH C₁₈ chromatographic column (2.1 mm×100 mm, 1.7 μm) was used as the stationary phase, and 2 μL of the testing solution were introduced to pass through the column. Mixtures with various ratios of 0.1% (φ) formic acid solution and acetonitrile were used as mobile phases in the programed gradient elution for separation of the 4 fungcides (thiophanate-methyl, myclobutanil, pyraclostrobin and iprodione together with their isotope-labelled internal standards) and their determinations by MS/MS. Modes of ESI⁺ and MRM were adopted in MS analysis. It was shown that linear relationships between values of peak areas of the fungicides and their respective mass concentrations were found in the same range of 0.1-50.0 μg·L^-1, and that their detection limits (3S/N) were in the range of 0.1-0.2 μg·kg^-1. Test for recovery was made by standard addition method, giving results of recovery in the range of 79.1%-112%, and RSDs (n=6) all less than 15%.