Determination of 21 Common Pesticides in Fish-Pond Water by SPE-New Mixed Adsorbent-UPLC-MS/MS

Four liters of fish-pond water sample were collected and placed into a 5 000 mL flask, and 100 μL of mixed internal standard solution (containing 0.5 mg·L^-1 of SKF525A and 2 mg·L^-1 of phenobarbital) were added. A mixture of 4 fillers, namely C₈, C₁₈, GDX403 and X-5, was prepared by mixing the 4 compounds by the mass ratio of 1:2:1:8, and 6 g of the mixture were taken and activated by soaking in 10 mL of methanol. After filtration, the activated mixed filler was added to the sample solution as adsorbent, and the mixture was shaked for 1 h with a shaker. After filtration, the solid adsorbent with the adsorbed pesticides and together with the filter paper was taken and extracted by shaking for 10 min with 100 mL each of benzene and ethyl acetate after adding 5 g of anhydrous magnesium sulfate. The mixture was then filtered through 1.0 μm filtering membrane. The filtrate was collected and evaporated to near-dryness by N₂-blowing. The residue was taken up and made its volume to 1.0 mL with methanol. The solution was filtered through 0.22 μm filtering membrane, and the filtrate was used for UPLC-MS/MS analysis under the optimized instrumental conditions. The use of mixture of the 4 filler as the adsorbent for enrichment of the pesticides improved the separation by giving an average recovery of 82% for the pesticides determined with averages above 90.0% for 11 pesticides. In the UPLC separation, InertSustain C₁₈ column was used as stationary phase, and 3 elution programmes were adopted using mixtures of (A) 20 mmol·L^-1 ammonium acetate solution containing 0.1% (volume fraction) formic acid of (A') 20 mmol·L^-1 ammonium acetate solution (without formic acid) and (B) acetonitrile of (B') methanol in various ratios as mobile phases. Under the prescribed elution programmes, retention times for the 21 pesticides together with the 2 internal standards were found in the range from 0.53 to 9.81 min. In MS/MS analysis, ESI with positive and negative ion modes and MRM mode were adopted. Internal standard method was used for quantification. Linearity ranges found were in the ranges from 0.005-0.5 μg·L^-1 for 17 among the 21 pesticides, and from 0.01-0.5 μg·L^-1 for the remainder. Dection limits (3S/N) found were in the range of 1-5 ng·L^-1. Test for accurary was made by standard addition method, giving results in the range of 92.5%-106% according to the ratio between determined mass concentration and the theoretical mass concentration. Values of RSDs (n=6) found were ranged from 1.3%-9.7% (intra day) and from 2.5%-13% (inter day).