Determination of Organophosphate Ester Metabolites in Soil by Solid Phase Extraction Combined with Ultra-High Performance Liquid Chromatography-Tandem Mass Spectrometry

The pretreated soil sample （0.2 g） was taken, and 100 μL of 0.1 mg·L⁻¹ surrogate （diphenyl phosphate-d₁₀) standard solution was added. The mixture was then ultrasonically extracted for 10 min after addition of 2 mL of methanol, followed by centrifugation for 10 min. This extraction process was repeated once. The combined supernatant was then concentrated to 1 mL by nitrogen-blowing, and diluted to 10 mL with water, and pH was adjusted to 5 with sodium acetate buffer solution. The extract was purified using Oasis WAX solid phase extraction column, which was preconditioned with 4 mL of methanol solution containing 5% （volume fraction, the same below） aqueous ammonia, 4 mL of methanol, and 4 mL of water. The target compounds were eluted with 4 mL of methanol solution containing 5% ammonia aqueous, and the eluate was concentrated to dryness by nitrogen-blowing at room temperature （approximately 25 ℃). The residue was reconstituted in 500 μL of methanol. An aliquot （200 μL） of the reconstituted solution was mixed with 200 μL of 0.02 mg·L⁻¹ isotope internal standard bis （1-chloro-2-propyl） phosphate-d₁₂ solution, and passed through a 0.22 μm filter membrane. The 12 organophosphate ester metabolites （mOPEs） in the filtrate were analyzed by ultra-high performance liquid chromatography-tandem mass spectrometry. The separation was performed on an Acquity UPLC BEH C₈ column using a mixed solution of methanol and 5 mmol·L⁻¹ ammonium acetate solution at different volume ratios as the mobile phase for gradient elution. Detection was carried out in the negative mode of electrospray ion source and multiple reaction monitoring mode, and quantification was carried out by the internal standard method. As shown by the results, linear relationships between the peak area ratios of the 12 mOPEs to the isotope internal standard and mass concentrations of the targets were kept in the range of 0.5–50 μg·L⁻¹, with detection limits in the range of 0.009–0.100 μg·g⁻¹. Test for recovery was made by the standard addition method, giving results in the range of 83.6%–123%, with RSDs （n=6） of the determined values in the range of 0.70%–11%. This method was applied to analysis of actual farmland soil samples, and it was shown by the results that 12 mOPEs were all detected to varying degrees in the soil samples, with detection ratio greater than 50% for 6 mOPEs, and total mass fractions in the range of 0.660–30.4 ng·g⁻¹ and the average value of 9.01 ng·g⁻¹.