Final Residues, Storage Stability, and Dietary Exposure of Pyroxasulfone and Diflufenican in Wheat Grain and Straw

This study developed a UHPLC-MS/MS method combined with a modified QuEChERS pretreatment technique to simultaneously determine diflufenican, pyroxasulfone, and their metabolites (M-1, M-3, M-25) in wheat grain and straw. The method exhibited good linearity within the concentration range of 0.0001–0.1 mg/L (r > 0.996), with a limit of quantification of 0.002 mg/kg. At three spiked levels, the average recoveries of the target analytes ranged from 82.0% to 110.1%, with relative standard deviations ≤ 13.9%. All analytes remained stable in both grain and straw matrices when stored at ≤−18 °C for 12 months. Field trials demonstrated that diflufenican residues in grains were all below 0.002 mg/kg, while residues in straw ranged from 0.002 to 0.018 mg/kg. The total residues of pyroxasulfone and its metabolites in grains ranged from 0.010 to 0.019 mg/kg, and in straw from 0.026 to 0.357 mg/kg. Dietary risk assessment results showed that the risk quotients for diflufenican and pyroxasulfone were 0.10% and 0.27%, respectively, both far below the 100% safety threshold. Based on the residue data and risk assessment results, it is recommended that the maximum residue limits for pyroxasulfone in wheat in China be set at 0.03 mg/kg for grains and 0.6 mg/kg for straw. These proposed values align with international standards while also accommodating the practical needs of domestic agricultural production and regulatory oversight.