Objective This study aimed to accurately assess the quality and safety level of aquatic products in the national rice-fishery symbiosis system, ensuring the safety of aquatic products for consumption and maintaining the healthy and sustainable development of the fishery industry.

Methods Rice paddy aquatic products such as Procambarus clarkii, Cyprinus carpio and Eriocheir sinensis were chosen as the sample matrix, and the organophosphorus insecticide methamidophos was selected as the research object. The good chromatographic separation and sufficient mass spectrometry response intensity of methamidophos were used to screen the suitable conditions for instrumental determination. Based on the modified QuEChERS technique, the optimal preparation method was preferred by comparatively analyzing the effects of different extraction solvents, such as acetonitrile, ethyl acetate, acidified acetonitrile, acidified ethyl acetate, and different combinations of purification reagents, including PSA, C₁₈, anhydrous magnesium sulphate, and Cleanert LipoNo, as well as different nitrogen blowing temperatures on the recoveries of spiked methamidophos. Finally, an ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was established for the accurate determination of methamidophos residues in aquatic products. Its measurement uncertainty introduced by sample preparation and instrumental analysis were evaluated at the same time, which provided the basis for optimizing the determination procedure and improving the accuracy of the test data.

Results The preparation optimization experiments suggested that the analytes were preferably extracted with acetonitrile, purified by Cleanert LipoNo, concentrated by nitrogen blowing at 40 ℃, separated on a Waters Acquity UPLC BEH C₁₈ column and quantified by external standard method. Under the optimal conditions, the standard curve was linear in the range of 0.1−50.0 µg·L⁻¹ with the correlation coefficient of 0.999 2. The limits of detection and quantification were found to be 0.25 µg·kg⁻¹ and 0.5 µg·kg⁻¹. Due to the thorough sample preparation, the recoveries were 82.1%−96.5% with relative standard deviations less than 11%. In this study, the main sources of uncertainty in measurement were standard curve fitting and recoveries, followed by standard solution preparation, sample repeated measurement, sample weighing and sample processing. The error caused by volumetric determination was the least.

Conclusion Due to the advantages of low matrix interference and better sensitivity than existing literature reports, this method aims to precisely determine the trace residues of methamidophos in aquatic products. During the detection process, it is necessary to control the influence of standard curve fitting and recoveries on the accuracy of the results, so as to scientifically evaluate the residual concentration of methamidophos in aquatic products and dietary risk.