## 3 Results and Discussion
### Optimization of Extraction Conditions  
To achieve a higher recovery for the target insecticides, different experimental conditions were optimized. Each parameter was optimized while all the others were fixed at their optimized value. Ultrapure water or methanol-water solvent with spiked insecticide concentration at 50 ng g\({}^{-1}\) was used to examine the extraction recovery. All the experiments were carried out in triplicate, and the means of results were reported.  
Desorption of the analytes from the magnetic sorbent was investigated using the commonly used organic solvent, i.e., ethyl acetate, acetone/chloroform (\(v\)/\(v\) = 1:1), acetone, acetone/ethyl acetate (\(v\)/\(v\) = 1:1), and acetonitrile. Among the potential eluent solvents, ethyl acetate as an aprotic solvent with intermediate polarity exhibited the highest eluting power for all insecticides, showing significant difference form the protic solvent acetonitrile (Figure S3a). Thus, ethyl acetate was selected as the desorption solvent. For completely desorbing the analytes from the 3D-rGOPFH, the influence of the desorption time from 2 to 40 min was also studied. The best desorption efficiency was achieved when 20 min was used (Figure S3b).  
To assure the maximum adsorption amount of analyte, the effect of extraction time on the extraction efficiency was optimized by increasing the time from 2 to 40 min. As shown in Figure S3c, for most of the insecticides, 20 min was needed to achieve the optimal adsorption amount. For mirex and bifenthrin, 2 min was enough to get equilibrium, nevertheless, extraction recovery decreased slightly when a longer time was used. The reduction in extraction efficiency may be attributed to back-extraction effect after the equilibrium due to the issue of solubility of these analytes in water [35]. To achieve an appropriate extraction efficiency for all the insecticides, 20 min was selected for subsequent experiments. Meanwhile, the effect of the amount of the 3D-rGOPFH on the extraction was studied by using different amounts ranging from 2.5 to 30 mg. As shown in Figure S3d, the maximum extraction efficiency was obtained at  
Figure 3: Evaluation of the extraction efficiency for different insecticides using different magnetic sorbents. The standard deviation (SD) bars were marked (\(n\) = 5).  
20 mg of 3D-rGOPFH. No further increase in the sample recovery rate can be observed when a larger absorbent dose was applied. In subsequent experiments, effects of the sample solution pH (from 3 to 11) and ionic strength (with NaCl addition from 0 to 15% (_m/v_)) on the extraction efficiency were explored. The results indicated that there was no need to adjust the pH of the sample solution (Figure S3e) during the extraction process, and the addition of NaCl was not necessary (Figure S3f).  
Because methanol was adopted in the real-world sample analysis, the effect of methanol content on the MSPE of insecticides should be discussed. The addition of methanol was investigated over the range of 0-50% (_v/v_). In Figure S3g, the results demonstrated that the use of a small amount of methanol (0-20%) did not affect the extraction efficiency.  
To examine whether the target analytes were desorbed sufficiently, the effect of the volume of ethyl acetate from 0.5 to 2.0 mL was further explored. Figure S3h clearly shows that 1 mL is the optimal desorption volume. No further increase in the sample recovery rate was noticed when a larger desorption volume was applied. Single time desorption was sufficient for achieving high recovery.