## 3 Results
### The Optimization of the Extraction Conditions  
Various parameters that influenced the extraction efficiency of the SPME of PAEs on the COF-TPB-DMTP coated fiber, such as the extraction temperature, extraction time, agitation speed and ionic strength, were optimized. SPME is a dynamic equilibrium extraction technique, so the number of adsorbed analytes increases with the lengthening of the extraction time until dynamic equilibrium is reached [58]. The effect of the extraction time was optimized from 5 to 35 min (Figure 5a). The peak areas of the PAEs rapidly increased with the variation in the extraction time from 5 min to 30 min. To achieve the best extraction efficiency, 30 min was chosen as the extraction time.  
The higher temperature could facilitate the diffusion of analytes, which enabled shortening the time that the analytes required to reach the adsorption equilibrium. However, an excessive temperature could decrease the distribution coefficient of PAEs between the coating and the sample matrix, which could result in a drastic drop in the extraction efficiency. The effect of the extraction temperature was examined from 25 \({}^{\circ}\)C to 60 \({}^{\circ}\)C (Figure 5b). It can be seen that the extraction efficiency dropped sharply when the extraction temperature was over 30 \({}^{\circ}\)C. Therefore, 30 \({}^{\circ}\)C was chosen as the optimized extraction temperature in the further studies.  
Figure 5: The effects of extraction parameters on the extraction efficiency for the PAEs: (**a**) extraction time (conditions: extraction temperature, 30 \({}^{\circ}\)C; desorption time, 2 min; desorption temperature, 280 \({}^{\circ}\)C; NaCl concentration, 5%; agitation speed, 250 rpm); (**b**) extraction temperature (conditions: extraction time, 30 min; desorption time, 2 min; desorption temperature, 280 \({}^{\circ}\)C; NaCl concentration, 5%; agitation speed, 250 rpm); (**c**) agitation speed (conditions: extraction time, 30 min; extraction temperature, 30 \({}^{\circ}\)C; desorption time, 2 min; desorption temperature, 280 \({}^{\circ}\)C; NaCl concentration, 5%); (**d**) NaCl concentration (conditions: extraction time, 30 min; extraction temperature, 30 \({}^{\circ}\)C; desorption time, 2 min; desorption temperature, 280 \({}^{\circ}\)C; agitation speed, 250 rpm). The extraction solution contained DIBP 30 \(\mathrm{\SIUnitSymbolMicro}\)g\(\cdot\)L\({}^{-1}\), DBP and DPP 10 \(\mathrm{\SIUnitSymbolMicro}\)g\(\cdot\)L\({}^{-1}\), and BBP and DEHP 50 \(\mathrm{\SIUnitSymbolMicro}\)g\(\cdot\)L\({}^{-1}\).  
The effect of the agitation speed was evaluated from 50 rpm to 550 rpm (Figure 5c). The results show that the peak areas of the five PAEs increased when the agitation speed varied from 50 rpm to 250 rpm. Once the agitation speed was over 250 rpm, a reduction in the extraction efficiency was observed. This could be attributed to the appearance of the vortex, which could lead to the removal of the fiber from the sample matrix. So, an agitation speed of 250 rpm was chosen in the following experiments.  
Since the ionic strength could cause a variation in the viscosity, the effect of the ionic strength was discussed, which was replaced with a certain concentration of NaCl (Figure 5d). With the concentration of NaCl up to 5%, the peak areas of the five PAEs increased due to the \"salting-out\" effect. Instead, the extraction efficiencies showed a negative result with the further improvement of the concentration of NaCl due to the increased viscosity, which could hinder the diffusion of PAEs [20]. Therefore, 5% NaCl concentration was added into the sample metrics for the SPME.