## 2 Results and Discussion
### Optimization of the SPME Extraction Using the Prepared PDMS Needle  
The prepared needle coated with PDMS was used for the isolation of pesticides from spiked solutions in order to evaluate and improve its extraction capability by adjusting the main parameters to achieve optimal removal of the target analytes from the aqueous sample by adsorption on the sorbent layer, then desorption into the injection port of the chromatographic system. This evaluation was first conducted on the effect of extraction volume; four distinct volumes of a standard solution of pesticides (with concentrations of 100 ng mL\({}^{-1}\)) were tested: 10, 30, 50, and 70 mL. The intensity of the signal increased from 10 to 50 mL, then diminished at 70 mL for most pesticides. This decrease was attributed to the limited capacity of the needle coating to absorb higher amounts of analyte. Thus, as can be seen in Figure 3A, the best extraction efficiency was achieved for an extraction volume of 50 mL, which corresponded to the highest peak areas. In the case of liquid samples, two extraction modes can be used in SPME: direct immersion (DI-SPME) and headspace (HS-SPME). In DI-SPME, the needle is completely immersed in the aqueous phase of the sample, and this is the method of choice for analyzing \"clean\" aqueous samples, because it is more sensitive. On the other hand, in HS-SPME, the needle is placed in the vapor phase (headspace) above the liquid solution for the extraction of the volatile compounds. The two techniques have been applied and compared; the DI-SPME allowed for the extraction of all the 13 investigated pesticides, and it exhibited  
Figure 3: Optimization of SPME extraction: effect of (**A**) volume of extraction; (**B**) extraction mode; (**C**) time of extraction. The peak area is given as a relative value for each pesticide, with the corresponding standard deviation. greater peak intensities than those obtained by HS-SPME, which could extract only six analytes, as shown in Figure 3B. In another step, the SPME optimum extraction time was assessed using the homemade PDMS needle in order to enhance the SPME method performance. The following extraction times were evaluated: 10, 20, 30, and 40 min. The results showed that almost all target pesticides reached their extraction equilibrium at 20 min. However, diazinon showed the lowest extraction yield during all the evaluated time ranges. On the other hand, as shown in Figure 3C, most analytes reached the maximum level of extraction at 20 min. Therefore, this extraction time was selected as the optimum value to allow for a faster extraction with better efficiency. In this study, the influence of stirring the water solution was also evaluated by comparing our results to those of previous studies and selecting the optimal conditions. It was shown that it was possible to reduce the time to achieve equilibrium between the aqueous phase and the needle coating by a thorough agitation of the water sample [47]. On the other hand, the accumulation of the analyte in the polymeric layer is regulated by its diffusion in both the aqueous matrix and the SPME coating, as stated by [PERSON] et al. [48]. Additionally, in the static situation, the aqueous layer generated on the surface of the sorbent slows down the absorption rate by restricting the transfer of the analyte from the aqueous phase to the solid phase. In contrast, in the dynamic mode (i.e., while being stirred) a thinner layer of water accumulates on the polymer surface, which favors analyte diffusion [48]. Another phenomenon has been observed by [PERSON] et al. [49] when the aqueous sample was stirred, and the needle immersed in the sample remained immobilized. This stirring mode caused the effect of washing the sorbent surface and induced a reduction in the extraction efficiency of the analytes as the stirring velocities increased. Additionally, another previous study reported that a rapid stirring could reduce the extraction yield because of the air bubble formation on the fiber surface [50].