## 3 Results and Discussion
### Optimization of the SPME Procedure  
The SPME procedure was optimized by evaluating salt addition, pH, and sample volume, which significantly influence analyte extraction efficiency. A constant stirring rate of 700 rpm and a polyacrylate (PA) coating were used, as PA has demonstrated good extraction efficiency for R in aqueous solutions [1]. Ethanol (6.6%) was added to enhance the extraction efficiency [37]. Standard mixtures of Q (3.0 \(\mathrm{\SIUnitSymbolMicro g}\)/mL) and R (1.0 \(\mathrm{\SIUnitSymbolMicro g}\)/mL) in 6.6% ethanol were used for optimization, with UV detection. Since the extraction efficiency of some analytes can be enhanced by increasing the ionic strength of the sample solution, leveraging the salting-out effect [38], four NaCl concentrations (0%, 10%, 20%, and 30%) were subjected to SPME and analyzed. As shown in Figure 2A, the presence of NaCl generally led to a significant increase in the amount of both the analytes. The best results were obtained using 10% NaCl, which was selected for the subsequent experiments. The pH is also a crucial factor for the extraction of certain analytes [30]. Consequently, its influence on the extraction capacity was studied conducting experiments with saline solutions (10%NaCl) at different pH values (2.3, 6, and 10). As shown in Figure 2B, the extraction efficiency of both analytes decreased as the pH increased. Therefore, an acidic pH was selected for the next optimization step. In addition, the effect of sample volume was evaluated, since variations in sample volume can significantly affect quantification and accuracy [39]. As shown in the insert in Figure 2C, among the volumes examined, 15 mL is the minimum sample volume ensuring that the extracted amount remains less than 1% of the total analyte present. In contrast, smaller volumes lead to greater depletions of the analyte, reaching up to 5% with a 1.5 mL sample. Once the extraction conditions were optimized, a comparative study on the performance of one homemade polydopamine (PDA) silica fiber and of four commercial LC fibers, i.e., polyacrylate (PA), polyethylene glycol (PEG), polydimethylsiloxane-divinylbenzene (PDMS-DVB), and polydimethylsiloxane (PDMS), was conducted. Figure 3A and Figure 3B reports the extraction time profiles obtained for quercetin and trans-resveratrol, respectively, using the PDA and PA fibers. Both of the fibers effectively extracted the target analytes, while the optimal extraction times were 60 min for Q and 30 min for R. An equilibrium time of 60 min was adopted for further experiments. Figure 2: Extraction efficiency of quercetin (dark grey) and trans-resveratrol(light grey). **(A)** Effects of salt addition (NaCl, g/100 mL); **(B)** effect of pH in the presence of 10% NaCl; **(C)** effect of the sample volume. The insert in figure **(C)** shows the percentage depletion. Fiber used: PA; extraction time: 60 min at room temperature; desorption mode: dynamic. Experiments were subsequently carried out under the same extraction conditions to compare the extraction efficiency of the PDA, PA, PEG, PDMS-DVB, and PDMS fibers. For this purpose, a mixed standard solution (1 \(\upmu\)g/mL) in 6.6% ethanol and 10% NaCl and at pH 2.3 was used. As shown in Figure 4, both analytes were successfully extracted by the PDA fiber, while the best results were observed for the PEG fiber, followed by the PA fiber; on the contrary, the PDMS-DVB and PDMS fibers were the least efficient. These findings were in good agreement with those of [PERSON] et al. [40], who compared the performance of PDMS-DVB and PDMS fibers with that of Carbowax/Templated Resin (CW/TPR) for determining quercetin and myricetin in grapes, vegetables, and red wine samples, with CW/TPR yielding the best results. Although CW/TPR is not currently commercially available, PEG exhibits a comparable degree of polarity. Data from [PERSON] et al. [1] for trans-resveratrol further support these results. The dynamic and static desorption modes [41] were evaluated.