## 2 Results
### Optimization of HS-SPME on the PDMS Fiber
Starting from the 0 level for %NaCl (20%), a moderate A\({}_{\rm TOT}\) decrease is observed when %NaCl both rises and decreases, whereas the total area of the chromatogram dramatically decreases when the temperature grows, regardless of the NaCl content in the sample. The surface model developed for A\({}_{\rm ED}\) (Figure 2) results are significant (\(p\) = 0.0064) according to ANOVA and exhibit a relatively high R\({}^{2}\)\({}_{\rm adj}\) value (0.876), but provides a noticeably worse predictive performance (Q\({}^{2}\)\({}_{\rm adj}\)= 0.5625) and a significant lack-of-fit (\(p\) = 0.035). In this regard, it must be noted that the T\({}^{2}\) term, despite being non-significant, was anyway retained in the surface model because its elimination resulted in further worsening of the Q\({}^{2}\)\({}_{\rm adj}\) value. Nevertheless, the so-obtained surface model, although it may be unsuitable for quantitative prediction of the response, can be accurate enough to describe the qualitative trend of A\({}_{\rm ED}\) within the experimental domain. It can be noted that the shape of the A\({}_{\rm ED}\) surface is not much dissimilar from that of A\({}_{\rm TOT}\), but the maximum of the first response is slightly shifted toward the center of the experimental domain (T = 37 \({}^{\circ}\)C and %NaCl = 13). The similarity between the two surfaces is not unexpected because ED is one of the volatile components contributing the most to the wine volatilome. In this regard, it must be noted that the relative contribution of the ED peak area to the total area of the chromatogram noticeably increases at the maximum temperature and minimum NaCl concentration. Development of the surface model for A\({}_{\rm MB}\) by quadratic regression revealed that all the coefficients describing the effect of t were non-significant. Therefore, this factor was removed, and the response was described bya second-degree polynomial model depending on T and %NaCl, that was further simplified because also T\({}^{2}\) and %NaCl\({}^{2}\) coefficients were non-significant. The final surface (Figure 3) suggests a progressive increase of the response when both T and %NaCl increase. The observed behavior of MB can be interpreted by the enhanced release of this compound from the liquid sample to the headspace, and then to  
Figure 3: Optimization of HS-SPME collection of wine volatile profiles with the PDMS fiber: (**a**) response surface for the peak area of 3-methyl-1-butanol (A\({}_{\rm MB}\)); (**b**) coefficients of the surface model; \(p\) < 0.05 (\({}^{\circ}\)), \(p\) < 0.001(***). the sorbent, promoted by temperature rising, and by the decrease of its solubility in the wine sample determined by the increase of ionic strength when NaCl is added [16]. Different from the behavior of MB, the total area of the chromatogram and, at high ionic strength, that of ED exhibit a noticeable decrease when temperature rises. In addition, a moderate salting out effect seems to only influence A\({}_{\text{TOT}}\) at low %NaCl values (roughly in the range 10-20%), whereas global extraction efficiency slightly decreases at higher salt concentrations. A qualitatively similar effect is observed for A\({}_{\text{ED}}\) at low and intermediate temperatures, whereas, at higher temperatures, the extraction efficiency of ED monotonically decreases when the salt content grows. The increase in temperature, as previously discussed, increases the concentration of the volatiles in the headspace, but partition between headspace and sorbent, which is an exothermic process, is reduced at higher temperature and, consequently, the extraction can be inhibited [16]. In the case of ED, the latter effect seems to prevail on the first, as already documented for several ethyl esters in raw spirits and model wine samples, including ED [30; 31]. Since the esters are the volatile components contributing the most to the wine volatilome, it is reasonable that the temperature effects on A\({}_{\text{ED}}\) and of A\({}_{\text{TOT}}\) are qualitatively similar.