## 2 Results
### Optimization of HS-SPME on the DVB/CAR/PDMS fiber
It can be seen that PC1 describes the greater variation in the volatile composition detected by HS-SPME/GC-MS within the experimental domain, which occurs when we move from the two maxima of the response surface, (%NaCl = 10, T = 50 \({}^{\circ}\)C) and (%NaCl = 30, T = 30 \({}^{\circ}\)C)  
Figure 6: Optimization of HS-SPME collection of wine volatile profiles with the DVB/CAR/PDMS fiber: (**a**) response surface for the peak area of 3-methyl-1-butanol (A\({}_{\rm MB}\)); (**b**) coefficients of the surface model: \(p<0.01\) (**), \(p<0.001\)(***). located at negative scores, towards the minimum region (represented by the central point and the other two points of the 2\({}^{2}\) DoE), mainly located at positive score values. Most of the volatile components with positive loadings are relatively polar compounds (alcohols, organic acids, and low molecular weight esters) including MB. On the other hand, the greater values of the A\({}_{\text{TOT}}\) response, identified by negative scores on PC1, are linked to a preferential extraction of the esters. Interestingly, the two maximum conditions, which are well separated along PC2, can be differentiated according to the kind of esters preferentially extracted. More specifically, higher molecular weight esters are preferably extracted at %NaCl = 10 and T = 50 \({}^{\circ}\)C, whereas extraction of low molecular weight esters is favored at %NaCl = 30 and T =30 \({}^{\circ}\)C. It seems that extraction of polar compounds, which is exalted at the intermediate and highest levels of both T and %NaCl factors, is favored by a salting out effect combined with the easier volatilization of these compounds at higher temperatures, both effects promoting partitioning of the volatiles into the fiber sorbent. Roughly, the distribution of the esters along PC2 follows the inverse order of volatility: long chain esters with higher boiling points have negative loadings, while lower molecular weight and more volatile esters have positive loadings. In this regard, previous investigations regarding the PDMS fiber [31] showed that salting out is more effective for volatile esters, whereas the positive effect of salting is limited to low added concentrations or produces a negative effect as volatility of the esters decreases. As concerning the effect of temperature, a previous investigation regarding extraction of wine components by SPME on PDMS and DVB/CAR/PDMS fibers [31] has highlighted that the increase of temperature in the range 30-60 \({}^{\circ}\)C produces a monotonic decrease in the extraction of the most volatile esters, a progressive increase for the less volatile ones, whereas the semi-volatile compounds exhibit a maximum in the extraction efficiency at intermediate temperatures. It is also known that less volatile esters need a longer exposure time to achieve the equilibrium between the sample and the sorbent [30; 31] compared to more volatile compounds. It is likely that an exposure time of 30 min, selected in this study, was insufficient to have equilibration of the less volatile esters at the lower temperature level here investigated. The combination of the above effects can explain the preferential extraction of less volatile esters, including ED, at high temperature and low NaCl content and the preferential extraction of more volatile esters at low temperature and high salt concentration. Figure 7: Scores (red triangles) and loadings (blue circles) projected on the plane of the first two principal components (PC1 and PC2) extracted from the GC chromatograms collected with the DVB/CAR/PDMS fiber in the DOE points. Experimental conditions of the DOE points are identified by %NaCl and T values reported in square brackets.