## 2 Results
### Optimization of HS-SPME on the PDMS Fiber  
To maximize chemical information on the volatile profile of the wine aroma, we primarily investigated, by DoE-RSM, the influence of the SPME conditions on the total area of the GC/MS chromatogram, A\({}_{\text{TOT}}\). This choice identifies a unique set of optimum conditions for the extraction of all the target analytes. All of the experiments were performed on the same wine sample belonging to the TRE variety. In the investigation of the performance of the PDMS fiber, the combinations of the exposure time (t) of the fiber to the headspace, the sample temperature (T), and the percentage (_w/v_) of NaCl (%NaCl) were defined according to a Box-Behnken DoE [20] with duplicate experiments in the central point. To evaluate the dependence of the fiber coating and SPME conditions on the extraction recovery of specific wine aroma components in more detail, we also modeled, by DoE-RSM, the dependence of the peak-area of two selected compounds, ethyl decanoate (ED) and 3-methyl-1-butanol (MB), from the SPME conditions. These two compounds, which can represent apolar and polar volatile components, respectively, were selected because they provide relative intense chromatographic peaks within the whole experimental domain. This condition minimizes experimental error in the collection of model responses. The matrix of the experiments and the experimental responses are displayed in Table 2. The surface models for the three responses were generated by quadratic regression after transformation of the values of the three factors (t, T, and %NaCl) into the corresponding levels (\(-1\), \(0\), or \(1\)). The final surfaces and the model coefficients for A\({}_{\text{TOT}}\), A\({}_{\text{ED}}\), and A\({}_{\text{MB}}\) responses are displayed in Figure 1, Figure 2, and Figure 3, respectively, whereas the calculated responses in the DoE points are given in Table 2. The reliability of the surface models was also evaluated by the adjusted coefficients of determination in fitting and leave-one-out cross-validation (R\({}^{2}\)adj and Q\({}^{2}\)adj, respectively) and by analysis of variance (ANOVA) [20], reported in Table S2 (Supplementary Material). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**t(min)**} & \multirow{2}{*}{**NaCl (\%)**} & \multirow{2}{*}{**T(\({}^{\circ}\)C)**} & \multicolumn{2}{c}{**A\({}_{\text{TOT}}\) 10\({}^{-6}\)**} & \multicolumn{2}{c}{**A\({}_{\text{ED}}\)\(\cdot\)10\({}^{-6}\)**} & \multicolumn{2}{c}{**A\({}_{\text{MB}}\)\(\cdot\)10\({}^{-6}\)**} \\ \cline{5-10}  & & & **exp** & **calc** & **exp** & **calc** & **exp** & **calc** \\ \hline
10 & 10 & 40 & 9.16 & 8.73 & 2.66 & 2.61 & 0.46 & 0.49 \\
30 & 10 & 40 & 13.18 & 12.37 & 4.98 & 4.54 & 0.60 & 0.49 \\
10 & 30 & 40 & 9.04 & 9.20 & 1.4  
Figure 2: Optimization of HS-SPME profiling of wine volatiles extracted with the PDMS fiber: (**a**) response surface for the peak area of ethyl decanoate (A\({}_{\text{ED}}\)) at t = 30 min; (**b**) coefficients of the surface model; \(p<0.05\) (*), \(p<0.01\)(**). Figure 1: Optimization of HS-SPME profiling of wine volatiles extracted with the PDMS fiber: (**a**) response surface for the total area of the GC chromatogram (A\({}_{\text{TOT}}\)) at t = 30 min; (**b**) coefficients of the surface model; \(p<0.05\) (*), \(p<0.01\)(**), \(p<0.001\) (***). Figure 1a displays the dependence of A\({}_{\rm TOT}\) from T and %NaCl, t being fixed at its maximum level (30 min). Since the t - -T and t-%NaCl interactions are non-significant, the shape of this surface does not depend on the exposure time of the fiber to the sample headspace, although the response decreases when t decreases because of the positive value of the t coefficient and non-significance of the t\({}^{2}\) term (Figure 1b). The maximum for A\({}_{\rm TOT}\) can be found at t = 30 min, T = 30 \({}^{\circ}\)C, and %NaCl = 20.