## 2 Results
### Optimization of HS-SPME-GC \(\times\) GC-TOFMS Methodology  
The efficiency of HS-SPME extraction depends on several experimental parameters, such as the fiber coating, extraction time, temperature, sample amount, sample dilution, and amount of salt (NaCI). So, to increase the effectiveness of this technique, a detailed optimization of each step was carried out and the best conditions have been chosen according to the total number of monoterenes, sesquiterpenes, and C\({}_{13}\)-norisorprenoids identified, because these compounds play a key role in the differentiation of the viniferous varieties. The extraction efficiency of the sample preparation technique greatly depends on the value of the distribution constant of analytes partitioned between the sample and the fiber coating material; consequently, the selection of a suitable fiber coating is crucial. The DVB/CAR/PDMS fiber has been described in the literature as having a good affinity for the monoterenes and C\({}_{13}\)-norisorprenoids with a lower and higher boiling point [36]. According to [PERSON] et al. 's 2010 study [37], these fibers ensure the extraction of compounds with an extensive range of molecular masses (40-275 m/z). Thus, the \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Year** & \
\begin{tabular}{c} **Potential Alcohol Degree** \\ **(\% vol)** \\ \end{tabular} & **pH** &
\begin{tabular}{c} **Total Acidity** \\ **(g L\({}^{-1}\))\({}^{1}\)** \\ \end{tabular} \\ \hline \multirow{3}{*}{Trinc} & 2021 & 12.9 & 3.17 & 5.51 \\  & 2022 & 13.0 & 4.70 & 3.60 \\ \multirow{3}{*}{CS} & 2021 & 13.1 & 3.11 & 7.82 \\  & 2022 & 13.2 & 3.40 & 6.72 \\ \multirow{3}{*}{TB} & 2021 & 13.6 & 3.66 & 4.74 \\  & 2022 & 14.3 & 3.59 & 4.27 \\ \multirow{3}{*}{Sy} & 2021 & 13.6 & 3.43 & 4.89 \\  & 2022 & 13.3 & 3.40 & 5.84 \\ \multirow{3}{*}{Cast} & 2021 & 12.9 & 3.31 & 5.35 \\  & 2022 & 13.0 & 3.66 & 5.44 \\ \hline \hline \end{tabular} \({}^{1}\)Tartaric acid.  
\end{table} 
Table 1: Chemical characterization of different red grape samples. 50/30 \(\upmu\)m DVB/CAR/PDMS fiber was automatically selected for the adsorption of the target compounds in this study. Initially, to optimize the sample quantity, 2, 4, and 6 g of red grapes were tested. Figure 1a shows the results obtained for each quantity in the study. Based on the findings, employing 2 g of the sample led to the identification of thirty-seven compounds (twenty-five sesquiterpenes, ten monoterpenes, and two C\({}_{13}\)-norisoprenoids). Increasing the sample amount to 4 g resulted in the detection of forty-three compounds (twenty-seven sesquiterpenes, fourteen monoterpenes, and two C\({}_{13}\)-norisoprenoids). Likewise, when utilizing 6 g of grapes, thirty-eight compounds were detected (twenty-five sesquiterpenes, eleven monoterpenes, and two C\({}_{13}\)-norisoprenoids). The decrease in the number of compounds when 6 g of grapes were used can be explained by the possible competition between the analytes of interest and the remaining volatile compounds in the fiber coating. Therefore, 4 g was selected as the sample amount in the following study. It is important to highlight that this study was performed in triplicate and the number of identified compounds was the same between repetitions. The use of NaCl increases the ionic strength of the samples, and affects the decrease in the solubility of the compounds and their partition coefficient, improving the extraction of the analytes [38]. So, in this work 2, 4, and 6 g of salt were tested, starting with 2 g of NaCl, because this amount prevents the grape's fermentation. In fact, the presence of fermentative compounds could decrease the sensitivity of the fiber by competing with the target analytes at the time of adsorption. Figure 1b shows that the increase in the amount of salt leads to a loss in the number of terpene compounds identified.