## 3 Results and Discussion
### SPME Time and Temperature Optimization  
To identify the optimal conditions in terms of time and temperature for the extraction of the model mixture, the TIC peak areas of four compounds representative of the chemical diversity of the model mixture, namely \(\alpha\)-pinene, furfural, \(\beta\)-caryophyllene and (E)-\(\beta\)-damascenone were specifically studied. Because extraction of volatile compounds onto an SPME fiber is influenced by their volatility and their polarity, these four compounds were selected according to these properties (Table 3). On the one hand, \(\alpha\)-pinene and furfural were chosen because they have two of the lowest boiling points among all molecules present in the model mixture and because they are relatively apolar and polar, respectively. On the other hand, \(\beta\)-caryophyllene and (E)-\(\beta\)-damascenone both exhibit high boiling points but have different polarities. Despite its higher polarity and boiling point than (E)-\(\beta\)-damascenone, vanillin's TIC peak area is not represented here because of a partial coalition, but its SIM area (\(m/z\) 152) exhibited the same behavior as (E)-\(\beta\)-damascenone's one. \begin{table}
\begin{tabular}{l l l} \hline \hline  & **Boiling Point** (\({}^{\circ}\)**C**) & **Log P (o/w)** \\ \hline \(\alpha\)-pinene & 155–156 & 4.830 1  \\ \hline furfural & 161–162 & 0.410 1  \\ \hline \(\beta\)-caryophyllene & 254–257 & 6.777 1  \\ \hline (E)-\(\beta\)-damascenone & 274–276 & 4.120 2  \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Selected properties of the four studied compounds. The extraction temperature was optimized by following the TIC signal of the four representative compounds at 20 \({}^{\circ}\)C, 30 \({}^{\circ}\)C, 45 \({}^{\circ}\)C, and 60 \({}^{\circ}\)C to maximize their peak areas (Figure 1). Increase in temperature is known to have two antagonistic effects on the extraction: it increases analytes' concentration in the headspace as well as their diffusion coefficient, but also decreases the distribution constant between the sample and the fiber [38]. For furfural, \(\alpha\)-pinene and \(\beta\)-caryophyllene as well as the total area of the chromatogram, the highest peak area was obtained at an extraction temperature of 30 \({}^{\circ}\)C. Because of its low volatility, a low concentration in the model mixture (0.0469 mg/L), and a presumably low affinity for the SPME coating, (E)-\(\beta\)-damascenone was particularly difficult to detect as its peak area was much lower than those of other compounds. Because the largest peak area for (E)-\(\beta\)-damascenone was found to be slightly higher at 45 \({}^{\circ}\)C than at 30 \({}^{\circ}\)C, an extraction temperature of 35 \({}^{\circ}\)C was chosen as the best compromise for the detection of the four compounds. Extraction kinetics were studied by measuring the peak areas after 5, 30, 60, and 120 min of extraction. As shown in Figure 2, at least 60 min of extraction were necessary to reach equilibrium for \(\alpha\)-pinene and furfural. With a decreasing peak area over time, \(\alpha\)-pinene presented a singular kinetic profile. This decrease in peak area over time can be explained by a competition occurring between compounds for their adsorption onto the fiber. Because of its high volatility, the concentration of \(\alpha\)-pinene in the headspace is expected to be high. \(\alpha\)-pinene is therefore extracted rapidly by the sorbent. Nevertheless, over time, \(\alpha\)-pinene might be replaced onto the fiber by other less volatile compounds exhibiting a higher affinity for the fiber, leading to a desorption of \(\alpha\)-pinene and a decrease in \(\alpha\)-pinene's signal during extraction. In the case of \(\beta\)-caryophyllene, equilibrium was not reached during extraction, but peak areas obtained at 60 and 120 min were close. With an extraction time of 60 min, this molecule showed a signal more intense than all three other compounds and was easily detected. On the contrary, the peak area of (E)-\(\beta\)-damascenone was much lower during the entire extraction.