## 2 Results and Discussion  
### HS-SPME Analysis of Area Nuts  
In the first part of this study, the HS-SPME method was optimized. To investigate the conditions of SPME extraction of volatile compounds, the main effects of extraction temperature and extraction time (these variables greatly influence the vapor pressure and equilibrium of the aroma compounds in the headspace of the sample) and their interactions were studied by using a central composite design (CCD) based on the response surface methodology (RSM), including four replicates at the central point. The extraction time and extraction temperature were studied in the range of 10-90 min and 40-120 \({}^{\circ}\)C, respectively. For each extraction run, 250 mg of crushed area nuts was placed in a 20 mL headspace vial sealed with a screw cap equipped with a Teflon septum. The entire matrix of the experimental design with experimental values is summarized in Table 1. Optimal extraction conditions were evaluated in STATISTICA software, version 12 (StatSoft CR, Prague, Czech Republic) based on the number of peaks in the chromatogram (NoP) as a response variable. The results of the multiple regression analysis are presented in Table 2, and the response surface model obtained for NoP is depicted in Figure 1. The model shows the effects of the extraction temperature and extraction time on the number of peaks detected in the chromatograms. The relationship between the independent variables and their responses depicted in Figure 1 is characterized by the following equation:  
\[\mathit{NoP}\ =\ -\ 55.22135+2.15T_{\mathit{ext}}\ -\ 0.00961T_{\mathit{ext}}^{2} \ +\ 0.85885t_{\mathit{ext}}\ -\ 0.00617t_{\mathit{ext}}^{2}\]  
where \(\mathit{NoP}\) is the number of peaks in the chromatogram, \(T_{\mathit{ext}}\) is the extraction temperature (\({}^{\circ}\)C), and \(t_{\mathit{ext}}\) is the extraction time (min). The adjusted root square value (R\({}^{2}\) = 0.994) shows the high reliability of the model. The data in Table 1 demonstrate an extremely close agreement between the experimental and predicted values of the response variables. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Run**} & \multicolumn{2}{c}{**Coded Factors**} & \multicolumn{2}{c}{**Decoded Factors**} & \multicolumn{2}{c}{**Response**} \\ \cline{2-7}  & \(T_{ext}\) & \(t_{ext}\) & \(T_{ext}\) (\({}^{\circ}\)C) & \(t_{ext}\) (min) & \multicolumn{2}{c}{**NoP**} \\ \cline{2-7}  & & & & & **Observed** & **Predicted** \\ \hline
1 & \(-1\) & \(-1\) & 40 & 10 & 24 & 23 \\
2 & \(-1\) & 1 & 40 & 90 & 44 & 43 \\
3 & \(-1\) & 0 & 40 & 50 & 41 & 43 \\
4 & 0 & \(-1\) & 80 & 10 & 63 & 63 \\
5 & 0 & 1 & 80 & 90 & 81 & 83 \\
6 (C) & 0 & 0 & 80 & 50 & 84 & 83 \\
7 (C) & 0 & 0 & 80 & 50 & 85 & 83 \\
8 (C) & 0 & 0 & 80 & 50 & 83 & 83 \\
9 (C) & 0 & 0 & 80 & 50 & 81 & 83 \\
10 & 1 & \(-1\) & 120 & 10 & 72 & 72 \\
11 & 1 & 1 & 120 & 90 & 92 & 92 \\
12 & 1 & 0 & 120 & 50 & 92 & 92 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimental conditions used for the central composite design analysis and the corresponding observed and predicted responses for HS-SPME (\(T_{ext}\) = extraction temperature; \(t_{ext}\) = extraction time; (C) = central point; \(NoP\) = number of peaks in the chromatogram).