## 3 Results and Discussion
### Method Validation  
The proposed method was validated and applied to determine the concentration of 119 trace volatile compounds in Baijiu using HS-SPME-GC\(\times\)GC-TOFMS. The quantitative method for 26 esters, 11 alcohols, six acids, 24 aldehydes and ketones, six furans, eight pyrazines, 11 sulfur compounds, seven phenols, 16 terpen, and four lactones was constructed using a model synthetic solution under the optimal conditions. The performance of the method regarding linearity, detection limits, LOD, LOQ, precision, and accuracy for each compound are shown in Table 2.  
Good linearity could be obtained for all volatile compounds at the concentration studied, with coefficients of determination (\(R^{2}\)) above 0.99. The developed method had good precision because all RSD values calculated for intraday precision varied between 0.14% and 9.34% and interday precision varied between 0.14% and 9.93%. Moreover, the recovery values varied from 86.79% to 111.94%, which indicated that the developed method was accurate for determining trace compounds in Baijiu. The lowest LOD and LOQ of all compounds were for methyl nonyl ketone, 0.04 ng/L and 0.14 ng/L, respectively, and the highest LOD and LOQ were for hexanol (12.93 ug/L and 43.09 ug/L). Some of these compounds were hundreds of times lower than those reported in the literature using HS-SPME-GC-MS analysis [35; 36]. For instance, the LOD of 3-octanol in this study was 6.98 ng/L, which was 27 times lower than that of HS-SPME-GC-MS, which had a LOD of 189.39 ng/L [37]. Eleven sulfur compounds were quantitatively analyzed using the optimized method, and the LOQ of 1.36 ng/L for dimethyl trisulfide was 198 times lower than the 0.27 ug/L achieved with the GC-PFPD [10]. These results indicated that this method had obvious quantitative analysis advantages compared with GC-MS and even specific element analysis instruments, and it was an effective method for quantitative analysis of trace compounds in Baijiu. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Volatile Compound**} & \multicolumn{4}{c}{**Peak Area Percentage (\%) a**} \\ \cline{2-5}  & **Water** & **SS** & **SS + HCV** & **Baijiu b** \\ \hline
**Esters** & & & & \\ Phenethyl butyrate & 100 & 90 & 52 & 48 \\ Ethyl cinnamate & 100 & 71 & 41 & 38 \\ Methyl benzoate & 100 & 98 & 95 & 88 \\
**Alcohols** & & & & \\
1-Nonanol & 100 & 47 & 20 & 18 \\
1-Octen-3-ol & 100 & 70 & 59 & 58 \\ Benzyl alcohol & 100 & 72 & 35 & 29 \\
**Aldehydes and ketones** & & & & \\
1-Octen-3-one & 100 & 82 & 56 & 43 \\ E-2-heptenal & 100 & 77 & 33 & 30 \\ Trans-2-nonenal & 100 & 87 & 47 & 49 \\
**Furans** & & & & \\
2-Acetylfuran & 100 & 62 & 23 & 16 \\ Ethyl 2-fluoroate & 100 & 90 & 62 & 52 \\
**Pyrazines** & & & & \\
2,3-Dimethylpyrazine & 100 & 52 & 16 & 13 \\
2-Ethyl-6-methylpyrazine & 100 & 59 & 25 & 16 \\
2,3-Diethylpyrazine & 100 & 68 & 75 & 69 \\
**Sulfur compounds** & & & & \\ Methyl thiobutyrate & 100 & 60 & 29 & 18 \\ Thiazole & 100 & 27 & 15 & 11 \\ Ethyl 3-methylhiopropionate & 100 & 88 & 44 & 44 \\
**Phenols** & & & & \\
4-Methylgluaiacol & 100 & 90 & 74 & 77 \\
4-Methylphenol & 100 & 96 & 74 & 69 \\
4-Ethylphenol & 100 & 88 & 87 & 86 \\
**Terpenes** & & & & \\ Isophorone & 100 & 90 & 57 & 52 \\ \(\beta\)-Cyclocitral & 100 & 82 & 77 & 69 \\ Geraniol & 100 & 83 & 76 & 71 \\
**Lactones** & & & & \\ \(\gamma\)-Valerolactone & 100 & 53 & 30 & 32 \\ \(\gamma\)-Butyrolactone & 100 & 56 & 39 & 13 \\ \(\gamma\)-Nononactone & 100 & 92 & 81 & 80 \\ \hline \hline \end{tabular} a: The ratio of the peak area of volatile compounds in different matrices to the peak area in water. b: The real Baijiu (Guotai, 53% vol) was diluted to 50% vol.  
\end{table} 
Table 1: Comparison of peak areas of volatile compositions in various matrices. Figure 4: Standard curves of different chemical classes of compounds in the quantitative analytes: (**A**) SS and (**B**) SS + HCV. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Volatile Compounds** & \begin{tabular}{c} **Linear Range** \\ **(\#g/L)** \\ \end{tabular}  &