## 2 Results and Discussion
### Comparison of Pretreatment Methods  
Figure 3 shows the 2D plot of HAB extracts obtained by three different pretreatment methods (HS-SPME, SPE, and SBSE). Table S1 lists the 606 compounds identified in this study, grouped according to different chemical classes. This result showed the most detailed characterization of volatile constituents in HAB for the first time. Figure 4 presents the correlation of HAB analytes identified by three pretreatment methods, and only 205 compounds were commonly identified, which shows the great difference among the three pretreatment methods. Table 1 compares the number of compounds identified in each class using HS-SPME, SPE, and SBSE. Figure 3: Total ion chromatogram (TIC) contour plot obtained from the HS-SPME-GC\(\times\)GC-TOFMS, SPE-GC\(\times\)GC-TOFMS, and SBSE-GC\(\times\)GC-TOFMS analysis of herbaceous aroma-type Baijiu, and 4 classes of compounds distributed in contour plot (red balls are nitrogenous compounds, green balls are sulfides, gray balls are terpenes, and blue balls are lactone compounds). HS-SPME is based on the establishment of partition equilibrium of the analytes between the polymeric stationary phase, which covers a fused silica fiber, and the matrix of the sample. It is a simple, rapid, and inexpensive technique in which the extraction and concentration processes are performed simultaneously; furthermore, only small sample volumes are required [29]. A total of 409 volatile compounds were identified by HS-SPME-GC\(\times\)GC-TOFMS, including esters, alcohols, sulfides, and terpen; however, lactones and nitrogenous compounds were poorly detected. The results indicated that HS-SPME was particularly beneficial for the analysis of volatile compounds, but defective for the extraction of some high-boiling volatile compounds. SPE has also been reported to have the functions of extraction, enrichment, and rinsing. Abundant adsorbent material can provide high extraction capacity when properly optimized [21]. SPE is supposed to be the complementary nature of the extraction techniques for HS-SPME, which is best exploited for the analysis of semi-volatiles. Several peaks present at the end of the contour plot by SPE were not detected when using HS-SPME, such as \(\gamma\)-dodecactone, ethyl _cis_-9-octadecenoate, ethyl linoleate, and ethyl vanillate. In addition, more nitrogenous compounds were detected when using SPE protocol. However, some volatile compounds may be lost, including terpenes and volatile sulfides, because the Baijiu sample was exposed to atmosphere during extraction. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Class**} & \multicolumn{4}{c}{**Number of Compounds**} \\ \cline{2-5}  & **SPME** & **SPE** & **SBSE** & **Total** \\ \hline Esters & 125 & 125 & 121 & 179 \\ Aldehydes \& Ketones & 71 & 66 & 66 & 111 \\ Terpenes & 62 & 30 & 45 & 82 \\ Alcohols & 57 & 47 & 50 & 81 \\ Sulfides & 29 & 20 & 27 & 37 \\ Furans & 20 & 18 & 21 & 29 \\ Nitrogenous compounds & 6 & 23 & 18 & 29 \\ Acids & 19 & 18 & 20 & 23 \\ Phenols & 13 & 14 & 15 & 18 \\ Lactones & 7 & 12 & 12 & 17 \\ Total & 409 & 373 & 395 & 606 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of volatile compounds detected in Chinese herbaceous aroma-type Baijiu using HS-SPME-GC\(\times\)GC-TOFMS, SPE-GC\(\times\)GC-TOFMS, and SBSE-GC\(\times\)GC-TOFMS, and SBSE-GC\(\times\)GC-TOFMS. Figure 4: Comparison of identification compounds obtained by HS-SPME-GC\(\times\)GC-TOFMS, SPE-GC\(\times\)GC-TOFMS, and SBSE-GC\(\times\)GC-TOFMS. (**A**) (Venn diagram) and (**B**) (Bar plot graph displaying compound distribution according to chemical class). SBSE was initially introduced in 1999 as a miniaturized and solvent-free extraction technique for aqueous samples. Compared with HS-SPME, SBSE provides greater analytical sensitivity and reaches much lower detection and quantification limits.