## 3 Results and Discussion
### GC-IMS Combined with PCA  
Figure 3 shows a top view of the SAEOs extracted using the four extraction methods in a three-dimensional (3D) topographic map of GC-IMS with a blue background. The red vertical line on the left represents the reactive ion peak, and each point on either side of the reactive ion peak represents a volatile organic compound. The colour indicates the concentration of the substances. White indicates a lower concentration and red indicates a higher concentration. It can be seen that the change in the organic compounds was significant with the retention time between 100 and 500 s. The difference in substance composition was not obvious, but mainly manifested as a clear difference in the content. The main compounds in SAEOs were as follows: 1 and 2, anethod; 3, alpha-terpineol; 4 and 6, terpinene; 5, linalool; 7, limonene; 8 and 9, 1-8-cineol (Figure 4). These results were consistent with those of the GC-MS analysis in this study (Table 2). As shown in Figure 5, a characteristic map with 85 separate signals was obtained using GC-IMS to analyse the volatile compounds of SAEOs extracted using different methods. Each row in the picture represents a sample of essential oil, consisting of all the volatile organic signal peaks. Each column shows a signal peak for an organic compound at the same retention time. SAEOs extracted using the four different methods showed  
Figure 4: Typical GC-IMS topographic map of SAEOs. The compounds of the SAEOs were labelled by searching the GC-IMS map library. A compound can produce multiple signals (monomer, dimer, or trimer), for example, 1 and 2 are anethod, and 1 is a monomer and 2 is a dimer. The whole spectrum represents the headspace composition of the sample. Figure 3: Top view of the GC-IMS 3D topographic map of the SAEOs extracted by different methods. The longitudinal coordinate is the gas phase retention time, and the abscissa is the ion migration time (drift time). significantly different amounts of compounds. The difference between the SE and the other three SAEO methods was mainly reflected in the components with retention times ranging from 100 s to 300 s, including 2-methylbutanal, acetate, hexanal, pentanal, hexanol, 1-pentanol, acetoin, 2-methylbutanol, 2-pentanone, 1-butanol, 3-methylbutanal, 2-butanone, isobutanol, and butanal. The different components obtained through ESE compared to those obtained from the other three SAEO methods were mainly 1-8-cineol, limonene, gamma-terpinene, delta-3-carene, alpha-terpinene, alpha-pinene, alpha-phellandrene, beta-pinene and mycrene. The different components obtained through SCD compared to those obtained from the other three SAEO methods were mainly benzene and 2-3-butanedione. The different components obtained through HD compared to those obtained through the other three SAEO methods were mainly 5-methyl-2-furanmethanol, 2-furfurral, isopropyl acetate, and 4-methyl-2-pentanone. These results suggested that GC-IMS can effectively characterise the differences in the volatile compounds of the SAEO extracted using different extraction methods. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**No. **} & \multirow{2}{*}{**RT**} & \multirow{2}{*}{**Volatile Compounds**} & \multicolumn{8}{c}{**Relative Content (\%)**} \\ \cline{3-14}  & & & **SCD1** & **SCD2** & **SCD3** & **SE1** & **SE2** & **SE3** & **ESE1** & **ESE2** & **ESE3** & **HD1** & **HD2** & **HD3** \\ \hline
1 & 17.94 & \(\alpha\)-Pinene & 0.2262 & 0.2252 & 0.2006 & 0.2508 & 0.2432 & 0.2516 & 0.2645 & 0.2662 & 0.2471 & 0.0378 & 0.0488 & 0.0351 \\
2 & 20.61 & Sabine  
Figure 5: Corridor diagram of volatile compounds of SAEOs extracted by four different methods. It was found that SAEOs extracted by different methods showed the same composition but with different amounts, which was consistent with the results of GC-MS analysis (Table 2). However, the GC-MS and GC-IMS results were quite different for specific relative contents.