## 3 Results and Discussion
### GC-MS Combined with PCA Analysis  
Overall, 70 volatile compounds extracted using the four methods were identified and quantified using GC-MS (Table 2). It was observed that the contents of 52 compounds in SAEOs extracted by HD, ESE, and SE constituted 99.58 \(\pm\) 0.08%, 99.47 \(\pm\) 0.30%, and 99.43 \(\pm\) 0.05% of the SAEOs, respectively. In addition, the content of 53 compounds of SAEOs extracted by SCD was 99.51 \(\pm\) 0.03%. The main components were trans-anethole, limonene, foeniculin 1-(3-methyl-2-butenoxy)-4-(1-propenyl) benzene, and anionic aldehyde, etc., which is consistent with the findings of [PERSON] et al. [29]. The composition of SAEOs extracted by HD was quite different from that of those extracted by other methods. All components are shown as trace compounds. There were no significant differences  
Figure 6: PCA scatter plot of pre-processed GC–IMS spectra for SAEOs extracted by four different methods.  
among the components of SAEOs extracted using SCD, SE, or ESE, and the components of SAEOs extracted by SCD were similar to those of SAEOs extracted by SE. The main compound in SAEOs extracted by the four methods was anethole, the content of which was greater than 80%. The limonene contents in SAEOs extracted by SCD and SE were similar and were higher than that of in SAEOs extracted by ESE and lower than that of in SAEOs extracted by HD, which is consistent with [PERSON] et al. [30]. The amounts of linalool and estragole in SAEOs extracted by HD were significantly higher than those of linalool and estragole in SAEOs extracted by the other methods, while the amount of foeniculin in SAEOs extracted by HD was lower than that in SAEOs extracted by the other methods. The results indicated that the content and composition of SAEOs extracted by HD were quite different from those of SAEOs extracted by the other three methods. The content and composition of SAEOs extracted by ESE were also different from those of SAEOs extracted by SCD and SE, whereas those of SAEOs extracted by SCD and SE were similar. The relative content of each compound was used as the characteristic value for PCA after standardisation. As shown in Figure 7, the total variance contribution of the two principal components was 87.97%, which meant that it represented the entire dataset. There was a high degree of polymerisation for SAEOs extracted by the same method and a highly dispersed distribution for SAEOs extracted by different methods, revealing that SAEOs extracted by different methods could be well distinguished.  
In addition, the distribution of 70 compounds was dispersed (Figure 8), indicating that the SAEOs extracted by the four methods were similar in terms of chemical composition but had different amounts of each compound. Meanwhile, the scattered points on the diagram illustrate that the specific components used as the basis for discrimination could be determined from the volatile compounds of the SAEOs, which included peak 22 (tR = 34.90 min) derived from 4-carvomenthenol and peak 23 (tR = 36.21 min) derived from \(\alpha\)-terpineol. PCA can compensate for the defects of GC-MS by quickly and conveniently determining the characteristic substances of SAEOs extracted using different methods. Therefore, GC-MS analysis combined with PCA analysis could be used to distinguish SAEOs extracted using different extraction methods, which is in line with the findings of [PERSON] et al. [31].  
Figure 7: The PCA diagram of GC\(-\)MS of the SAEOs extracted by the four different methods.