## 3 Results and Discussion
### Hierarchical Cluster Analysis and Principal Component Analysis  
Statistical and chemometric methods were applied for further analysis and interpretation of the obtained results, which can be a useful tool in the analysis of large data sets [39; 40; 41]. In this work, the pattern recognition methods HCA and PCA, a ranking approach SRD, and Wilcoxon matched pair test were applied. The HCA resulted in two double dendrograms. The dendrogram that represents the clustering of the control and CO\({}_{2}\)-NADES systems based on the percentage of each detected compound is given in Figure 2. The presented vertical dendrogram indicates the following findings: (1) the most abundant compounds in almost all extracts were carvacrol and thymol (placed in the purple vertical cluster), except in the Bet/EG extracts, in which thymoquinone was the most abundant compound; (2) the significant presence of borneol, terpinene-4-ol, cryophyllene oxide, linalool, carvacrol methyl ether, _trans_-caryophyllene, and _p_-cymene was observable in the majority of the extracts (placed in the separate vertical sub-cluster of the red-colored cluster); (3) the majority of the compounds were minor compounds, located in another sub-cluster of the vertical red cluster. The horizontal dendrogram in Figure 2 shows the grouping of the CO\({}_{2}\)-NADES systems and control samples. It indicates that all three control samples were placed in different clusters and sub-clusters, suggesting that they were potentially different and, therefore, not very stable over time. Bet/EG samples were placed in a separate cluster (green horizontal cluster), which indicated that they were distinctive from the other CO\({}_{2}\) -NADES systems, as previously explained. The main reason for their separation into different clusters was the content of thymoquinone, which was higher than in other CO\({}_{2}\)-NADES systems, and the significantly lower abundance of thymol and carvacrol in comparison with other extracts. The distribution of other NADES in the purple and red horizontal clusters showed that some samples monitored at the start and after 3 months were placed in the same cluster (e.g., Gly/Glu/Sor/W0 and Gly/Glu/Sor/W3, as well as Pro/Gly/Sor/W0 and Pro/Gly/Sor/W3, which belong to the red horizontal cluster), as well as some samples  
Figure 2: Clustered heat map (double dendrogram) of the control and CO\({}_{2}\)-NADES systems and the compounds detected in the extracts. The blue–red scale presents the percentage distribution of compounds in each sample. monitored after 3 and 6 months (Bet/Gly/Suc/W3 and Bet/Gly/Suc/W6). The main observations were scattering of the control samples in the dendrogram and significant separation of Bet/EG samples from other samples but in the same cluster, indicating that the Bet/EG samples had not suffered significant changes over time. Another aspect of HCA of CO\({}_{2}\)-NADES systems and control is presented in Figure 3, where the clustered heat map shows the clustering of the samples in regard to the groups of compounds (oxygenated monoterepenes (OM), monoterepene hydrocarbons (MH), non-terpenes (NT), oxygenated sesquiterpenes (OS), and sesquiterpene hydrocarbons (SH)). The vertical dendrogram indicates the significant separation of the OM compounds considering their highest contents (it can be considered as an outlier since it is placed outside the main cluster), whereas the other groups of compounds (MH, NT, OS, and SH) belonged to the same cluster considering their abundance, which was significantly lower in comparison to the OM compounds. The clustering of the CO\({}_{2}\)-NADES and control samples is presented in the horizontal dendrogram, which indicates the scattering of the control samples in different clusters and sub-clusters. Moreover, it also demonstrates the grouping of Bet/EG samples in a similar way as in the previous analysis, with the difference that they were placed in the same cluster (green horizontal cluster) with the CO\({}_{2}\)-NADES systems monitored at the start of the experiment (Gly/Glu0).