## 3 Results and Discussion
### Chemometrics of the DES-Based Extracts
#### 3.2.2 Hierarchical Cluster Analysis (HCA)  
Hierarchical cluster analysis (HCA) was utilized for classification of the tested extracts prepared by the DES-UAE procedure on a similar basis. According to the results presented in Figure 4a as a dendrogram, different extracts of herbs and agro-food residues can be classified into two major categories accounting for their antioxidant properties determined by three analytical methods.  
The dendrogram depicted a clear separation of twelve extracts (all LB extracts (except LB-DES1), three M extracts (M-DES4, M-DES5, and M-DES6), three ChoP extracts (ChoP-DES3, ChoP-DES4, and ChoP-DES6), and N-DES3 extract) with high antioxidant potential from the other analyzed samples. LB, M, and ChoP samples obtained using betaine-based DES6 (B:U) with the highest CUPRAC values, as well as LB, N, and ChoP samples prepared with DES3 (Gu:U-30) having high ABTS results, created two inter-clusters (Figures 1a,b and 4a). As can be seen in the dendrogram, the second main group was divided into two sub-groups. Fourteen extracts (extracts from all herbs prepared with ChCl-based, BH extracts obtained with all DESs, N and Cha after extraction by DES2 and DES5 containing CitA, and ChoP-DES5) were arranged in one sub-group characterized by the lowest AC values determined by three analytical methods. However, the second sub-group clustered ten extracts (ChoP-DES1, N-DES6, ChoP-DES2, M-DES2, Cha-DES3,  
Figure 4: Dendrograms of hierarchical cluster analysis for (**a**) the studied extracts obtained by six DESs and (**b**) and the applied analytical methods for AC determination.  
M-DES3, Cha-DES6, N-DES4, Cha-DES4, and BH-DES4), which exhibited moderate antioxidant potential. It is noteworthy that DES4 (Gu:U-50) extracted bioactive compounds with a high ability to scavenge DPPH\({}^{\bullet}\) radicals from Cha, N, and BH, which differentiated them in this sub-group (Figures 1c and 4a).  
Importantly, various DESs can highly efficiently extract antioxidants and further distinguish the different plant materials.  
Additionally, HCA depicted two clusters and connections between three assays utilized for the AC determination of herbs and waste extracts (Figure 4b). The ABTS and DPPH methods formed a distinct group and were more closely connected to each other, while the CUPRAC test clearly separated from this cluster.  
This can be explained by the fact that ABTS and DPPH tests are classified as mixed-mode assays, where hydrogen atom transfer (HAT), electron transfer (ET), and proton-coupled electron transfer (PCET) mechanisms play different roles in varied proportions, depending on the corresponding reaction conditions (pH and solvent) [33]. These analytical methods are based on the scavenging of a stable radical chromophore (ABTS\({}^{\bullet+}\) and DPPH\({}^{\bullet}\)) by antioxidants; thus, they allow for the measuring the radical scavenging activity of the samples. However, CUPRAC is a well-known ET-based method associated with Cu(II) reduction to Cu(I) by antioxidants in the presence of neocuproine as a ligand and generation of a yellow-orange complex with a maximum absorption peak at 450 nm. Moreover, CUPRAC and ABTS methods can be applied to matrices containing hydrophilic and lipophilic antioxidants, whereas the DPPH test has a higher affinity toward lipophilic than hydrophilic antioxidants.