## 4 Discussion  
The principal component analysis has clustered essential oil samples into three groups according to their similarity. Highly concentrated extracts were clustered according to the botanical family, while commercial essential oil blends were grouped together regardless of the botanical family they belong. In the first group, 4 samples of concentrated essential oils from the Asteraceae plant family were clustered together; the second group was made up with 9 samples of concentrated extracts from the Lamiaceae plant family, while the last group contained 17 samples (three remaining concentrated extracts and commercial essential oils) from different sources regardless of the claimed purity and plant source. Thus, all samples from the Lamiaceae (Mint) family and all samples from the Asteraceae (Sunflower) family were clustered together except for arrica essential oil (sample 2), that belongs to the Asteraceae family but was not clustered together with the rest of the oils from this family. Instead, arrica was clustered in the same group with concentrated frankincense and primrose oils. Arnica oil exhibits a similar ATR-FTIR spectrum to the primrose oil with a strong sharp C=O band at 1743 cm\({}^{-1}\) and a medium wide band at 1159 cm\({}^{-1}\) that are not present in frankinense essential oil. Interestingly, commercial lavelander, rosemary, frankinense and peppermint essential oil samples did not cluster with their corresponding concentrated extracts despite their similar spectral patterns. PCA of commercial blends could not be explained by 2D or 3D PC plots, indicating no correlation between two principal components characteristics. Commercial essential oils are usually produced by using around 20% of the named species of plant and adding in natural extractions from other essential oils to meet the standardized oil profile requirements. In contrast, the highly concentrated FLAVEX(r) essential oils used in this study were of 70-90% purity. Although same oil samples produce very similar spectral fingerprints, there is a significant difference in in the ATR-FTIR spectra from concentrated essential oils and commercial-grade essential oils. Dilution of an essential oil with a non-polar organic solvent not participating in hydrogen bonding leads to significant differences in terms of the shape and intensity of the respective bands, particularly in the region from 1000-1300 cm\({}^{-1}\). The peaks in the FTIR spectra of commercial oils samples are better separated with less overlapping IR bands. The area between 1100 and 1300 cm\({}^{-1}\) corresponds to stretching vibrations of the C-O group, while the O-C-O band originating from primary alcohols appears in the region from 1100 to 1020 cm\({}^{-1}\). For example, concentrated lavelander oil sample shows a clearly defined shoulder peak for the peak at 1740 cm\({}^{-1}\) (corresponding to the vibrations of the C=O group) on the lower wavenumber side, with a clear maximum at 1685 cm\({}^{-1}\)[29], which can be associated with the formation of a hydrogen bond between C=O and -OH groups (Figure 4). Thus, there is more similarity in spectra from the same botanical family than between spectra of the concentrated and diluted oil samples of the same origin. To confirm the results obtained by the PCA, a hierarchical cluster analysis (HCA) of spectra was performed (Figure 5). Unlike PCA, HCA considers all the data variability and shows the similarity or dissimilarity among each pair of samples. The goal of the clustering algorithm is to partition the objects into homogeneous groups, so the within-group similarities are large compared to similarities between the groups. The HCA results supported the results of PCA, in which concentrated essential oils samples extracted from the Lamiaceae family (samples 8-16) and from the Asteraceae family (samples 3-7) were close to each other according to the hierarchical clustering result.