## 3 Results and Discussion
### Principal Component Analysis and Discriminant Analysis  
Figure 4 shows the principal component scores plot of the whisky samples analysed at different scanning temperatures (Panel A: country of origin; Panel B: scanning temperature). The score plot showed a clear separation between different whisky samples according to country of origin, and this separation could be achieved independently of the temperature used during the spectra collection. Whisky samples belonging to the same country of origin clustered together independently of the scanning temperature along the first principal component (PC1), while samples belonging to the same  
Figure 2: Second derivative of UV-VIS spectra of whisky samples scanned at different temperatures (25, 35, 45, and 55 \({}^{\circ}\)C).  
commercial brand were arranged in a linear pattern in the direction of the PC2 following the increase in the scanning temperature.  
The PCA loadings are reported in Figure 5. The highest loadings in the first (92% of the total variation explained) and the second PC (6% of the total variation explained) were related to chromophores either associated with the presence of phenolic compounds or HMF in the UV region between 250 nm and 300 nm [27; 28]. As described above, the UV-VIS region might contain information about different chemical compounds present in the whisky samples analysed. Reports by other authors also indicated that compounds present in whisky such as benzoic acids could be measured in the wavelength range between 235 nm and 305 nm, and hydroxycinnamic acids were measured between 227 nm and 245 nm and between 310 nm and 32 nm [8; 9; 27; 28; 29]. Both flavonoids and phenolic compounds could be measured between 250 nm and 275 nm, between 350 and 390 nm, and between 475 nm and 545 nm, whereas catechins could be measured at 280 nm [8; 9; 27; 28; 29].  
PLS-DA was used to classify the whisky samples according to the country of origin based on their UV-VIS spectra. Figure 6 shows the optimal loadings for the PLS-DA regression models used to predict the origin and the scanning temperature in the set of samples analysed. The coefficient of determination (R\({}^{2}\)) and the standard error in cross validation (SECV) obtained for the prediction of origin were 0.84 and 0.97, respectively. For the prediction of temperature, the R\({}^{2}\) and the SECV were 0.87 and 4.5, respectively. Overall, the PLS-DA models correctly classified 100% of the whisky samples belonging to Canada and USA and 98% of the whisky samples belonging to Scotland and Ireland, respectively. The loadings (six latent variables or factors were used to develop the model) also indicated that the models used similar wavelengths for the prediction of whisky and temperature, as described in the PCA analysis. The main absorbances highlighted in the PLS-DA loadings corresponded with wavelengths at 353 nm, 301 nm, 263 nm, and 228 nm. The chemical description of these wavelengths agreed with the absorbances of HMF and polyphenol compounds, as discussed in the section above [27; 28].  
Figure 3: Linear regression between absorbance at 280 nm and scanning temperature. Please note that each dot represents the average of all whisky samples scanned at that temperature.  
Figure 4: Principal component score plot of whisky samples analysed UV-VIS spectroscopy. **(A)**: label by whisky, **(B)**: label by scanning temperature.  
Figure 5: Principal component loadings of whisky samples analysed UV-VIS spectroscopy.