## 2 Results and Discussion
### Key Aroma Marker Quantitation by Standard Addition (SA) and IS-SPME-GC-MS
The first principal component (F1) explains 57.70% of the total variance (74.85%), contributing most to the discrimination of samples, while the second principal component (F2) has a minor informative influence (17.15%). Three main groupings (ellipses with dotted lines) arbitrarily delineated by the authors can be observed in the score plot of Figure 1a: from left to right with increasing F1 score values, the Darjeeling Castleton, Azores, and Yunnan, followed by Assam, Ceylon, and Kenya, and finally Darjeeling Testa Valley (TV) with higher score values along F1. The distribution of variables as a function of the first two principal components is reported in the loadings plot of Figure 1b. Analytes providing the most information on the F1 axis, and likely more abundant in related infusions, directly correlate with Ceylon, Assam, Kenya, and Darjeeling TV teas. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Compound Name**} & \multirow{2}{*}{**Odor Threshold** (\(\mu\)/L)**} & \multirow{2}{*}{**Ceylon**} & \multirow{2}{*}{**Assam**} & \multirow{2}{*}{**Azores**} & \multirow{2}{*}{**Darjeeling Castleton**} & \multirow{2}{*}{**Darjeeling Testa Valley**} & \multirow{2}{*}{**Kenya**} & \multirow{2}{*}{**Yunnan**} \\  & & & & & & & & \\ \hline
3-Methyl butanal & 1.2 & 37.00 & 49.12 & 52.32 & 25.22 & 27.45 & 84.89 & 17.02 \\
2-Methyl butanal & 4.4 & 45.44 & 63.12 & 46.69 & 29.50 & 21.22 & 89.97 & 18.29 \\ Hexanal & 10 & 45.25 & 23.04 & 14.84 & 15.30 & 63.49 & 31.75 & 16.88 \\ (\(Z\))-4-Heptrenal & 0.06 & 0.98 & 0.68 & \(<\)LOQ & 0.20 & 0.82 & 0.41 & 0.47 \\ Phenyl acetaldehyde & 6.3 & 32.73 & 62.73 & 29.76 & 10.77 & 77.52 & 38.76 & 16.11 \\ Linalool & 0.6 & 25.75 & 9.72 & 10.19 & 7.86 & 54.48 & 27.24 & 0.41 \\ (\(E\),\(Z\))-2,6-Nonadien & 0.03 & 0.56 & 0.39 & \(<\)LOQ & 0.50 & 0.28 & 0.14 & \(<\)LOQ \\ (\(E\))-2-Nonenal & 0.4 & 0.40 & 0.24 & \(<\)LOQ & 0.02 & 1.21 & 0.60 & 0.15 \\ (\(E\),\(E\))-2-A+Nonadien & 0.2 & 0.29 & 0.39 & 0.14 & \(<\)LOQ & 1.46 & 0.73 & 0.29 \\ Geraniol & 3.2 & 13.20 & 1.07 & 16.38 & 11.31 & 24.83 & 12.41 & 2.78 \\ (\(E\),\(E\))-2,4-Decadien & 0.16 & 0.51 & 0.15 & \(<\)LOQ & 2.29 & 0.21 & 0.11 & \(<\)LOQ \\ \(\beta\)-Damascenone & 0.004 & 0.26 & 0.29 & \(<\)LOQ & \(<\)LOQ & 0.38 & 0.19 & \(<\)LOQ \\ \(\beta\)-Ionone & 0.2 & 2.16 & 0.78 & \(<\)LOQ & 0.18 & 3.84 & 1.92 & 0.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: SA quantitative results on key aroma compounds of tea infusion; data are expressed in \(\mu\)g/L. Odor thresholds are from reference literature [21; 27] or from Leibniz-LSB@TUM Odorant Database [43]. Figure 1: PCA results. (**a**) Score plot on the first and the second principal components (F1 F2 plane) based on key aroma compound quantitative data determined in infusions prepared from tea leaves (Table 2). For Ceylon teas, an average profile obtained from four lots was considered; (**b**) loadings plot with the distribution of quantified analytes. Interestingly, Darjeeling teas, although produced in the same region of India, showed distinctive yet different aroma-active compound patterns, with Darjeeling TV characterized by higher amounts of these analytes. On the other hand, teas from Ceylon, Assam, and Kenya were clustered together, suggesting similar aroma features. Experimental results indicate that the most potent odorants characterizing these kinds of tea are present in a wide range of concentrations, to be specific, 0-4 ug/L for _(Z)-4-heptenal_, _(E,Z)-2,6-nonadienal_, _(E)-2-nonneal_, _(E,E)-2,4-nonadienal_, _(E,E)-2,4-decadienal_, _\(\beta\)-damascenone_, and _\(\beta\)-ionone_, while _hexanal_, _linaloal_, _genaiol_, _phenyl acetaldehyde_, and \(2\) and _3-methyl butanal_ are in the range 0-100 ug/L. Darjeeling TV has a peculiar profile described by the floverp terpen _linalool_ (54.48 ug/L) and _geraniol_ (24.83 ug/L), the green-grassy note from _hexanal_ (63.49 ug/L), and fatty nuances likely modulated by unsaturated aldehydes _(E)-2-noncal_ and _(E,E)-2,4-nonadienal_.