## 2 Results and Discussion
### Key Aroma Marker Quantitation by Standard Addition (SA) and IS-SPME-GC-MS  
The standard addition procedure, widely used as a quantitation approach, consists of a series of experiments in which the original sample and a suitable number (at least four concentration levels) of aliquots of the sample, spiked with increasing and known amounts of reference compounds, are submitted to the analytical process. When using the single addition method, the analyte concentration in the sample can be estimated from Equation (1):  
\[A_{(0+a)}=\frac{A_{0}}{W_{0}}W_{a}+A_{0} \tag{1}\]  
where \(W_{0}\) is the amount of analyte in the matrix, \(W_{a}\) is the amount of analyte added to the sample, \(A_{0}\) is the instrumental response obtained from the analysis of the original sample, and \(A_{(0+a)}\) is the instrumental response of the analyte obtained from the analysis of the spiked sample. A preferable and more accurate procedure, which was applied in this study, includes multiple standard additions. With multiple SAs, a linear regression analysis evaluates the terms \(W_{a}\) and \(A_{(0+a)}\) so the amount of analyte in the matrix (\(W_{0}\)) is given by the ratio between the intercept and the slope, Equation (2):  
\[\frac{b}{a}=\frac{A_{0}}{\frac{A_{0}}{W_{0}}} \tag{2}\]  
Standard addition is a quantitation approach that can be carried out in different ways: (\(a\)) by spiking the target analyte(s), in a gaseous state, into the sample headspace (gas phase addition--GPA); (\(b\)) by spiking the analyte(s), dissolved in a suitable solvent, directly onto the sample (sample phase addition--SPA); or (\(c\)) by spiking the stable-isotope-labeled analyte(s) dissolved in a suitable solvent (stable isotope dilution analysis--SIDA) onto the sample. The present study adopted the SPA protocol, suitable for its ease of implementation and automation and its cost-effectiveness compared to isotopically labeled standards. The analytical protocol, rationalized in the Experimental section, consisted of (\(a\)) three replicate infusions prepared from each tea sample and (\(b\)) three standard addition levels for each infusion (plus the analysis of the original sample). Acetone was selected as a solvent for spiking solutions because it guaranteed the full solubilization of all target analytes, being miscible with water. For each calibration step, two analytical replicates were acquired. Thirteen key odorants were accurately quantified: 3-methyl butanal, 2-methyl butanal, hexanal, (\(Z\))-4-heptenal, phenyl acetaldehyde, linalool, (\(E\),\(Z\))-2,6-Nonadien, (\(E\))-2-nonenal, (\(E\),\(E\))-2,4-nonedienal, geraniol, (\(E\),\(E\))-2,4-decadienal, \(\beta\)-damascenone, and \(\beta\)-ionone. They are listed in Supplementary Table S1 (ST1) together with chromatographic information on retention times (\(t_{R}\) min), Target/Qualifier Ions \(m/z\), calibration functions and determination coefficients (\(R^{2}\)), and precision and accuracy results. For the method's performance parameter evaluation, see the experimental section, Section 3.9. Table 2 lists quantitation results, obtained using the SA calibration technique and IS-SPME-GC-MS (SIM) for key aromas in tea infusions. The results on Ceylon teas are reported as the mean value obtained from four commercial batches from the same harvest year. Data are expressed as \(\mu\)g/L in the infusion. Experimental results are consistent with those obtained by [PERSON] and [PERSON] [21], who adopted a more complex procedure for the isolation and accurate quantification of potent odorants, i.e., SIDA and solvent-assisted flavor evaporation (SAFE) followed by GC-olfactometry (GC-O) and aroma extract dilution assay (AEDA). An unsupervised multivariate approach (i.e., Principal Component Analysis--PCA) provides prompt information on samples' natural clustering based on compositional similarities. Figure 1a shows the score plot on the first and the second principal components (F1-F2 plane) obtained by analyzing the distribution of targeted odorants in all sample replicates.