## 3 Results and Discussion
### Identification of Volatile Compounds by HS-GC-IMS Analysis
This is why the color of certain odorants in block 3, including ethyl lactate, butyl propionate, pentyl acetate, 2-octanone, 1-hexanol, 2-butoxyethanol, 2-cyclohexenone, dimethyll trisulfide, and (_E_)-3-hexenol, appears intense in Figure 3.  
Figure 3: Graphical depiction of the GC-IMS data. (**A**). Gallery plot of the signal peak areas of identified volatile compounds with a visual difference. (**B**). All odorants of different blocks enlarged from plot A.  
PCA was conducted in this study to cluster the samples and quickly determine the types of unknown samples. The result in Figure 4 was highly consistent with the above Figure 3 analysis, especially for samples no. 1 and no. 3 of green tea, which were the two exception outlier samples. Thus, the PCA result confirmed that GC-IMS could be used as one of the effective approaches to cluster the samples. Identification of Aroma-Active Compounds in 18 kinds of Teas by Two-Dimensional Comprehensive Gas Chromatography-Olfactometry-Mass Spectrometry Analysis (GC \(\times\) GC-O-MS) Analysis  
In this study, over 300 kinds of aroma compounds in all 18 tea samples were identified (Supplementary Material Table S3) using the GC \(\times\) GC technique. The total ion chromatogram (TIC) plots shown in Figure 5 could directly discriminate green tea from oolong tea and black tea because of the amounts of aroma compounds in samples no. 1-9 were less than those in no. 10-18. In addition, the density of the TIC in no. 6 (Huan shan mao feng) was greater than that in other green teas. This might be because the processing of Huan shan mao feng originally belongs to roasted green tea [33]. Meanwhile, the number of aroma compounds in samples no. 10 and no. 14 was highest among the four oolong teas. This easily explains the importance of the roasting procedure for Wuyi Rock tea [34]. During the fermentation of black tea, more aroma compounds could be generated from the corresponding precursors, such as fatty acids, amino acids, glycosides, etc. [35]. Therefore, the plots of samples no. 15 to no. 18 were also rich and complex. Figure 4: PCA plot based on the results of the HS-SPME-GC-IMS analysis. The color of signal green, yellow, and red colored signals represented green teas, oolong teas, and black teas, respectively. Figure 5: The TIC plots of the 18 tea samples in 2D mode. The areas enclosed by green, blue, and red represent green tea, oolong tea, and black tea, respectively. Although GC \(\times\) GC possesses strong separation capabilities, only one-third of the 300 aroma compounds could be detected (labeled as \"O\" in Supplementary Material Table S3) when GC \(\times\) GC was equipped with an olfactometry system, which was a new approach to carrying out flavor analysis [36]. These volatiles were regarded as aroma-active compounds that could contribute to the aroma profiles of the 18 kinds of tea. These aroma-active compounds exhibited some differences in the aroma strength in each sample, mainly due to variations in their contents. For instance, compound no.121, (_E,E_)-3,5-octadecan-2-one could be detected in all 18 samples but could only be identified in oolong tea, black tea, and samples no. 3 and no. 7 of green tea. However, even though compounds no. 7 3-methylbutanal, no. 8 2-methylbutanal, no. 11, 1-penten-3-one; no. 60, 1-octen-3-one; no.89, acetic acid; no. 245, geraniol; and no. 262, 2-phenylethanol, etc. could not be identified in some tea samples, they could be detected in all samples.