## 4 Conclusions  
In summary, a total of 85 aroma compounds were detected in 18 kinds of tea by GC-IMS, and 318 aroma compounds were identified by GC \(\times\) GC-O-MS. The results of GC-IMS were more visible, but those of GC \(\times\) GC-O-MS were more comprehensive. Moreover, one-third of these were aroma-active compounds that contributed to the overall profile of the tea. Green tea (no. 1-no. 9) contained the least amount of aroma compounds due to the fermentation process, but roasted green tea no.1 had a more abundant aroma profile. Oolong tea (no. 10-no. 14) could be classified into two groups because of the roasting process of no. 10 and no. 14 of Wuyi Rock tea. Black tea (no. 15-no. 18) had the richest aroma profile because carotenoids, lipids, and glycosides in the tea could be fully degraded or oxidized during fermentation. The reason for the different types of tea was mainly attributed to the varieties of aroma compounds. However, the same type of tea can also have different flavors, mainly due to the different content of each odorant. The PCA and OPLS-DA revealed that green tea, oolong tea, and black tea could be clearly separated based on their peak areas. Moreover, the 49 aroma compounds with VIP values > 1.0 could be considered as the potential indicators for classification of the tea types, and these compounds included linalool, nerol, trans-nerolidol, phenylethanal, geraniol, methyl salicate, benzaldehyde, indole, (Z)-linalool oxide, 2-phenylethanol, (_E,E_)-2,4-heptadienal,\(\beta\)-myrcene, oimene, (_E_)-linalool oxide, 6-methyl-5-hepten-2-one, (_E_)-linalool oxide (pyranoid), \(\beta\)-Lonone, hexanal, (_E_)-2-hexenal, benzyl alcohol, dehydrolinalool, (+)-limonene, (_E_)-3,5-octadiag-2-one,\(\beta\)-cyclocitral, 3-hexenyl hexanoate, (_E_)-cital, \(\beta\)-franesene, 5-methylfurral, nerv-lactone, creamy lactone, 3,5-octadiag-2-one, hexyl hexanoate, 2-pentylfuran, 1-hexanol, safranal, furural, nonanal, toluene, cis-3-hexenyl butyrate, (_Z_)-3,7-dimethylocta-2,6-dienal, 1-ethyl-1h-pyrrole-2-carboxaldehyde,\(\alpha\)-farmesene, (_E_)-2-hexen-1-yl hexanoate, (_Z_)-3-hexenyl-2-methylbutanoate, (_Z_)-3-hexen-1-ol, 2-methylbutanal, 3-methylbutanal, (_E_)-2-octal, and 2,2,6-trimethyl-cyclohexanone. These aroma compounds are commonly found in various tea leaves. Overall, the identification by GC-IMS and GC \(\times\) GC-O-MS and the confirmation by the PCA and OPLS-DA could be considered as a method for identifying tea grades or varieties. Therefore, further research applying these preliminary results to actual tea production or classification is also needed.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/foods12132433/s1](https://www.mdpi.com/article/10.3390/foods12132433/s1)]([https://www.mdpi.com/article/10.3390/foods12132433/s1](https://www.mdpi.com/article/10.3390/foods12132433/s1)), Table S1: The result of the aroma profile evaluation; Table S2: The qualitative results of 18 kinds of teas by HS-GC-IMS; Table S3: The aroma compounds in 18 kinds of teas detected by GC \(\times\) GC-MS.  
Y.L.: Methodology, resources, writing--original draft; Y.W.: data curation, writing--revision draft; Y.H.: resources, investigation; H.S.: conceptualization, funding acquisition, supervision, writing--review and editing; P.Y.: data curation, methodology, writing--original draft. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 32172335.  
Data Availability Statement: Data is contained within the article (or Supplementary Material).  
Sincere thanks Ali Raza who is come from the San Diego State University, for polishing the English language and providing constructing advice to the manuscript.  
The authors declare no conflict of interest.