## 3 Results and Discussion
### Identification of Volatile Compounds by HS-GC-IMS Analysis  
GC-IMS was used in this study to initially detect aroma compounds and separate aroma profiles of 18 types of tea. A total of 85 aroma compounds were detected by GC-IMS, as listed in Table S2. The volatile flavor fingerprints for all samples are presented in Figure 3. It clearly showed that oolong tea and red tea contained more abundant aroma compounds if compared to the green tea. For the convenience of discussing the results, Figure 3 was divided into six regions (Blocks 1-6) based on the characteristics of the regions where the compounds were located. The result revealed that green tea could be distinctly separated from oolong tea and red tea. This was mainly due to the deficiency of block 3, and this shows that green tea has less sweet and fermented aromas compared to the sensory evaluation results. Meanwhile, volatile compounds that could directly distinguish black tea from oolong tea based on the missing areas of block 1 included (_E_)-2-nonenal, limonene, \(\beta\)-pinene, \(\alpha\)-pinene, and 3-methylpentanol, while those based on block 2 included aroma compounds furfural, 5-methylfurfural, ethyl octanoate, 2-acetylfuran, 2,5-dimethylpyrazine, and 2-methylpyrazine. Terpene and heterocyclic compounds were ignored in previous studies [10; 13]. The aroma compounds in block 5 were abundant in all kinds of tea, indicating that they are potent volatile compounds in tea types. Furthermore, almost all types of green tea had same fingerprint, except for the samples No. 1 and No. 3, which was mainly because of the individual areas of block 6 (including odorants ethyl 3-methylbutanoate, 2,3-pentanedione, 2-hexanol, acetic acid, methyl benzoate, 1-hydroxy-2-propanane, and (_E_)-3-hexanol) and block 4 (including odorants acrolein, diallyl disulfide, (_E_,_E_)-2,4-heptadienal, cyclohexanone, octanal, methyl heptanone, (_E_)-2-octenal, and nonanal), respectively. For the oolong tea, samples no. 10 and no. 14 were significantly different from other samples. The oolong tea samples no. 10 and no. 14 belong to Wuyi Rock tea. This type of tea undergoes a unique roasting process that contributes to a more distinct roast aroma [18]. This is the main reason why the odorants in block 2 and parts of block 3 had higher intensities compared to other oolong teas (the related compounds contained trimethylpyrazine, o-xylene, ethyl 2-methylbutanoate, propyl acetate, ethyl acrylate, ethyl lactate, butyl propionate, propyl acetate, 2-octanone, (_Z_)-3-hexenyl acetate, 1-octen-3-ol, 2-heptanone, 1-propanol, propyl acetate, 2-pentylfuran, 2-butanone, 2,3-butanedione, benzaldehyde, and ethyl propanoate). These odorants could present the two sensory attributes of fruity and roast-like aromas because their scores for both were higher than those of other oolong teas, as shown in Figure 1. On the one hand, the roast-like aroma could be attributed to heterocyclic compounds mainly formed by the Maillard reaction. On the other hand, the fruity aroma was the result of alcohols, ketones, and esters mainly formed by the degradation of lipids or glycosides [2]. The no. 12 sample (Tie guan yin) has been previously reported as being famous for its Yin flavor and distinct orchid fragrance [30]. According to the PCA plot, the odorants (including aroma compounds (_E_)-2-nonenal, limonene, \(\beta\)-pinene, \(\alpha\)-pinene, and 3-methylpentanol) in block 1 might be used as markers to separate no. 12 tea from other oolong teas. The No.18 sample (Zheng shan xiao zhong) from the black tea group, known for its strong burning aroma notes, was separated from other three black teas. This is mainly because the processing of no. 18 requires a special smoking procedure using pine needles or pine firewood, which differs from the processing of other black teas [31; 32].