## 3 Results and Discussion  
### Identification of VFCs in WRTs with Different Cultivars  
The VFCs in WRTs with different cultivars were analyzed using HS-SPME/GC-MS, and more than 900 peaks were preliminarily detected for each sample. After identification as described in \"2.6. _Data processing_\", 171 VFCs that could be classified into 11 categories according to their chemical structures, including esters, ketones, alkenes, aldehydes, nitrogen compounds, alcohols, aromatic compounds, oxygen heterocyclic compounds, lactones, sulfur compounds, and organic acids, were tentatively identified from the four different cultivars of WRTs, as listed in Table S1 of the Supplementary Material. More specifically, 166, 169, 166, and 169 VFCs were determined for DHP, RG, SX, and JF, respectively. Comparing the previous studies on SX [14] and RG [6] oolong teas, it was found that the number of VFCs was more abundant in our study, which may be due to the differences in extraction methods and instrument precision. However, due to the sole stationary phase of the GC column applied in the study, the identification of VFCs would be further checked using another GC column with a different stationary phase in the further work [23]. In this study, the comparison of retention times and MS data with authentic standards of some important VFCs in WRTs were applied for further identification. The number and concentration distributions of the different categories of VFCs among the four different cultivars of WRTs were further investigated (Figure 1). Overall, the number distributions of the different chemical categories of VFCs among DHP, RG, SX, and JF were similar, and esters (17%), alkenes (17%), and ketones (16%) presented the largest proportions in all types of WRTs, followed by aldehydes (14-15%), nitrogen compounds (9-10%), alcohols (9%), aromatic compounds (6-7%), and oxygen heterocyclic compounds (5\(-\)6%). Conversely, lactones (2%), sulfur compounds (2%), and organic acids (1%) comprised VFCs with minimum number proportions in all types of WRTs. The vast majority of nitrogen compounds belonged to nitrogen heterocyclic compounds (Table S1), which are sometimes referred to as heterocyclic compounds together with oxygen heterocyclic compounds. Esters, ketones and heterocyclic compounds were reported to be the most numerous compounds in RG WRT by using the consistent HS-SPME/GC-MS approach [6], which is in good agreement with our study. In addition, other research indicated that alcohols and heterocyclic compounds were dominant in SX oolong teas samples [14]. However, in our study, the proportion of alcohols was not as high as reported, possibly because the steam distillation extraction method could extract more low-boiling alcohols, and a larger number of VFCs were identified in this study, resulting in a less prominent proportion of alcohols. Figure 1: Number distribution and the compounds with top concentrations on the 11 chemical categories of volatile fragrant compounds (VFCs) in Dahongpao (DHP; (**A**)), Rougui (RG; (**B**)), Shuixian (SK; (**C**)) and Jinfo (JF; (**D**)) samples. Although it was inappropriate to compare the contents of the VFCs directly using their corresponding peak areas due to the great abundance differences presented in the GC-MS chromatogram brought about by the different functional groups, the peak area of the compounds in the same categories with the same functional groups might provide some references for their concentration distribution orders. Limonene, 1,1,6-trimethyl-1,2-dihydronaphthalene, 2-acetylfuran, dihydroactinidiolide, dimethyl disulfide, and 4-hydroxybtivric acid, which belong to alkenes, aromatic compounds, oxygen heterocyclic compounds, lactones, sulfur compounds, and organic acids, showed the highest concentrations in the corresponding chemical categories in all types of WRTs. Some differences in compounds with the highest concentrations in the other five chemical categories were observed among the four types of WRTs.