## 2 Results and Discussion
### Volatile Component Analysis  
In order to establish the experimental conditions, the mixed sample was analyzed via GC\(\times\)GC-QTOFMS in triplicate. The intraday precision was evaluated by analyzing three equivalent mixed samples on the same day, and this was then repeated for three consecutive days to determine the interday precision. As shown in Table S2, the intraday and interday precision were expressed as the relative standard deviation (RSD). RSD values of no more than 25% were found in each compound in the mixed sample, demonstrating the good repeatability of the GC\(\times\)GC-QTOFMS method. Subsequently, the four flower species were analyzed by the established method. The relative contents (%) of compounds in each sample were calculated based on the ratio of the area of the corresponding peak to the total peak area; the averages of the relative contents of each compound in the _Rhododendron_ flower samples are tabulated in Table S2. Figure 3 presents the distribution (%) of the major compounds in the four different species of _Rhododendron_. Among them, benzeneacetaldehyde was found in all flower species with high content in _R. irroratum_ (6.255% \(\pm\) 0.951%), _R. delavayi_ (7.013% \(\pm\) 0.059%), and _R. annae_ (6.349% \(\pm\) 0.062%), whereas it presented with relatively low content in _R. agastum_ (2.987% \(\pm\) 0.357%), Citronellal presented the highest content in _R. annae_ (7.004% \(\pm\) 0.028%) and _R. agastum_ (7.722% \(\pm\) 0.303%), and was the second most abundant component in _R. delavayi_ (7.944% \(\pm\) 0.225%), but was slightly low in _R. irroratum_ (4.178% \(\pm\) 0.654%). Both benzeneacetaldehyde and citronellal contribute to the sweet floral profile of these samples. Benzeneacetaldehyde has a grassy odor, while citronellal has a slight hyacinth odor. 1,2,3-trimethoxy-5-methyl-benzene was detected in all species with content ranging from trace (0.243% \(\pm\) 0.023% in _R. annae_) to abundant (6.046% \(\pm\) 0.623% in _R. irroratum_). On the other hand, isopulegol was detected only in _R. irroratum_ and _R. agastum_, with a highest content of 7.722% \(\pm\) 0.407% in _R. agastum_. Thus, this compound can be used to discriminate _R. agastum_ or _R. irroratum_ from other _Rhododendron_ species. Phenylethyl alcohol accounted for a significantly high content in _R. delavayi_ (up to 8.922% \(\pm\) 0.061%) compared to in the other three species and was characterized  
Figure 2: Diagram illustrating the process of compound confirmation in GC\(\times\)GC-QTOFMS.  
as a dried rose floral aroma. Similarly, 2-nonen-1-ol, with a sweet melon odor, also presented the highest content in _R. delavayi_ (5.633% \(\pm\) 0.813%) but slightly low in _R. iroratum_ (4.299% \(\pm\) 0.288%) and _R. agastum_ (4.071% \(\pm\) 0.378%). Limonene and isopulegol presented in all species with relatively low content compared with other major compounds and with no significant differences between species. Limonene has a sweet citrus or orange odor, while isopulegol has a minty or woody odor. Although the rest of the compounds had relatively low threshold values due to their low contents, they all play a certain role in the odor characterization and finally form the special odor types of different _Rhododend_ varieties.