## 2 Results and Discussion
### Identification of Common Volatile Components  
GC\(\times\)GC-QTOFMS was used to characterize the detailed chemical composition of all the samples. Several hundred peaks were generated in the GC\(\times\)GC contour plot with a peak detection threshold of S/N \(>\) 3. In total, 129 volatile compounds were tentatively identified in four _Rhododendron_ samples based on (1) spectral similarity (both match and reverse match scores of \(>\)750), (2) comparison with molecular ions (within 5 ppm), if they existed, and (3) retention index (RI, \(\pm\)35). Table S1 lists the complete information of the 129 volatile constituents detected by GC\(\times\)GC-QTOFMS.  
Figure 2 introduces the identification process of two examples (1,2-dimethoxybenzene and lilac aldehyde D). First, the National Insititute of Standards and Technology (NIST) library search for Peak 162 and Peak 189 resulted in 7 and 5 possible compounds, respectively, with match factor \(>\)750. Then, only exact mass analyses within a mass accuracy of \(<\)5 ppm were considered. For Peak 162, the measured accurate mass was 138.0676, which corresponds to a formula of C\({}_{8}\)H\({}_{10}\)O\({}_{2}\). The accurate mass reduced the number of possible compounds to two isomers (1,2-dimethoxybenzene and 1,4-dimethoxybenzene). Last, their retention indices were reviewed for further confirmation. The GC\(\times\)GC analysis provided an experimental RI value of 1151 for this peak, which matched 1,2-dimethoxybenzene (literature RI value of 1151) rather than 1,4-dimethoxybenzene (RI\({}_{\text{lit}}\) = 1168). Therefore 1,2-dimethoxybenzene was the final identified compound for Peak 162.  
Taking Peak 189 as another example: The NIST library search provided several possible compound matches. Among them, seven possible compounds were screened out according to their relatively high match scores. Subsequently, the mass spectrum provided a measured mass of 168.1148, corresponding to a chemical formula of C\({}_{10}\)H\({}_{16}\)O\({}_{2}\). This indicated that 2-methyl-2-(2-oxopropyl) cyclohexanone, lilac aldehyde D, and 2-hydroxy-4,4,6,6-tetramethyl-2- cyclohexen-1-one were three  
Figure 1: Chromatographic analysis of _Rhododendron_ by GC-quadrupole time-of-flight mass spectrometry (QTOFMS) and a comprehensive two-dimensional gas chromatography–mass spectrometry (GC\(\times\)GC)-QTOFMS color diagram (1: linalool; 2: 2-nonen-1-ol; 3: linalool oxide; 4: \(p\)-cymenene; 5: benzoic acid, methyl ester).  
possible compounds with the theoretical molecular ion mass of 168.1145. Lastly, the experimental RI value of the peak (RL\({}_{\rm exp}\) = 1190) confirmed that lilac aldehyde D with RI\({}_{\rm lit}\) of 1169 was the final identified compound for Peak 189, while the other two candidates, 2-methyl-2-(2-oxopropyl) cyclohexanone (RI\({}_{\rm lit}\) = 1360) and 2-hydroxy-4,4,6,6-tetramethyl-2-cyclohex-1-one (RI\({}_{\rm lit}\) = 1272), were screened out. The results emphasis the importance of applying further confirmation on the base of the spectral library match, since the compound with the highest match factor might be mistaken for the identity of the component [31]. In conclusion, with complementary identification processes, GC\(\times\)GC coupled with high-resolution QTOFMS produces more precise compound identification results.