## 3 Results and Discussion
### GC-MS Analysis  
The VOCs in rice samples were extracted using the optimum extraction parameters and analyzed by GC-MS. Overall, 30 samples, including 10 from Heilongjiang, 10 from Jilin, and 10 from Liaoning, were analyzed using HS-SPME-GC-MS (Figure 2A). The VOCs are expressed as the chromatographic peaks in the total ion chromatogram (TIC) (Figure S1). A total of 33 VOCs were identified based on their respective mass spectra and RI. The detected VOCs are listed in Table S2. Heilongjiang and Jilin have 32 identical VOCs, Jilin and Liaoning samples have 30 identical VOCs, Heilongjiang and Liaoning samples have 31 identical VOCs (Figure 2B). Detected VOCs were classified into 10 chemical classes, including alkane hydrocarbons, aromatic hydrocarbons, other volatile hydrocarbons, aldehydes, aromatic aldehyde, alcohols, organic acids, fatty acids, phenol-containing, and others. The qualitative analysis was performed by class percentage, as shown in Figure 3A.  
Figure 1: Effect of (**A**) sample weight, (**B**) extraction temperature, and (**C**) extraction time on the extraction efficiency. Different small letters a, b, c in the figure indicate the significant difference between different treatment groups (\(p<0.05\)).  
Alkanes are the primary class in the rice from three areas. Among them, alkane hydrocarbons were the highest concentration, up to 41.3% in Liaoning rice samples, compared to 27.9% and 18.6% in Heilongjiang and Jilin rice samples. Aldehydes are the second largest group in Heilongjiang and Liaoning rice samples, with 14.2% and 25.3%. At the same time, Jilin rice samples showed the highest composition at 61.5% of aldehydes. Aldehydes  
Figure 3: (**A**) Qualitative analysis of VOCs, alkane hydrocarbons, aromatic hydrocarbons, other volatile hydrocarbons, aldehydes, aromatic aldehydes, alcohols, organic acids, fatty acids, phenol-containing, other compounds in a % pie chart. (**B**) Quantitative analysis of VOCs compounds analyzed by HCA. (**C**) K-means clustering of VOC compounds.  
Figure 2: (**A**) Differentiation of Heilongjiang, Jilin, and Liaoning rice. (**B**) Sample gathering provinces from Heilongjiang, Jilin, and Liaoning. The number in the figure represents the number of VOCs detected in rice in this region, and the numbers with overlapping circles represent the number of VOCs common to rice in these regions.  
are critical substances in the formation of rice flavor and play an important role in rice flavor [19]. Samples from Heilongjiang were found to be enriched in other volatile hydrocarbons (23.0%), aldehydes, aromatic aldehydes (6.7%), and alcohols (9.1%) in comparison with Liaoning and Jilin rice samples. Similarly, phenol-containing (1.5%) was more abundant in samples from Heilongjiang than in the Liaoning and Jilin rice samples. At the same time, aromatic hydrocarbons were found in more significant amounts in Liaoning (2.0%) and Heilongjiang (1.8%) than in Jilin (0.7%). The relative levels of organic acids and fatty acids were the same in all rice samples.  
The unsupervised hierarchical clustering and K-means clustering quantitative analysis of 10 chemical classes in rice samples from Heilongjiang, Jilin, and Liaoning provinces are shown in Figure 3B. Heilongjiang and Jilin rice samples clustered together and away from Liaoning rice samples. The Liaoning rice samples showed a higher quantity of other volatile hydrocarbons, alkane hydrocarbons, aromatic hydrocarbons, alcohols, aromatic aldehyde, and fatty acids. Higher amounts of phenol-containing and others were found in the Heilongjiang rice samples. Higher quantities of aldehydes and organic acids were present in the Jilin rice sample.