## 3 Results and Discussion
### Volatile Compounds of Foxtail Millet Analyzed by HS-SPME/GC-MS  
To fully understand the variations between volatile compounds in twenty foxtail millet varieties, the volatile compounds were examined using HS-SPME/GC-MS. A total of 18 volatile compounds were tentatively identified in foxtail millet samples (Table 3), including aldehydes (6), ketones (3), alcohols (3), alkanes (4), and heterocyclic compounds (2), with hexanal, heptanal, 1-pentanol, 2-heptanone, nonanal, and 1-hexanol comprising the predominant volatile compounds determined in the 20 foxtail millet varieties under study. Compared with GC-IMS, GC-MS detected fewer volatile compounds, and most of them were macromolecular compounds. Figure 4: PCA plots of different varieties of foxtail millet. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & **NO.** & **Compound** & **Aroma Description** & \multicolumn{2}{c}{**RI**} & \multicolumn{2}{c}{**CAS**} & \multicolumn{2}{c}{**Molecule Formula**} & \multicolumn{2}{c}{**Characteristiction**} & \multicolumn{2}{c}{**Identification Method**} \\ \cline{5-10}  & 1 & Hexanal & green, grassy & 800 & 796 & 66-25-1 & C\({}_{4}\)H\({}_{2}\)O & 56,72 & MS,RI \\  & 2 & Benzaldehyde & bitter diamond, cherry, nut & 962 & 995 & 100-52.7 & C\({}_{4}\)H\({}_{2}\)O & 77,106 & MS,RI \\ Aldehydes & 3 & Heptanal & rancol, pungert & 901 & 900 & 111-71.7 & C\({}_{4}\)H\({}_{2}\)O & 70,41 & MS,RI \\  & 4 & Octanal & orange peel, green & 1003 & 1000 & 124-13.0 & C\({}_{4}\)H\({}_{2}\)O & 43,84 & MS,RI \\  & 5 & Nonanal & cucumber & 1104 & 1103 & 124-19.6 & C\({}_{4}\)H\({}_{2}\)O & 57,70 & MS,RI \\  & 6 & Decanal & waxox, floral, citrus & 1206 & 1203 & 112-31-2 & C\({}_{4}\)H\({}_{2}\)O & 43,70 & MS,RI \\  & 7 & 2-Heptano & fruity, scipy & 891 & 886 & 110-43.0 & C\({}_{4}\)H\({}_{2}\)O & 43,71 & MS,RI \\ Ketones & 8 & 3-Octeno-2-none & fruity, lemon & 1040 & 1039 & 1609-64.9 & C\({}_{4}\)H\({}_{2}\)O & 55,111 & MS,RI \\  & 9 & Acetophenone & sweet & 1005 & 1053 & 98-66.2 & C\({}_{4}\)H\({}_{2}\)O & 105,77 & MS,RI \\  & 10 & 1-Pranath & oil, vanilla sweet & 765 & 755 & 721-41.0 & C\({}_{4}\)H\({}_{2}\)O & 42,70 & MS,RI \\ Alcohols & 11 & 1-Hexanal & green, fruity & 868 & 867 & 111-27-3 & C\({}_{4}\)H\({}_{2}\)O & 56,69 & MS,RI \\  & 12 & 1-Octeno-3-ol & mushroom-like, rubbery & 980 & 979 & 339-86.4 & C\({}_{4}\)H\({}_{2}\)O & 57,22 & MS,RI \\  & 13 & Tetradecane & gasoline-like, mild waxy & 1400 & 1398 & 629-59.4 & C\({}_{4}\)H\({}_{2}\)O & 57,71 & MS,RI \\  & 14 & Pentadecane & 1500 & 1496 & 629-62.9 & C\({}_{4}\)H\({}_{2}\)O & 57,71 & MS,RI \\  & 15 & Hexadecane & & 1600 & 1592 & 544-76.3 & C\({}_{4}\)H\({}_{2}\)O & 57,71 & MS,RI \\  & 16 & Nonadecane & & 1900 & 1885 & 629-92.5 & C\({}_{4}\)H\({}_{2}\)O & 57,71 & MS,RI \\ Heterocyclic & 17 & Benzowicz & rose, vegetable & 1229 & 1224 & 95-16.9 & C\({}_{4}\)H\({}_{2}\)O & 135,108 & MS,RI \\ compounds & 18 & Benzone & 1,3-dimethyl-yl & 866 & 855 & 108-38.3 & C\({}_{4}\)H\({}_{2}\)O & 91,106 & MS,RI \\ \hline \hline \end{tabular} Note: ref: RI literature value; cal RI calculated value. Identification method: MS, mass spectrum comparison using NIST4 library; RI calculation in agreement with literature value. The arena description of compounds were referred from The Good Sorms Company Information System.  
\end{table} 
Table 3: Volatile compounds tentatively identified in the 20 foetal millet varieties determined by HS-SPME/CC-MS.  
Aldehydes generally possess green, fatty, and other odors, and play a significant role in the distinctive aroma of cereals due to their low thresholds [15; 33]. Aldehydes accounted for 35.41-66.36% of the total volatile compounds (Figure 5A), and they were the main volatile components in fortail millet. These compounds include hexanal, benzaldehyde, heptanal, octanal, nonanal and decanal, and the content of hexanal was the highest compared to other aldehydes (5.49-7.36 \(\upmu\)g/g) (Figure 5B, Table S2). Hexanal is one of the main volatile components in grains, such as rice [16], barley [34], and oats [35].