## 3 Results
### Analysis of HS-GC-IMS Test Results  
#### 3.2.1 Comparison of Differences in VOCs of Silage Based on HS-GC-IMS  
Using the Reporter plugin, a comparison of the spectral differences between the two groups of silage feed samples is shown in Figure 3. The 3D spectrum (Figure 3a) illustrates the ion migration time (normalized), gas chromatography retention time (s), and ion peak intensity along the X-, Y-, and Z-axes, respectively. However, due to the high degree of similarity between the VOCs in the two groups, visually comparing the differences using the 3D spectrum is challenging. Therefore, a two-dimensional plane-view spectrum was obtained through dimensionality reduction. The two-dimensional plane-view spectrum (Figure 3b) normalizes the ion migration time and reaction ion peak (RIP) position. The entire spectrum background is colored blue, while the red vertical line at 1.0 on the horizontal axis represents the RIP peak. The horizontal axis represents the ion migration time, and the vertical axis represents the gas chromatography retention time (in seconds). Each point on the spectrum corresponds to a VOC, with its intensity representing the VOC content. The color of each point indicates the intensity, where white represents low intensity, red signifies high intensity, and deeper colors indicate higher ion peak intensities. Figure 3c displays a GC-IMS two-dimensional plane-view difference spectrum created by selecting the normal silage spectrum as the reference and subtracting the signal peaks of the abnormal silage spectrum. This approach provides a clearer comparison of the differences between the two VOC profiles. If the VOCs in both groups are consistent, the background after subtraction will appear white. Conversely, if the VOC concentration in the abnormal group is higher than that of the reference group, the resulting background will be red, and vice versa. The GC-IMS spectrum results indicate that a significant number of VOCs are concentrated within the retention time range of 1000 s in the feed. The spectrum clearly illustrates the differences in the properties and signal strength of the VOCs between the two groups. Most VOC ions in abnormal silage exhibit longer migration times and significantly higher concentrations than those in normal silage. Figure 3c shows the differences in both the quantity and composition of flavor compounds in the abnormal group compared to the normal group.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sensor**} & \multicolumn{2}{c}{**Group (Mean \(\pm\) SD)**} & \multirow{2}{*}{**\(p\)-Value**} \\  & **YCQZ (\(n\) = 9)** & & \\ \hline W1C & 3.36 \(\pm\) 0.54 & 2.17 \(\pm\) 0.71 & 0.001 ** \\ W5S & 43.71 \(\pm\) 23.40 & 7.54 \(\pm\) 7.19 & 0.000 ** \\ W3C & 3.51 \(\pm\) 0.59 & 2.37 \(\pm\) 0.88 & 0.005 ** \\ W6S & 1.75 \(\pm\) 0.24 & 1.35 \(\pm\) 0.45 & 0.032 * \\ W5C & 3.51 \(\pm\) 0.62 & 2.49 \(\pm\) 1.02 & 0.020 * \\ W1S & 5.14 \(\pm\) 1.38 & 4.61 \(\pm\) 3.30 & 0.665 \\ W1W & 2.96 \(\pm\) 0.80 & 1.57 \(\pm\) 0.65 & 0.001 ** \\ W2S & 4.37 \(\pm\) 1.12 & 4.26 \(\pm\) 3.25 & 0.926 \\ W2W & 3.83 \(\pm\) 0.76 & 2.36 \(\pm\) 1.17 & 0.006 ** \\ W3S & 1.10 \(\pm\) 0.09 & 1.10 \(\pm\) 0.10 & 0.970 \\ \hline \hline \end{tabular}  
* \(p<\) 0.05 **\(p<\) 0.01.  
\end{table}
Table 2: Analysis of variance (ANOVA) was conducted on the 10 sensors of E-nose for two sets of samples.  
Figure 3: Three-dimensional (**a**) and 2D top view (**b**) HS-GC-IMS map of VOCs of silage. (**c**) HS-GC-IMS 2D plane-view difference spectrum.