## 3 Results and Discussion
### Feature Extraction and Selection  
The selection of characteristic parameters of gas-sensitive signals has a considerable influence on the accuracy of the detection model. In this paper, six eigenvectors were selected to represent fish meal samples with different qualities via response curves. These eigenvalues were the integral value, wavelet energy value, maximum gradient value, average differential value, relation steady-state response average value, and variance value.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Retention** & **Molecular** & \multirow{2}{*}{**Volatile Compounds**} & \multicolumn{2}{c}{**RI**} \\ \cline{4-6}  & **Time** & **Formula** & & **Calculated** & **Literature** & **Identification** \\ \hline
1 & 2.45 & C\({}_{3}\)H\({}_{0}\)N & N,N-dimethyl-methylamine & - & 502 & MS, RI \\
2 & 3.4 & C\({}_{5}\)H\({}_{0}\)O & Pentanal & - & 699 & MS, RI \\
3 & 4.61 & C\({}_{3}\)H\({}_{0}\)O\({}_{2}\) & Propanoic acid & 700 & 700 & MS, RI \\
4 & 5.35 & C\({}_{5}\)H\({}_{10}\)O & (Z)-2-penten-1-ol & 767 & 767 & MS, RI \\
5 & 6.31 & C\({}_{6}\)H\({}_{20}\)O & Hexanal & 800 & 800 & MS, RI \\
6 & 7.65 & C\({}_{4}\)H\({}_{0}\)O\({}_{2}\) & Butanoic acid & 832 & 805 & MS, RI \\
7 & 8.44 & C\({}_{6}\)H\({}_{0}\)O & (E) -2-hexanal & 851 & 854 & MS, RI \\
8 & 9.28 & C\({}_{5}\)H\({}_{10}\)O\({}_{2}\) & 3-Methyl-butanoic acid & 871 & 863 & MS, RI \\
9 & 9.71 & C\({}_{5}\)H\({}_{10}\)O\({}_{2}\) & 2-Methyl-butanoic acid & 882 & 861 & MS, RI \\
10 & 10.47 & C\({}_{5}\)H\({}_{10}\)O\({}_{4}\) & Heptan & 900 & 901 & MS, RI \\
11 & 11.19 & C\({}_{6}\)H\({}_{8}\)NO\({}_{3}\) & Methoxy-phenyl-oxime & 918 & - & MS \\
12 & 12.89 & C\({}_{7}\)H\({}_{0}\)O & Benzaldehyde & 958 & 962 & MS, RI \\
13 & 13.47 & C\({}_{6}\)H\({}_{12}\)O\({}_{2}\) & 4-Methyl-pentanoic acid & 972 & 949 & MS, RI \\
14 & 13.83 & C\({}_{6}\)H\({}_{16}\)O & 1-Octen-3-ol & 980 & 980 & MS, RI \\
15 & 14.14 & C\({}_{6}\)H\({}_{8}\)NO\({}_{2}\) & Methyl-carbamic acid phenyl ester & 987 & - & MS \\
16 & 14.54 & C\({}_{6}\)H\({}_{12}\)O\({}_{2}\) & Hexanoic acid & 997 & 990 & MS, RI \\
17 & 14.65 & C\({}_{7}\)H\({}_{10}\)N\({}_{2}\) & 2-ethyl-5-methyl-pyrazine & 1000 & 1005 & MS, RI \\
18 & 14.75 & C\({}_{6}\)H\({}_{10}\)O & octanal & 1002 & 1003 & MS, RI \\
19 & 15.73 & C\({}_{6}\)H\({}_{10}\)O\({}_{3}\) & 3-Hydroxy-2-dimethyl-butanoic acid ethyl ester & 1027 & - & MS \\
20 & 16.11 & C\({}_{7}\)H\({}_{0}\) & Benzyl alcohol & 1036 & 1036 & MS, RI \\
21 & 16.34 & C\({}_{6}\)H\({}_{0}\)O & Benzeneacetaldehyde & 1042 & 1045 & MS, RI \\
22 & 16.84 & C\({}_{6}\)H\({}_{10}\)O\({}_{2}\) & 5-Ethyldihydro-2(3H)-furanone & 1054 & 1057 & MS, RI \\
23 & 18.79 & C\({}_{6}\)H\({}_{18}\)O & Nonanal & 1103 & 1104 & MS, RI \\
24 & 21.46 & C\({}_{6}\)H\({}_{8}\)NO & 2-Perepidinone & 1175 & 1174 & MS, RI \\
25 & 21.67 & C\({}_{6}\)H\({}_{8}\) & Naphthalene & 1181 & 1182 & MS, RI \\
26 & 22.1 & C\({}_{10}\)H\({}_{20}\)O & 2-Decanone & 1192 & 1193 & MS, RI \\
27 & 22.56 & C\({}_{10}\)H\({}_{20}\)O & Decanal & 1205 & 1206 & MS, RI \\
28 & 25.67 & C\({}_{11}\)H\({}_{20}\)O & 2-Undecanone & 1293 & 1294 & MS, RI \\
29 & 26.46 & C\({}_{11}\)H\({}_{18}\)N\({}_{2}\) & 2,5-Dimethyl-3-(3-methyl-butyl) pyrazine & 1320 & 1315 & MS, RI \\
30 & 28.58 & C\({}_{14}\)H\({}_{30}\) & Tetradecane & 1400 & 1400 & MS, RI \\
31 & 30.47 & C\({}_{15}\)H\({}_{20}\)O & Butylated hydroxytoluene & 1519 & 1513 & MS, RI \\
32 & 34.78 & C\({}_{16}\)H\({}_{32}\)O\({}_{2}\) & n- Hexadecanoic acid & 1960 & 1968 & MS, RI \\ \hline \hline \end{tabular}
\end{table}
Table 4: The identified volatile flavor compounds of fish meal headspace using SPME-GC-MS.