## 3 Results and Discussion
### Validation of MHS-SPME-Arrow Method  
The analytical performance of the MHS-SPME-arrow-GC-MS was evaluated in terms of LOD, LOQ, inter-day precision, intra-day precision, and recovery, as shown in Tables 1 and 2. This method allows the analysis of a variety of different pyrazines, and the selected internal standard is pyridine with a similar structure to pyrazines. Therefore, good selectivity is a necessary requirement for the correct identification and quantification of all analytes. The identification and quantitation of target compounds, including internal standard, were based on the retention time and different selected ions, which are listed in Table 1.  
For the LOD and LOQ determinations, the refined rapeseed oil (\"pyrazine-free\") was used as the matrix to prepare standard solutions. The suitable concentration was estab \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & **Retention** & \multirow{2}{*}{**Selected Ions 1**} & **LOD 2** & **LO 2** & **Intra-Precision** & **Inter-Precision** & **Concentration** \\  & **Time (min)** & & **(ng/g)** & **(ng/g)** & **RSD (\%)** & **RSD (\%)** & **Level 1 (ng/g)** \\ \hline
3-methyl, pyridine 4 & 15.93 & 66, 92, 93 & – & – & – & – & – \\
2-methyl-pyrazine & 14.78 & 67, 53, 94 & 3 & 6 & 7.13 & 8.66 & 907 \\
2,5-dimethyl-pyrazine & 16.62 & 42, 81, 108 & 5 & 20 & 5.11 & 5.67 & 5011 \\
2,6-dimethyl-pyrazine & 16.90 & 40, 42, 108 & 8 & 20 & 13.68 & 12.14 & 264 \\ Ethly-pyrazine & 17.05 & 80, 108, 107 & 2 & 6 & 12.28 & 14.77 & 192 \\
2-ethyl-6-methyl-pyrazine & 18.93 & 94,122, **121** & 3 & 6 & 9.64 & 12.27 & 363 \\
2-ethyl-3-methyl-pyrazine & 19.21 & 94, 122, **121** & 3 & 10 & 7.83 & 9.56 & 586 \\
2-trimethyl-pyrazine & 19.80 & 42, 81, **122** & 3 & 6 & 10.61 & 11.01 & 622 \\
2,6-diethyl-pyrazine & 21.04 & 108, 136, **135** & 3 & 10 & 13.69 & 15.10 & 43 \\
3-ethyl-2-3-dimethyl-pyrazine & 21.41 & 108, 136, **135** & 4 & 20 & 11.28 & 10.78 & 1837 \\
2-ethyl-3-5-dimethyl-pyrazine & 22.28 & 108, 136, **135** & 5 & 20 & 13.21 & 15.08 & 129 \\
3,5-diethyl-2-methyl-pyrazine & 23.63 & 122, 135, **149** & 6 & 20 & 13.09 & 12.99 & 177 \\ Isopropyl-pyrazine & 29.02 & 67, 120, **119** & 60 & 180 & 12.67 & 14.98 & 190 \\ SH-5-Methyl-6-7- & - & - & - & - & - & - \\ dihydrocyclopenta-pyrazine & 30.18 & 119, 133, **134** & 25 & 50 & 10.00 & 14.02 & 94 \\ \hline \hline \end{tabular}  
* Quantification ions are present in bold. 2 LOD and LOQ were obtained by standard solution of pyrazines, and precision was evaluated by commercial flavor rapeseed oil. 3 The concentration of pyrazines in the sample during the repeatability test. 4 Internal standards. Abbreviations: RSD—relative standard deviation.  
\end{table} 
Table 1: List of retention time, selected ions, limits of detection (LOD), limits of quantification (LOQ), inter-day precision, intra-day precision of 13 target pyrazines in oils. Figure 2: The number of pyrazines that met the following conditions: (**A**) R2 \(>\) 0.75; (**B**) 0.4 \(<\beta<\) 0.95, using solid phase microextraction (SPME)-arrow fiber and traditional SPME fiber, respectively. lished as LOD and LOQ of pyrazines with the signal-to-noise ratio (S/N) of pyrazines being 3.0 and 10.0, respectively [38]. As shown in Table 1, the LODs and LOQs of 13 pyrazines were in the range of 2-60 ng/g and 6-180 ng/g, respectively, indicating that MHS-SPME can be used in routine quantitation of pyrazines. To evaluate the precision of the method, five parallel experiments were carried out on commercial flavor-enhanced rapeseed oils to calculate the intra-day precision. The inter-day precision was determined by analyzing the same sample five times a week for three weeks. Both intra-day precision and inter-day precision were expressed as the relative standard deviation (RSD). The results showed that the RSD of intra-day and inter-day were both lower than 16%, verifying the good precision of the MHS-SPME-arrow method. Model experiments were also carried out to evaluate accuracy.