## 3 Results and Discussion  
### Optimization of HS-SPME Conditions  
The experiments were performed on the flavor-enhanced rapeseed oil. Variables such as type of SPME-arrow fiber, pre-incubation temperature, extraction temperature, and time were studied to optimize the performance of HS-SPME. The HS-SPME conditions were optimized on the basis of peak areas of analytes and performed three times repeatedly. Figure 1 shows that the influence of the type of the fiber coating, pre-incubation temperature, extraction temperature, and time on the total peak area of target pyrazines. The sensitivity and selectivity of the extraction method were determined by the properties of adsorbents on SPME fibers. The selection of a suitable fiber was the first step in developing the HS-SPME method. The performance of SPME fibers for the target compounds depends on the polarity of the analytes and the physical and chemical properties of coating types [16]. In this study, the extraction efficiency of the four commercial SPME fibers with different polar coatings was compared under the same conditions. The results showed significant differences among the various fiber coatings (\(p<0.05\)) (Figure 1A). PDMS/DVB/CAR, which is produced with a cross-linked co  
Figure 1: Influence of (**A**) the type of the fiber, (**B**) pre-incubation temperature, (**C**) extraction temperature, (**D**) extraction time on the total peak area of target compounds. coatings thus that can be used for the extraction of polar and non-polar VOCs, had the best performance (\(p<0.05\)). The single-phase of PDMS (non-polar) and PA (polar) showed the poorest performance. These results were consistent with previous studies [36]. Therefore, a PDMS/DVB/CAR (120 \(\upmu\)m \(\times\) 20 mm) fiber was chosen for the optimization of the HS-SPME conditions and further used in this study. After the type of coating was selected, the second important parameter was pre-incubation temperature. The role of the pre-incubation procedure is to volatilize the aroma compounds from the sample and build equilibrium between the sample and headspace. At the same pre-incubation time, the increase of temperature can promote the distribution of weak volatile components in the headspace and accelerate the building of equilibrium. However, higher temperatures may lead to the conversion and degradation of unstable substances. Five pre-incubation temperatures were studied: 20, 40, 60, 80, and 100 \({}^{\circ}\)C. The results (Figure 1B) showed that the higher pre-incubation temperature indeed contributed to the volatilization and equilibrium of pyrazines. However, the total peak area was not significantly different between 80 \({}^{\circ}\)C and 100 \({}^{\circ}\)C. Thus, 80 \({}^{\circ}\)C for pre-incubation temperature was chosen as the SPME condition for further studies. Extraction temperature is an important factor affecting the extraction efficiency of aroma compounds. The increasing temperature is conducive to the release of aroma compounds from the matrix at the same extraction time. However, high temperatures may also cause the degradation of components in the food matrix and decrease the absorbability of SPME fibers. Five extraction temperatures were studied: 40, 50, 60, 70, and 80 \({}^{\circ}\)C. According to Figure 1C, the extraction temperature of 50 \({}^{\circ}\)C was selected. The effect of different extraction times (30, 40, 50, 60, and 70 min) was also evaluated at 50 \({}^{\circ}\)C. The optimum time was required to reach equilibrium in three phases: The fiber coating, the headspace, and the sample. SPME under equilibrium is an important condition for carrying out the MHS-SPME operation. Compared with extraction without equilibrium, extraction under equilibrium has better repeatability [32]. As shown in Figure 1D, 50 min was chosen as the optimal extraction time, which ensured extraction efficiency and established the equilibrium of pyrazines in three phases.