## 3 Results and Discussion
### Method Development and Optimization
#### 3.2.3 Optimization of Extraction Time  
The effect of extraction time on the results was more obvious. The extraction time for HS-SPME usually ranges from a few minutes to an hour or longer, depending on factors such as the solvent matrix, analytes, extraction fibers, and solution volume [25]. In this study, the extraction of 200 ng/L mixed standard solution with 50/30 \(\upmu\)m DVB/CAR/PDMS extraction fibers was carried out at an ion concentration of 20% and an extraction temperature of 65 \({}^{\circ}\)C. The extraction of the three DMSs was assessed using extraction times of 20 min, 30 min, 40 min, 50 min, 60 min, 90 min, and 120 min.  
As can be seen in Figure 2b, the peak areas of the three DMSs varied significantly with the increase in extraction time and showed a gradual increase until 90 min, and only the peak area of DMTS showed inflection points at 50 min and 90 min. The correlation was fitted to DMS, and the fitted image in Figure 2b shows that the peak area of DMS increased rapidly up to 60 min, after which the rate of increase slowed down until 90 min, after which the peak area was less affected by the extraction time. The fitted DMDS curve showed that the peak area of DMDS increased exponentially with an increase in the extraction time up to 120 min, and there was a tendency to continue increasing. The extraction time had the most significant effect on the DMDS peak area. The curve fitting of DMTS showed that the peak area of DMTS exhibited a parabolic trend with the increase in extraction time and reached the maximum value when the extraction time was 87 min, and the sudden drop in the peak area of DMTS at 60 min was probably due to the internal competition among the three DMSs for adsorption. With the increase in extraction time, DMTS was heavily adsorbed by the extraction fibers before 90 min, and the large drop in DMTS at 120 min indicated that the extraction of DMTS by the fiber was oversaturated, and the competitive adsorption of DMS, DMDS, and DMTS caused DMTS to fall off the fiber again.  
In this study, although the GC-FPD was highly selective and sensitive to DMSs, the extraction fibers did not exhibit this property. Therefore, when the 50/30 \(\upmu\)m DVB/CAR/PDMS extraction fiber reached the saturated state after an extended extraction time, DMDS and DMS, which have higher affinity for this fiber, replaced DMTS, which has lower affinity, through competitive adsorption [18]. The peak areas of different odorants require different times to reach the inflection point of extraction change, and the time point before reaching the equilibrium point was chosen as the extraction time to efficiently and rapidly extract  
Figure 2: Effect of ionic strength, extraction time, and extraction temperature on the detection of DMS, DMDS, and DMTS during extraction (the blue line is the trend line and the red line is the fitted curve).  
and adsorb odorants [18; 28]. In this study, to ensure that DMSs can be extracted and adsorbed more stably and to reduce the peak area error affected by the change in extraction time due to the experimental operation, according to the fitting results of the three DMSs, a time gradient of 87 min and 10 min before and after it, that is, 77, 87, and 97 min, were chosen as the extraction times in the orthogonal optimization experiment.