## 3 Results and Discussion
### Method Development and Optimization  
#### 3.2.1 Selection of Extraction Fiber  
In HS-SPME, the extraction fibers are central throughout the experiment. The amount and rate of analyte adsorption by the extraction fiber are related to parameters such as the polarity, volatility, and boiling point of the analyte [23]. To enable the analytes to be quickly adsorbed and equilibrated by the coating on the surface of the extraction fibers, and to enable them to be detached as soon as possible during thermal desorption, it is necessary  
Figure 1: Peak shape of 100 ng/L dimethyl sulfur-containing compounds by 50/30 μm DVB/CAR/PDMS extraction fiber (**a,c,e,g**) and 75 μm CAR/PDMS extraction fiber (**b,d,f,h**).  
to select the appropriate extraction fibers according to the specific analytes to achieve the optimal HS-SPME effect [24].  
In this experiment, 50/30 \(\upmu\)m DVB/CAR/PDMS and 75 \(\upmu\)m CAR/PDMS extraction fibers were coated with a bipolar map layer, which provided better extraction results than unipolar PDMS, PDMS/DVB, and polyamide (PA)-coated extraction fibers [25]. Considering the comparison in terms of the limiting value of the adsorption effect of the extraction fibers on a particular DMS, in this study we compared the extraction effect of the two extraction fibers for three DMSs at an ionic concentration of 20%, an extraction time of 30 min, an extraction temperature of 65 \({}^{\circ}\)C, and a lower concentration (100 ng/L). The results of the extraction peak shapes using 50/30 \(\upmu\)m DVB/CAR/PDMS and 75 \(\upmu\)m CAR/PDMS extraction fibers for 100 ng/L of DMS, DMDS, and DMTS in a single standard and 100 ng/L in a mixed standard are shown in Figure 1.  
As seen from Figure 1a,c, the chromatographic peaks indicated that DMS and DMDS present a good separation in the column. The chromatographic peak height of DMS after extraction by 75 \(\upmu\)m CAR/PDMS (Figure 1b) was higher than that of DMS after extraction by 50/30 \(\upmu\)m DVB/CAR/PDMS fiber (Figure 1a). The extraction effect on DMS was superior with 75 \(\upmu\)m CAR/PDMS extraction fiber than 50/30 \(\upmu\)m DVB/CAR/PDMS extraction fiber. However, the signal-noise ratio of DMTS was weak and it was difficult to identify the peak after being extracted by 75 \(\upmu\)m CAR/PDMS extraction fibers. Hence, it indicated that DMTS was not easily adsorbed by 75 \(\upmu\)m CAR/PDMS extraction fibers at 100 ng/L. As shown in Figure 1g, at the concentration of 100 ng/L, although the adsorption capacity of 50/30 \(\upmu\)m DVB/CAR/PDMS for DMS was weak, all three DMSs were able to produce distinct peaks, i.e., at least ensure that the DMSs could be detected at 100 ng/L. Therefore, comprehensively examining the half peak width, peak height, and peak area of the chromatographic peak, the 50/30 \(\upmu\)m DVB/PDMS fibers were more sensitive to DMTS adsorption. The 50/30 \(\upmu\)m DVB/CAR/PDMS fiber had a better adsorption effect for DMSs. The 50/30 \(\upmu\)m DVB/CAR/PDMS fiber had also been used for the extraction of olfactory substances in eutrophic lakes [18; 26; 27]. The extraction effect was confirmed and applied to the extraction of DMSs in this study.