## 3 Results and Discussion
### Method Development and Optimization
#### 3.2.5 Orthogonal Optimization  
Most current optimizations of HS-SPME conditions use a single factor as a variable to investigate the optimal conditions for the variation of this single factor when other factors are fixed [18; 19; 20; 21]. This method often ignores the influence of interactions between multiple factors on target extraction. Therefore, in this study, on the basis of the previous single-factor optimization, the ion concentration (16%, 18%, and 20%), extraction time (77 min, 87 min, and 97 min), and extraction temperature (45 \({}^{\circ}\)C, 55 \({}^{\circ}\)C, and 55 \({}^{\circ}\)C) were selected as the three factors and three levels for orthogonal optimization experiments using 200 ng/L mixed standard working solution, which comprehensively considered the three values of the three factors to ensure the selection of the most accurate and stable HS-SPME conditions. The specific arrangements of the orthogonal experiments are listed in Table 2.  
Figure 3 shows the results of the orthogonal experiments. As can be seen from the figure, under the nine different orthogonal experimental conditions, there was little difference in the peak areas of the three DMSs, but it was still necessary to refine the comparison and analysis of the three DMSs peak areas separately. The peak area of DMS was the largest in group 9 (20%, 97 min, and 55 \({}^{\circ}\)C), followed by group 8 (20%, 87 min, and 50 \({}^{\circ}\)C) and group 6 (18%, 97 min, and 50 \({}^{\circ}\)C). The peak area of DMDS was larger in groups 8, 6, and 9. The comparison of the peak areas of DMDS was as follows: group \(8>\) group 6 \(>\) group 9. The interaction of larger ion concentration (18% and 20%) with different extraction times (87 min and 97 min) and extraction temperatures (50 \({}^{\circ}\)C and 55 \({}^{\circ}\)C) contributed to the increase in the peak areas of DMS and DMDS. The peak area sizes of DMTS were more evenly distributed among the nine orthogonal experiments, but the peak area sizes of DMTS were largest in groups 6 and 7 (20%, 77 min, and 60 \({}^{\circ}\)C), followed by group 5 (18%, 87 min, and 60 \({}^{\circ}\)C) and group 8. The comparison of the extraction effect of the three DMSs peak areas showed that the best experimental results were obtained for groups 6 and 8.  
Further comparative analysis of groups 6 and 8 showed that although the extraction peak areas of DMTS in group 8 were smaller than those in group 6, the extraction peak areas of DMS and DMDS were larger than those in group 6. Comparing the extraction efficiencies of the two groups, the extraction time of group 8 (87 min) was less than that of group 6 (97 min). Therefore, the extraction process was completed faster under the experimental conditions of group 8. Therefore, group 8 (20% ion concentration, 87 min extraction time, and 50 \({}^{\circ}\)C extraction temperature) was selected for the final HS-SPME conditions for this optimization experiment.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Experimental Group** & **Ion Concentration/\%** & **Extraction Time/min** &
\begin{tabular}{c} **Extraction** \\ **Temperature/\({}^{\circ}\)C** \\ \end{tabular} \\ \hline
1 & 16 & 77 & 50 \\
2 & 16 & 87 & 55 \\
3 & 16 & 97 & 60 \\
4 & 18 & 77 & 55 \\
5 & 18 & 87 & 60 \\
6 & 18 & 97 & 50 \\
7 & 20 & 77 & 60 \\
8 & 20 & 87 & 50 \\
9 & 20 & 97 & 55 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Orthogonal experiments.  
Figure 3: Effect of different orthogonal experimental treatment groups on the detection of DMS, DMDS, and DMTS.