## 2 Results
### Optimization of Solid-Phase Microextraction (SPME) Parameters
#### 2.1.3 Effect of Extraction Time and Temperature  
The time of extraction is one of the most important parameters during the SPME procedure. Three different extraction times were analyzed, i.e., 15, 30, and 45 min. The extraction was carried using DVB/CAR/PDMS fiber in 2.5% _v/v_ ethanol solution, with the addition of 20% _w/v_ of NaCl and EDTA in the total sample volume of 10 mL. The temperature of extraction was set at 50 \({}^{\circ}\)C. As can be seen from Figure 3, the total peak area increased significantly for 30 min extraction time when compared to 15 min. Further extension of the exposure time to 45 min did not significantly increase the sensitivity of extraction but only extended the overall analysis time. Figure 3: Effect of extraction time on the total peak area of analyzed VSCs. Ethanol concentration 2.5% _v/v_; NaCl addition 20% _w/v_; EDTA addition 1% _w/v_; extraction temperature: 50 \({}^{\circ}\)C; and SPME fiber: DVB/CAR/PDMS. Figure 2: The effect of the addition of EDTA on volatile sulfur compounds (VSCs) extraction without (0% _w/v_) and with (20% _w/v_) NaCl treatment at different alcohol concentrations (2.5%, 5%, and 10% _v/v_). Extraction temperature: 50 \({}^{\circ}\)C, extraction time: 15 min; SPME fiber: DVB/CAR/PDMS. In order to estimate the influence of the temperature of extraction on the efficiency of the SMPE process, five distinct temperatures were evaluated, namely: 20, 35, 50, 65, and 75 \({}^{\circ}\)C. The preconcentration was conducted with use of DVB/CAR/PDMS fiber in 2.5% _v/v_ ethanol solution enriched in NaCl (20% _w/v_) and EDTA for 30 min, as was established in previous experiments. The obtained results are presented in Table 2. It can be seen that the best overall results were obtained when the temperature of 35 \({}^{\circ}\)C was used. The extraction at lower temperature (20 \({}^{\circ}\)C) gave higher signal response for lower boiling compounds (ethanethiol, dimethyl-sulfide, 1-propanethiol, thiophene, diethyl-sulfide, and 1-butanethiol) (Table 2), while the rest of compounds exhibited higher signals at elevated temperatures (mostly 35 or 50 \({}^{\circ}\)C). These results are in agreement with those reported by [PERSON] et al. [4], where the sensitivity of adsorption of dimethyl sulfide, diethyl sulfide, and dimethyl disulfide decreased with increasing the temperature in the range of 25-75 \({}^{\circ}\)C. In this study, application of temperatures above 50 \({}^{\circ}\)C (i.e., 65 and 75 \({}^{\circ}\)C) resulted in a stark decrease in the obtained peak areas, which may be due to formation of artefacts caused by thermal degradation. Another important issue when VSCs are taken into consideration is the formation of artifacts during SMPE analysis coupled to GC-MS detection. The research conducted on _Allium_ plants shows that some of the compounds detected by the SPME/GC-MS technique were not present in the original sample but were formed during analytical procedures as artifacts [30; 31; 32]. Sulfur compounds are often unstable and reactive and can undergo transformation to other compounds, e.g., methanethiol can be oxidized to dimethyl disulfide and dimethyl sulfide to dimethyl sulfoxide [33]. Several solutions were proposed to overcome this problem. [PERSON] et al. [30] reported that the use of SPME coupled to liquid chromatography with UV detection allowed them to avoid thermal degradation of the compounds present in raw, cooked, and distilled garlic samples on the injector. [PERSON] et al. [34] applied the use of static headspace injection and cool-on-column gas chromatography coupled with sulfur chemiluminescence detector to analyze 68 various samples of Australian wines in which reductive aroma was observed. The authors were focused on such compounds as hydrogen sulfide, methanethiol, ethanethiol, methyl thioacetate, ethyl thioacetate, dimethyl sulfide, diethyl sulfide, carbon disulfide, dimethyl disulfide, and diethyl disulfide. Among them, only ethyl thioacetate and diethyl disulfide were not detected in any of 68 tested samples.