## 3 Results and Discussion
### Method Validation  
Once the parameters of the SBSE technique for volatile sulphur compounds in distillates were optimized, the method was analytically validated. First, the linearity range of each compound was determined from its calibration curve. For calibration, six concentration levels were used, in triplicate, at 7% (\(v/v\)) alcoholic grade of each volatile sulphur compound to be studied and covering the range of concentrations expected to be found in the samples (Appendix A, Table A2). In general, good correlation coefficients were obtained, with values (\(\mathbf{r}^{2}\)) greater than 0.991 in eight of the ten compounds studied. EMS and P showed erratic relative chromatographic area values and their calibration curves could not be established. The calibration curve equations for the remaining eight sulphur compounds are shown in Table 4. 1-pentanetthiol together with 1-propanethiol and 1-butanethiol had previously shown poor linearity in a study of the determination of volatile sulphur compounds in fruit brandes by SPME [11], whereas ethyl methyl sulphide exhibits the lowest boiling point and octanol-water partition coefficient (Log K\({}_{\mathrm{ow}}\)) of all of the sulphur compounds considered in this work, which could explain these anomalous chromatographic values (Table 1).  
Subsequently, the detection and quantification limits (DL and QL) for each sulphur compound were calculated as three and ten times, respectively, the standard deviation of the y-intercept of the regression lines divided by its slope [28]. As can be seen, the DL and QL values found were between 0.04-0.93 \(\mu\)g/L and 0.13-3.1 \(\mu\)g/L, respectively (Table 4). In general, lower DL and QL values were obtained for those volatile sulphur compounds with higher Log K\({}_{\mathrm{ow}}\) values (Table 1). This is logical considering the apolar character of PDMS.  
Finally, the inter-twister and inter-day repeatability values for each compound were evaluated (Table 4). For this purpose, the same sample prepared in absolute ethanol at intermediate concentrations in the linear range for each compound was analyzed under the optimized SBSE conditions on different days (3 days) and with five different twisters each day. The corresponding RSD values obtained for the inter-twister and inter-day repeatability are shown in Table 4. The RSD values obtained were from 5.9 to 21.8% for the inter-twister repeatability and from 7.2 to 27.5% for the inter-day values. 1-Pentanetthiol exhibited high RDS values, which could be due to possible instability problems that were observed during the calibration phase. The values obtained demonstrate the high precision of the SBSE method optimized with values around 20% or lower.  
For recovery, a non-aged distillate sample was spiked with a global standards solution at five levels of concentration. These concentrations coincided with those employed in the calibration phase. The slopes of the lines obtained for each compound were compared with those corresponding to the calibration (\(t\) criterion). In general, there was no significant difference between both slopes at \(p\leq 0.05\). The recovery values ranged from 84.8% for TC to 119.4% for DEDS, which corroborate the accuracy of the SBSE method developed.