## 2 Results and Discussion
### Optimization of the BA\(\mu\)E-\(\mu\)LD Efficiency
#### 2.2.2 Microextraction Parameters  
In this section, the first parameter optimized was the effect of the agitation stirring, in which three different speeds were tested, i.e., 750, 990, and 1250 rpm. From the results achieved (data not shown), it was observed that the best results occurred when 990 rpm are used, in line with previous reports [29; 30; 31; 32; 33; 34; 35; 36] and having been selected for further assays. Afterwards, the solution pH was also adjusted in order to promote partially or completely non-ionized TCAs, which may condition the the recovery yields through reverse-phase interactions [8; 29; 39]. The pK\({}_{\mathrm{a}}\) values for the six TCAs under study vary between 6 and 10, noticing that at pH 12, all of them are in the non-ionized form. Figure 4a,b show the results obtained from assays performed at several pH values (2.0, 5.5, 8.0 and 12.0), where the best data are attained by using the C\({}_{18}\) coating at pH 12.0, once the six target TCAs become non-ionized [40]. However, when using the SX phase, the recovery yields did not vary significantly with pH variation. This observation can be attributed to additional chemical interactions promoted by the latter sorbent phase, other than reverse-phase type,  
Figure 3: Effect of MeOH (**a**) and MeOH:ACN mix (**b**) stripping solvents on the back-extraction of the six TCAs from the different polymer phases by BA\(\mu\)E-\(\mu\)LD/LVI-GC-MS(SIM). The error bars represent the standard deviation of three replicates. e.g., dipole-dipole. For this reason, the optimization was pursued using only C\({}_{18}\) polymer at pH 12. The next optimization step was to study the effect of the medium polarity on the recovery yields for the six TCAs, varying the content (0 to 20%) of an organic modifier (MeOH). It is usually employed to minimize the possible adsorption of the analytes to the glass walls of the sampling flask (\"wall-effect\"), which generally occurs for compounds with log \(K_{\text{O/W}}\) higher than 4. However, it can also lead to a solubility increment of the analytes in the aqueous media, hindering the microextraction process [8; 28]. As expected, and once the six TCAs present log \(K_{\text{O/W}}\) lower than 5, the best results were attained without MeOH addition. Afterwards, the matrix ionic strength was evaluated by increasing amounts of NaCl (0 to 20%). An inert salt is usually added to the aqueous matrix to promote the \"salting-out\" effect, reducing the target compounds solubility in water. Commonly, for analytes presenting a log \(K_{\text{O/W}}\) higher than 3.5, the addition of an inert salt does not improve the recovery yields through the BA\(\mu\)E approach. This can be caused by several reasons; it can caused by the \"oil-effect\" phenomena, promoting the migration of non-polar compounds to the surface of the aqueous media, minimizing the contact between the analytes and the microextraction device; it can enhance the viscosity by decreasing the equilibrium kinetics; and the polymeric surface area can be blocked through the salt ions. Nevertheless, discrepancies are sometimes observed, and therefore, this parameter should be always evaluated [8; 28]. Figure 4c depicts the obtained results, where the highest recovery yields were obtained by using 5% of NaCl. A possible explanation to this finding is that there is a maximum efficiency until that value, where the \"salting-out\" effect takes place, i.e., the  
Figure 4: Effect of matrix pH using SX (**a**) and C\({}_{18}\) (**b**) sorbent phases, percentage of NaCl (**c**) and equilibrium time (**d**) on the microextraction efficiency of six TCAs in aqueous media by BA\(\mu\)E-\(\mu\)LD/LVI-GC-MS(SIM). The error bars represent the standard deviation of three replicates. available water to dissolve the analytes is reduced with the consequent increment on the recovery yields.