## 3 Results and Discussion
### Microextraction Stage  
Since BA\(\mu\)E is a static microextraction technique, the equilibrium between the matrix and the sorbent phase depends on the affinity of each analyte to the PS-DVB polymer. Therefore, the main experimental conditions that may affect the microextraction stage, including the kinetic (extraction time and agitation speed) and thermodynamic (pH, ionic strength, and polarity of the matrix) parameters, were evaluated to achieve the best extraction efficiency for the two target analytes.  
Since the beginning, it was noticed that pH is a very important parameter, as the UV filters under study are extremely hydrophilic compounds. Thus, the effect of the matrix pH was assessed, ranging from 1.0 to 8.0. As the extraction efficiency decreased with the increase of the pH (Figure 2a), pH 2.0 was chosen for further studies. Due to the very low and even negative p\(K_{\mathrm{a}}\) values involved (Table 1), the analytes were deprotonated even in low values of pH. However, maximum adsorption at more acidic pH values indicated that the low pH contributed to the increase of H\({}^{+}\) ions on the adsorbent surface, resulting in strong electrostatic attraction between the positively charged anionic PS-DVB surface and the deprotonated analyte ions. Subsequently, the equilibrium time and agitation speed were evaluated, where the former has a great effect on the microextraction kinetics, as it may limit the distribution of the target compounds between the matrix and the sorbent phase. On the other hand, the later influences the mass transfer (diffusion) of both UV filters towards the analytical device through the floating sampling technology approach. Therefore, equilibrium times between 1 and 16 h were studied, in which the results obtained revealed that 16 h was needed (Figure 2b), ensuring the best equilibrium conditions. Besides the slow kinetics demanding a substantial period of equilibrium time, this analytical approach can be performed overnight without any special requirements. Agitation speed was then assessed through assays performed at 750, 1000, and 1250 rpm, and, according to the data obtained (data not shown), an agitation speed of 750 rpm provided much higher stability on the analytical devices during the microextraction process and was chosen for the subsequent studies.  
Finally, the matrix characteristics such as ionic strength and polarity were also evaluated, as they can significantly affect the performance of the sample enrichment. To check the effect of the ionic strength, assays were performed, with the addition of NaCl having concentrations ranging from 0 to 30%. From the data achieved (Figure 2c), the increment of the ionic strength up to 20% helped the microextraction process, decreasing the solubility of both organic compounds in aqueous media (\"salting-out effect\"), favoring their migration towards the sorbent phase. On the other hand, the matrix polarity was studied through the addition of MeOH up to 15%. The data obtained in Figure 2d showed that the average recoveries for both UV filters decreased significantly with the increment of the MeOH content, once it favored their solubility in the bulk solution.