## 2 Results and Discussion
### Optimization of the Recovery Assays  
For the present work, several parameters that affect the extraction and back-extraction efficiency yields were evaluated. Therefore, systematic assays were performed to optimize parameters such as, equilibrium time, agitation speed, matrix characteristics (pH, organic modifier and ionic strength) and LD conditions. In a first approach, the LD conditions that ensure complete back-extraction for five analytes from the PS-DVB phase were optimized. Solvents such as MeOH, ACN and a mixture (Mix) with equal volumes of both were assayed to survey the desorption performance, followed by solvent switch to a more suitable for HPLC-DAD analysis. Figure 2a depicts the data obtained (extraction time: 16 h (1000 rpm), pH 5.5 and back-extraction time: 45 min under sonification). From the data obtained, the ACN was selected as back-extraction solvent, since it presents the highest ability to desorpt the five phenols from the PS-DVB phase. After the selection of the most effective solvent for back-extraction, desorption times of 30, 45 and 60 min were also assayed, in which 45 min is enough and no advantages were obtained by using longer periods of time (data not shown). Consequently, 45 min was established for the back-extraction process.  
According to the literature [18; 19; 20; 21], the equilibrium time and stirring speed are extremely important parameters to be optimized for better extraction conditions. Concerning the stirring rate, it can have great influence in the mass transfer kinetics of the analytes towards to sorbent phase, as well as the floating sampling process. Thus, these two parameters had been evaluated for the PS-DVB devices extraction process. Figure 2b shows that 1,250 rpm gave a better performance on the recovery yields of the five analytes and was selected for further assays. Subsequently, the equilibrium time was carried out by experiments within 1, 2, 3, 4 and 16 h. As illustrated in Figure 2c, the recovery yields still increased after 16 h of the extraction. Although the extraction process is a bit slow, this analytical approach can be performed overnight. Thereby, the extraction time was set at 16 h for further experiments.  
As demonstrated in previous works [18; 19; 20; 21], the matrix characteristics, _i.e._, pH, ionic strength and polarity are also important parameters that significantly affect the extraction efficiency. It must be emphasized that, for the phenol compounds, the pH plays an important role since these types of analytes are ionisable.  
The results obtained are presented in Figure 2d, and show, as expected, that the pH influences the extraction process and plays an important role in the recovery yields. From the data obtained, it is possible to see that pH 5 is the best value for the interaction of phenols under study with the PS-DVB phase, whereas a more acidic or basic matrix media reduces the recovery yields since the analytes became charged, reducing therefore the selectivity.  
The ionic strength and polarity were also modified through the addition of NaCl and MeOH (5%, 10% and 15%) to the matrix media, respectively. In a first approach, the addition of 5% NaCl increased slightly the recovery yields, whereas the progressive addition of MeOH decreased significantly the recovery yields of the five phenols under study (data not shown). As a consequence, the matrix pH 5 and 5% of NaCl were established for further experiments in the absence of MeOH.