## 3 Results and Discussion
### Evaluation of the Extraction Process  
First, the best pH was studied, varying between 4 and 9. Solutions of HCl and NaOH 1 M, 0.1 M, and 0.01 M were used to lower or to raise the pH value, respectively. The percent recoveries obtained at pH 4 and those obtained at pH 5 were analytically significant; therefore, we can state that at acid pH the extraction of the analytes took place quantitatively. However, the authors noted that the percentage of errors increased at lower values of pH. Therefore, the experiments were carried out at pH 5. Figure 2 shows the recoveries, together with the relative error bars in the pH range between 4 and 7. Figure 1: Master scheme of the extraction process procedure. Figure 2: Percentage recoveries obtained at different pH extraction solutions. The relative standard deviations (RSDs) of each measurement are reported as error bars. It should be noted that Figure 2 does not report recoveries above pH 7 because they are very low with respect to this pH value. The reason should be due to the I.S. ionization [64; 65; 66] and thus, the Area\({}_{\text{analyte}}\)/Area\({}_{\text{IS}}\) ratio is completely altered. The methodology examined in this paper is based on the dispersion of the extraction solvent in the solution. In this way, micro-droplets are formed containing the extraction solvent and the solution becomes opalescent; the greater the opalescence, the greater the dispersion, but at the same time the contact surface between the solvent and the analyte is smaller. In this way, a stable dispersion is obtained and, very importantly, it does not change for the whole analytical procedure. In the experiments, the authors deeply investigated the analytical conditions, in particular the choice of the extraction solvent and the relative volume to be used. Four different solvents were tested (i.e., \(n\)-hexane, \(n\)-heptane, \(iso\)-octane and benzene) at three different volumes (150 \(\upmu\)L, 200 \(\upmu\)L, 250 \(\upmu\)L). The choice of the extraction solvents was based on their density, which had to be lower than that of water; thus, they could be directly recovered on the solution using the Hamilton syringe. Solvent volumes were studied based on two considerations. Volumes lower than 150 \(\upmu\)L do not allow to recover any extraction solvent because the solvent layer shows a real small thickness; on the other hand, a large drop is formed using extraction solvent volumes greater than 250 \(\upmu\)L (a bad condition for microextraction). Using an extraction solvent volume between 150 and 250 \(\upmu\)L, the volume recovered at the top of the solution is between 90 and 160 \(\upmu\)L. Figure 3 shows the recoveries obtained using the four solvents together with the relative standard deviation (RSD) reported as error bars for each measurement. The best recoveries are obtained by 200 \(\upmu\)L of \(n\)-hexane. Subsequently, two further important parameters for improving the extraction procedure, such as the stirring time and the ultrasonic bath duration, were studied. Mixing times ranging between 5 and 25 min were studied, whereas experiments were performed with different ultrasound times of between 6 and 30 min to obtain the best analytical conditions. In fact, different tests were performed by varying both parameters (individually or simultaneously), but no significant increase in analyte recoveries was achieved. Consequently, the authors decided to perform 5 min of stirring time and 6 min of ultrasonic bath; these conditions were sufficient to obtain a reliable dispersion and a quantitative PAEs/BPA recovery. The other fundamental step concerns the breaking of the emulsion, where a salt addition is essential. Actually, the best method is to centrifuge the solution and obtain the separation of the two phases; starting from 1 L, this procedure was difficult. The key already tested in previous papers [67; 68]  
Figure 3: Percentage recoveries obtained in relation to the volume and the solvent used: (**a**) \(n\)-hexane; (**b**) \(n\)-heptane; (**c**) \(iso\)-octane; and (**d**) benzene.