## 3 Results
### Sorption Studies
The effect of pH on the recovery of target drugs was also tested in dynamic conditions. In each experiment, 250 mL of a water  
Figure 7: Sorption efficiency of ketoprofen (KETO) and ibuprofen (IBU) on studied copolymers in pH 3, 7, 11; (mean (bar) \(\pm\) standard deviation (whisker) (\(n=3\))). solution of ibuprofen and ketoprofen with a concentration of 1.1 ug/mL was used. As it is seen from Figure 8, the impact of sample pH has the same trend as in the case of static sorption. The maximum recoveries and the highest preconcentrations were obtained for IBU and KETO in acidic pH on both 4 VP sorbents. However, on poly(4 VP-_co_-TRIM), the recovery of KETO and IBU was almost 100%, whereas the recovery of KETO on poly(4 VP-_co_-14 DMB) sorbent was slightly lower and reached about 81%. Moreover, the effect of the sample volume was investigated. In the analysis of environmental samples, large sample volumes are often required to achieve high preconcentration factors and obtain better sensitivity. The experiment was performed at pH 3, as it was verified as the most appropriate for the sorption of tested drugs. The results presented in Figure 9 show that the recovery for ibuprofen and ketoprofen is almost unchanged when the sample volume was from 50 mL to 200 mL and then slightly decreased with the increasing sample volume. The maximum recoveries obtained for ibuprofen and ketoprofen were 92% and 81%, respectively. However, even at a sample volume of 400 mL, the recoveries were still at an acceptable level of about 70%. The observed drop in the recovery along with the increasing sample volume is most probably connected with the exceeding drug sorption capacity. At this point, it has to be emphasized that the effect of sample volume on the recovery was investigated by reusing the same SPE column after its regeneration with 10 mL of methanol after each sorption-desorption experiment. Based on these results, conclusions concerning the stability and reproducibility of sorbents, which are key factors in evaluating their adsorption performance, can be made [40]. As can be seen in Figure 9, the recoveries were stable when the concentration of KETO and IBU in dosed solutions was lower than the sorbent capacity. Both poly(4 VP-_co_-14 DMB) and poly(4 VP-_co_-14 DMB) solvents reused four times (to a volume of 200 mL, Figure 9) do not show a reduction in the extraction efficiencies. Additionally, to estimate the sorbent selectivity to selected drugs from the NSAIDs group that are acidic compounds, aspirin with the mixture of ibuprofen and ketoprofen was also tested. From Figure 9, it follows that both copolymers indicate the highest selectivity towards ibuprofen, slightly lower towards ketoprofen, and the lowest towards aspirin. All these drugs are acidic compounds with aromatic rings in their structures. However, a significant difference in the chemical structures of these three compounds should be considered. The carboxyl group in the aspirin molecule is directly connected to the aromatic ring, thanks to which the \(\pi\) electrons and free-electron pairs from the carboxyl group can participate in resonance with the delocalized electrons of the aromatic ring. In the case of ketoprofen and ibuprofen molecules, such resonance is not possible. Figure 8: Recoveries obtained for ketoprofen and ibuprofen on 4 VP copolymers from solutions of pH = 3, 7, 11; (mean (bar) \(\pm\) standard deviation (whisker) (\(n\) = 3)). In order to estimate the possibility of using 4 VP sorbents as SPE beds for preconcentration of ibuprofen and ketoprofen from acidic water samples, the adsorption-desorption was made from solutions of low drug concentration, as target compounds have been detected in the water at low \(\mu\)g/L levels [18; 19]. In this section, the initial solution concentration of ketoprofen and ibuprofen was between 50 \(\mu\)g/L and 190 \(\mu\)g/L, respectively. In total, 250 mL of solution was dosed to SPE columns filled with 50 mg of sorbents.