## 3 Results and Discussion
### Effect of the Crosslinker on DSPE Efficiency  
The effect of differently crosslinked polymers on the extraction efficiency towards phenolic acids is displayed in Figure 3 and the results are summarized in Table S10. All solid sorbents exhibited different percentage recoveries indicating the significant influence of the crosslinker. All sorbents showed the same systematic increase in recoveries following the order: CinA \(<\) FerA \(<\) CaffA \(<\) ChlorA \(<\) DHB. This circumstance could be due to the common basic structure of all three polymers, which is due to the use of the same monomer VIM. The reason why the extraction recoveries followed this particular order can probably be explained by the different pKa values. The lower the pKa, the stronger the acid, which means that the substance dissociates more strongly in water. Therefore, DHB and ChlorA with the lowest pKa values should also exhibit the highest proportion of dissociated molecules in anionic form. Since the sorbent used is an anion exchanger, more analytes could be bound to the sorbent via ionic interactions compared to the other phenolic acids. This consequently also led to higher extraction recoveries. CaffA, FerA and CinA have similar pKa values, however they exhibited different extraction recoveries. A reason could be the different number of hydroxyl groups in their chemical structures. Occurring hydrogen bonds may increase the adsorption force between the phenolic acids and sorbent, which could explain the previously stated order of recoveries. The EGDMA crosslinked polymer exhibited the lowest extraction recoveries (52.4-85.3%) for all phenolic acids except the CinA. This can probably be explained by the absence of an aromatic moiety. Significantly better recoveries (71.9-95.1%) were obtained with the DVB crosslinked polymer. It was assumed that the EGDMA crosslinked polymer would be better suited for the extraction of phenolic acids due to its more polar structure and the possibility of forming hydrogen bonds. On the contrary, DVB has an aromatic ring structure, which enables the formation of \(\pi\)-\(\pi\) interactions with phenolic acids. This was probably the main reason for the higher recoveries with the DVB crosslinked polymer. The polymer synthesized with [C\({}_{6}\)-bis-VIM] [Br] showed promising recovery results especially for DHB, ChlorA and CaffA, ranging between 91.1 and 99.3%. In contrast, the recoveries for FerA (71.4%) and CinA (50.8%) were significantly lower. In comparison to the EGDMA crosslinked polymer, much higher recoveries were obtained for all analytes except the CinA. In detail, 10.2% higher recoveries were obtained for FerA, 13.8% for ChlorA, 14.0% for DHB and 17.9% for CaffA. Only the recovery for CinA was 1.7% lower. Compared to the sorbent synthesized with DVB, the crosslinked [C\({}_{6}\)-bis-VIM] [Br] polymer showed significantly higher recoveries for ChlorA, CaffA and DHB. Slightly lower recoveries were found for FerA, and much lower recoveries were obtained for CinA. It is very likely that  
Figure 3: Analyte recovery results obtained from DSPE experiments with three differently crosslinked vinylimidazole polymers. Standard deviations are given as error bars. FerA and CinA, due to their higher pKa values, were mainly present as uncharged species and therefore were more affected by the non-polar interactions of the DVB. In summary, the recovery with poly(n-VIM/C\({}_{6}\)-bis-VIM) in comparison to the DVB crosslinked polymer for ChlorA was higher by 3.4%, for DHB by 4.2% and for CaffA by 5.6%. In contrast, the recoveries were 3.4% smaller for FerA and 21.1% for CinA. Phenolic acid loading capacities (q) of the three differently crosslinked vinylimidazole polymers were calculated according to the following equation:  
\[\mathrm{q}=[(\mathrm{c_{i}}-\mathrm{c_{eq}})*\mathrm{V}]/\mathrm{m} \tag{1}\]  
where \(\mathrm{c_{i}}\) is the initial loading concentration, \(\mathrm{c_{eq}}\) the concentration at equilibrium, \(\mathrm{V}\) the sample volume and \(\mathrm{m}\) the dry mass of the applied polymer.