## 3 Results and Discussion
### DSPE Method Optimization  
An efficient DSPE protocol for the extraction of phenolic acids was established by optimizing the extraction time, the conditioning and eluting solution. Subsequently, the EGDMA, DVB and [C\({}_{6}\)-bis-VIM] [Br] crosslinked vinylimidazole polymers were compared regarding their phenolic acid extraction efficiency. It was observed that the type of crosslinker had a significant influence on the extraction recoveries. Furthermore, the influence of the monomer to crosslinker ratio of the poly(n-VIM/C\({}_{6}\)-bis-VIM) polymer on the extraction efficiency was investigated.  
It was shown that by using a phosphate buffer for the conditioning, a greater percentage of analytes could be bound to the polymer than by using an aqueous solution containing 0.5% LiCl (\(w\)/\(w\)) and 5 vol.% MeOH or a solution of 5 vol.% MeOH in water. All results are summarized in Table S2. In particular, the CaffA, FerA and CinA were strongly affected by the type of conditioning solution. This can probably be explained by their similar pKa values, which are significantly higher compared with ChlorA and DHB.  
The amount of bound CaffA, FerA and CinA could be increased by 3.2, 6.2 and 7.7%, respectively, when using a phosphate puffer (pH 7.0) instead of 5 vol.% MeOH in water. The effect of conditioning pH on the bonding of phenolic acids was tested between 6.0 to 7.7. Higher pH values were avoided due to the instability of phenolic acids [51].  
With increasing pH value of the phosphate buffer, the adsorption of CaffA, FerA and CinA slightly increased, whereby this trend was more pronounced for FerA and CinA than for CaffA; however, the difference between pH 7.0 and 7.7 was not very pronounced. Basedon the knowledge that phenolic acids can degrade at basic pH, a phosphate buffer with pH 7.0 was selected as the conditioning solution for further DSPE experiments.  
The adsorption equilibrium was investigated to minimize the duration of the extraction process during the DSPE experiments. All results are given in Table S9. As can be shown in Figure 1 the quantity of bound analytes did not significantly change after 10 min. We decided to use an extraction time of 15 min for following the DSPE experiments to be certain that a complete adsorption equilibrium was established.  
In order to optimize the elution of analytes from the solid sorbent, different eluting conditions were applied. In the preliminary experiments, MeOH and ACN with and without different concentrations of TFA were tested. Furthermore, the addition of NaCl and LiCl was investigated. It was found that higher recoveries could be achieved with ACN compared to MeOH and LiCl compared to NaCl. By adding water to the organic solvent, the solubility of the LiCl was improved and the analyte recoveries could further be increased. Based on these results, six different eluting solutions containing various quantities of TFA, LiCl, ACN and water were tested. The highest recoveries for all analytes (except DHB) could be obtained with 2 vol.% TFA, 1.5% (\(w/w\)) LiCl and 50 vol.% ACN in water (Figure 2). All results are provided in Table S3.  
Figure 1: Analyte recovery results obtained from DSPE experiments with the poly(n-VIM/C\({}_{6}\)-bis-VIM) polymer with a monomer to crosslinker ratio of 11.50 to 1, by applying different extraction times. Standard deviations are given as error bars.  
Figure 2: Analyte recovery results obtained from DSPE experiments with the poly(n-VIM/C\({}_{6}\)-bis-VIM) polymer with a monomer to crosslinker ratio of 11.50 to 1, by applying different eluting solutions. Standard deviations are given as error bars.