## 3 Results and Discussion
### Polymer Characterization  
The poly(n-VIM/C6-bis-VIM) polymer with a monomer to crosslinker ratio of 11.50 to 1 was investigated regarding its specific surface area and surface structure. The BET  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**SPE Sorbent** & **ChlorA** & **CaffA** & **DHB** & **FerA** & **CinA** \\ \hline poly(n-VIM/C6-bis-VIM) & 98.5 \(\pm\) 3.3 & 97.2 \(\pm\) 2.0 & 97.4 \(\pm\) 3.1 & 97.4 \(\pm\) 1.9 & 97.4 \(\pm\) 1.7 \\ Oasis® MAX & 93.8 \(\pm\) 0.6 & 92.5 \(\pm\) 0.3 & 92.8 \(\pm\) 0.3 & 92.3 \(\pm\) 0.2 & 91.6 \(\pm\) 0.2 \\ \hline \hline \end{tabular} Extraction recovery: mean \(\pm\) SD /% n = 3.  
\end{table}
Table 2: Recovery comparison of the developed poly(n-VIM/C6-bis-VIM) polymer with a monomer to crosslinker ratio of 11.50 to 1 and the commercially available SPE sorbent Oasis® MAX.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Sorbent/** & \multicolumn{3}{c}{**Sample Concentration/**} & \multicolumn{3}{c}{**Sample Volume/**} & \multicolumn{3}{c}{**Recovery /\%**} & \multirow{3}{*}{**Reference**} \\
**Amount** & & & **mg L\({}^{-1}\)** & & **mL** & **ChlorA** & **CaffA** & **DHB** & **FerA** & **CinA** \\ \hline C18/500 mg & SPE & 25 & - & - & 14.0 & - & 14.5 & - & [44] \\ Sax/200 mg & SPE & 25 & - & - & 29.9 & - & 30.5 & - & [44] \\ Oasis® HLB/200 mg & SPE & 25 & - & - & 102.5 & - & 102.7 & - & [44] \\ Magnetic MIP/ & & 0.1 & 10 & 95–100 & 95–100 & - & 65–70 & - & [47] \\
10 mg & & & & & & & & & \\ Poly(n-VIM/C6-bis-VIM) & SPE & 37.5-55 & 0.4 & 96.9 & 101.9 & 97.8 & - & - & [46] \\
30 mg & & & & & & & & & \\ Oasis® MAX/30 mg & SPE & 100 & 1 & 93.8 & 92.5 & 92.8 & 92.3 & 91.6 & This work \\ Poly(n-VIM/C6-bis-VIM) & SPE & 100 & 1 & 98.5 & 97.2 & 97.4 & 97.4 & 97.4 & This work \\ Poly(n-VIM/C6-bis-VIM) & DSPE & 100 & 1 & 92.5 & 92.0 & 92.5 & 90.6 & 84.1 & This work \\ \hline \hline \end{tabular} Extraction recoveries obtained from aqueous standard solutions.  
\end{table}
Table 3: Comparison of the developed DSPE and adapted SPE procedure with other solid-phase extraction methods for phenolic acids.  
measurement of the sample gave a specific surface area of 49.3 m\({}^{2}\) g\({}^{-1}\). The adsorption and desorption isotherm are provided in Figure S1. Additionally, the pore size distribution calculated from the desorption branch (20 points from 0.95 to 0.3 p/p\({}_{0}\)) of the isotherm is given in Figure S2. The results showed that the pore size distribution ranges between 25 and 50 A with a maximum at 35 A. The SEM measurements provided an image of the surface structure of the co-polymer, which is given in Figure S3. The surface is composed of many small, round, spherical components that were all about the same size. No macroscopic structural elements were observed. The recorded ATR-FTIR spectra (Figure S4) confirmed the presence of the functional groups incorporated by the monomer and the crosslinker. The following signals in the spectra of poly(n-VIM/C\({}_{6}\)-bis-VIM) were observed and assigned: 3083 cm\({}^{-1}\) N-H stretching; 2932 cm\({}^{-1}\) C-H stretching (aromatic); 2859 cm\({}^{-1}\) C-H stretching (aliphatic); 1553 cm\({}^{-1}\) C-N stretching; 1414 cm\({}^{-1}\) C-H stretching (ring); 1284 cm\({}^{-1}\) C-H in plane bending; 1227 cm\({}^{-1}\) C-H in plane bending and C-N stretching; 1156 cm\({}^{-1}\) C-N (aliphatic); 1083 cm\({}^{-1}\) C-H in plane bending (ring) and C-C stretching; 908 cm\({}^{-1}\) C=C bending and stretching; 818 cm\({}^{-1}\) C-H bending (ring); 740 cm\({}^{-1}\) C-H bending (ring); and 662 cm\({}^{-1}\) N-C stretching (ring). Especially the signal at 3083 cm\({}^{-1}\) is of importance as it is the characteristic frequency of imidazolium salts. Furthermore, a signal at 1176 cm\({}^{-1}\) was observed in the spectra of the crosslinker, which was also found in the spectra of the polymer (1156 cm\({}^{-1}\)) and not in the spectra of the VIM. The obtained results are consistent with the structural analysis of [PERSON] et al. and the IR spectrum table from Merck [52; 53].