## 3 Results and Discussion
### Reusability of the Sorbent  
The reusability of the sorbent was studied on three independent sorbents. Each of the tested sorbents was used 5 times for extraction of water samples spiked at 50 ng mL\({}^{-1}\). The results given in Table 4 show the good stability of the DES sorbent and the possibility of its repeatable usage. Figure 8: A chromatogram of a standard solution of formaldehyde derivative. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample of Natural** & **Initial** & **Spiked** & **Measured** & **RSD [\%]** & **RR** \\
**Water** & **[ng mL\({}^{-1}\)]** & **[ng mL\({}^{-1}\)]** & **[ng mL\({}^{-1}\)]** & **(_n = 4_)** & **[\%]** \\ \hline \hline \multirow{3}{*}{Malta Lake} & \multirow{3}{*}{nd} & 0.5 & 0.41 \(\pm\) 0.06 & 14.6 & 82.0 \\  & & 5.0 & 4.55 \(\pm\) 0.19 & 4.2 & 91.0 \\  & & 10.0 & 9.88 \(\pm\) 0.45 & 4.6 & 98.8 \\ \hline \multirow{3}{*}{Baba Lake} & \multirow{3}{*}{nd} & 0.5 & 0.39 \(\pm\) 0.07 & 17.9 & 78.0 \\  & & 5.0 & 4.58 \(\pm\) 0.22 & 4.8 & 91.6 \\  & & 10.0 & 9.74 \(\pm\) 0.40 & 4.1 & 97.4 \\ \hline \multirow{3}{*}{Körnickie Lake} & \multirow{3}{*}{nd} & 0.5 & 0.48 \(\pm\) 0.09 & 18.8 & 96.0 \\  & & 5.0 & 4.34 \(\pm\) 0.43 & 9.9 & 86.8 \\  & & 10.0 & 9.91 \(\pm\) 0.38 & 3.8 & 99.1 \\ \hline \multirow{3}{*}{Cybina River} & \multirow{3}{*}{nd} & 0.5 & 0.43 \(\pm\) 0.07 & 16.3 & 86.0 \\  & & 5.0 & 4.45 \(\pm\) 0.14 & 3.2 & 89.0 \\  & & 10.0 & 9.67 \(\pm\) 0.35 & 3.6 & 96.7 \\ \hline \multirow{3}{*}{Warta River} & \multirow{3}{*}{nd} & 0.5 & 0.45 \(\pm\) 0.08 & 17.8 & 90.0 \\  & & 5.0 & 4.21 \(\pm\) 0.17 & 4.0 & 84.2 \\ \cline{1-1}  & & 10.0 & 9.54 \(\pm\) 0.31 & 3.3 & 95.4 \\ \hline \hline \end{tabular} nd—not detected; RR—relative recovery.  
\end{table} 
Table 3: The results for the determination of formaldehyde in surface water samples. Comparison with Different Microextraction Methods Used for Determination of Formaldehyde in Real Samples  
Formaldehyde, due to its toxic properties, is often analyzed in various samples (Table 5) [8; 9; 10; 15; 16; 17; 18; 23]. Several sample preparation methods were used in these studies based mainly on SPME and DLLME. Nash reagent and 2,4-dinitrophenylhydrazine were two main derivativizing agents. Compared to these methods, the one proposed in the present study offers LOD similar to other methods based on HPLC-UV detection [7; 8] as well as micro-spectrophotometric detection [17]. Much better LOD was obtained compared to spectrofluorimetric detection [18] and surprisingly to GC-MS detection [23]. Furthermore, the results obtained for the new DES-based sorbent show new possibilities offered by this new class of sorbents that due to the ease of their synthesis and modification of their properties enable synthesis directed to the intended purpose. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Extraction Cycle**} & \multicolumn{3}{c}{**Concentration [ng mL\({}^{-1}\)]**} \\ \cline{2-4}  & **Sorbent 1** & **Sorbent 2** & **Sorbent 3** \\ \hline
1 & 41.5 & 41.9 & 42.0 \\ \hline
2 & 41.9 & 41.7 & 41.6 \\ \hline
3 & 41.4 & 41.3 & 41.7 \\ \hline
4 & 41.2 & 41.4 & 41.1 \\ \hline
5 & 38.4 & 38.9 & 39.1 \\ \hline Average & 40.9 & 41.0 & 41.1 \\ \hline RSD [\%] & 3.4 & 3.0 & 2.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Reusability of DES sorbents.