## 3 Results and Discussion
### Analysis of Real Samples  
We applied the established method for analysis of real environmental water samples. Our analysis results show that the six targets were not detectable in the reservoir water using the current established method. To test the applicability of our optimized method to real samples, spiked reservoir samples were used instead. As shown in Table 4, we added two spiked levels of the target analytes, one spiked at 0.05 \(\upmu\)g/L of NAP and 0.5 \(\upmu\)g/L of KEP, DIF, MEF, DIC, and NIF and another spiked at 0.5 \(\upmu\)g/L of  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Analyte**} & **Liner Range** & **Correlation of** & **LOD \({}^{\text{a}}\)** & **LOD \({}^{\text{a}}\)** & **Intra-Day RSD** & **Inter-Day RSD** \\  & **(\(\upmu\)g L\({}^{-1}\))** & **Determination (R\({}^{\text{2}}\))** & **(\(\upmu\)g L\({}^{-1}\))** & **(\(\upmu\)g L\({}^{-1}\))** & **(\(\upmu\)g L\({}^{-1}\))** & **(\(\upmu\)g, n = 3)** & **(\%, n = 3)** \\ \hline naproxen & 0.01–10.0 & 0.995 & 0.011 & 0.032 & 0.3 & 3.3 \\ ketoprofen & 0.05–10.0 & 0.994 & 0.025 & 0.044 & 0.5 & 2.7 \\ difulisal & 0.05–10.0 & 0.996 & 0.018 & 0.076 & 0.9 & 3.1 \\ metemamic acid & 0.05–10.0 & 0.995 & 0.016 & 0.088 & 0.9 & 2.8 \\ diclofenac & 0.05–10.0 & 0.997 & 0.008 & 0.006 & 1.3 & 4.0 \\ nifulamic acid & 0.05–10.0 & 0.997 & 0.002 & 0.003 & 2.1 & 3.3 \\ \hline \hline \end{tabular}  
* \({}^{\text{a}}\) LODs and LOQs were estimated as the concentration where S/N = 3 and 10, respectively.  
\end{table}
Table 3: Liner regression, LODs, LOQs, and RSDs data for the poly(DEAEMA-co-DVB) based WAX SPE/HPLC-UV method.  
Figure 7: Effects of sample volume on the recoveries (%) of selected pharmaceuticals, \(n\) = 3.  
NAP and 5 \(\upmu\)g/L of KEP, DIF, MEF, DIC, and NIF. The satisfactory recoveries of 88.3-101.7% show the reliability of our established method. We also compared the SPE performance of the laboratory-made WAX poly(DVB-co-DEAEMA) with a commercial C18 and MAX for extraction of six NSAIDs from spiked reservoir water. The 500-mL reservoir water sample was spiked at 0.05 \(\upmu\)g/L of NAP and 0.5 \(\upmu\)g/L of KEP, DIF, MEF, DIC, and NIF. Table 5 shows that, compared with C18, our current WAX gives much higher recoveries for all NSAIDs, which can be ascribed to the stronger retention from both a dual interaction mechanism and a high specific surface area.  
For most pharmaceuticals, our results were comparable or even superior to Oasis MAX, especially for NIF. NIF is a strong acid that remains ionized across the whole pH range. Thus, NIF has a very strong interaction with the SAX Oasis MAX so that it is not possible to disrupt the ion exchange mechanism by pH adjustment, thus resulting in a low recovery of 30%. However, our WAX SPE utilized a hydrophobic backbone functionalized with a tertiary amine with a pKa of approximately 10.6, which allows easy control of the ionization of the stationary phase by modifying the pH. At the same time, a pKa of approximately 10.6 guaranteed that the stationary phase was positively charged across the investigated pH range. As a result, our WAX SPE can be applicable to both strongly and weakly acidic compounds. It was demonstrated that the laboratory-made poly(DVB-co-DEAEMA) can be applied to the extraction of NSAIDs from NIF (strongly acidic) to KEP, DIF, MEF, DIC, and NIF (weakly acidic), and the detection limit can be as low as 0.01 \(\upmu\)g/L. In addition, our results are comparable to the performance of those mixed-mode SPE sorbents used in other previous reports. In conclusion, the laboratory-made poly(DVB-co-DEAEMA) can be an excellent mixed-mode SPE material for sample pretreatment of acidic organic pollutants in environmental samples.