## 4 Conclusions  
For the first time, a mixed-mode WAX poly(DVB-co-DEAEMA) utilizing a hydrophobic backbone functionalized with a tertiary amine (with a pKa of approximately 10.6) was successfully used for SPE  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Analyte**} & \multicolumn{2}{c}{**Spiked Levels a**} & & & \\  & **(1)** & & **(2)** & \\  & **Eluate b  & **RSD\%** & **Eluate** & **RSD\%** \\ \hline naproxen & 101.3 & 2.7 & 92.0 & 3.6 \\ ketoprofen & 93.3 & 1.8 & 101.7 & 4.4 \\ diflunisal & 89.5 & 7.7 & 98.4 & 5.6 \\ mefenamic acid & 92.4 & 1.4 & 93.6 & 7.2 \\ diclofenac & 96.6 & 2.2 & 88.3 & 2.7 \\ niflumic acid & 90.6 & 4.6 & 90.1 & 3.5 \\ \hline \hline \end{tabular}  
* (1) Spiked at 0.05 \(\upmu\)g/L of NAP and 0.5 \(\upmu\)g/L of KEP, DIF, MEF, DIC, and NIF; and (2) Spiked at 0.5 \(\upmu\)g/L of NAP and 5 \(\upmu\)g/L of KEP, DIF, MEF, DIC, and NIF.
* Eluate was obtained using 2 mL of 2\(\%\) NH\({}_{3}\)-H\({}_{2}\)O/MeOH as eluent.  
\end{table}
Table 4: Recoveries (%) and RSDs after the WAX based SPE of 500 mL of reservoir water samples at two different spiked levels.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Analyte**} & \multicolumn{1}{c}{**Poly(DEAEMA-co-DVB) a**} & \multicolumn{1}{c}{**Oasis MAX a**} & \multicolumn{1}{c}{**C18 a**} \\ \hline naproxen & 90 & 93 & 70 \\ ketoprofen & 98 & 93 & 73 \\ diflunisal & 92 & 90 & 66 \\ mefenamic acid & 89 & 88 & 83 \\ diclofenac & 88 & 90 & 62 \\ niflumic acid & 93 & 7 & 69 \\ \hline \hline \end{tabular}  
* Spiked at 0.05 \(\upmu\)g/L of NAP and 0.5 \(\upmu\)g/L KEP, DIF, MEF, DIC, and NIF.  
\end{table}
Table 5: Comparison of recovery performance between poly(DEAEMA-co-DVB) with commercial SPE columns.  
extraction of widely acidic pharmaceuticals (pKa values ranging from 1.68 to 4.45) from real water samples. This result was difficult to achieve by mixed-mode SAX and mixed-mode WAX containing weaker primary and secondary amines. The former cannot perform well for strong acids while the latter was not a good choice for weakly acidic analytes. In contrast to both, our mixed-mode WAX SPE can be successfully applied in simultaneous efficient enrichment of both strongly and weakly acidic NSAIDs with satisfying recoveries for the first time. The other importance of this work lies in that the functionalized sorbent was synthesized in a well-defined micro-sphere format by simple one-step Pickering emulsion polymerization. The method is process-simplified, as only one step was included in the fabrication. Finally, the application of our mixed-mode WAX not only allows selective extraction of the target analytes but also cleans up the sample, which are important particularly when the sample is complex and impurities can interfere with quantification.  
X.G., conceptualization, data curation, investigation, and writing--original draft; Y.L., supervision, writing--review and editing, and validation; C.H., methodology; X.Z., supervision and writing--reviewing and editing; and J.C., funding acquisition. All authors have read and agreed to the published version of the manuscript. This research was funded by the Nation Natural Science Foundation of China (grant number 21705149). The authors declare no conflict of interest.