## 3 Results
### Quantitative Comparison  
Water samples with pH values of 4.0, 5.0, 6.0, 7.0 and 8.5 were spiked with natural products, and an aliquot of each sample was extracted using EtOAc as well as resin extraction, as described above. For the quantitative comparison, the response of the protonated pseudo-molecular ions of the respective standard injections was used as reference to calculate the concentrations of the respective spike compounds. For determination of a calibration curve, LOQ and LOD, we injected 0.05, 0.25, 0.5 and 1 ug of cyclosporine, gentamycin, colchicine and tetracycline using the analytical protocol described above. We also injected 0.25 ug and 0.05 ug of ampicillin to determine the LOD of ampicillin that was not detected in the qualitative study. The calibration curves are shown in Figure 2. The LOD for ampicillin was <0.05 ug; LOD for tetracycline, cyclosporine and colchicine was <0.05 ug; LOD for gentamycin was <0.25 ug, and a proper quantification of gentamycin failed (see Figure 2). We also failed to detect gentamycin at 0.05 ug in negative electrospray. The extracts were dissolved in 1.0 mL of 80% MeOH aq., and the final yields were determined by multiplying the concentration with the volume; the final yields are given in Appendix A. Relative yields were calculated as ratios of the initial quantity of spike compound in the 100 mL of aqueous solution to be extracted. The relative yields are given in Figure 3.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Ret. Time** & **Mw** & \multicolumn{2}{c}{**Present in Extracts:**} \\
**Compound** & **(min)** & **(u)** & **EtOAc** & **Resin** \\ \hline Rifampicin & 6.06 & 822.9 & + & + \\ Vancomycin & 0.97 & 1449.2 & + & + \\ Tetracycline & 2.09 & 444.4 & + & + \\ Colchicine & 3.30 & 399.4 & + & + \\ Cyclosporine & 9.60 & 1202.6 & + & + \\ Paclitaxel & 6.51 & 853.9 & + & + \\ Ampicillin & 1.51 & 349.4 & - & - \\ Gentamycin & 0.46 & 482.5 & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 2: Retention times and presence of the natural products in EtOAc and resin extracts.  
Figure 3: Yield of the natural products in percentage recovery from the spiked solution. A: Results for the liquid–liquid phase extraction. B: Results for the liquid–solid phase extraction. The different pH levels for the aqueous solutions/extraction conditions are given on the \(x\)-axis. Note that gentamycin (\({}^{\star}\)) was not detected in any of the two extracts; however, the LOD for gentamycinin would be around 5.0% of the initially spiked quantity; given the extract was dissolved in 1.0 mL of solvent, for all the other compounds LOD would equal <1.0% of the initially spiked compound quantity.  
Figure 2: Calibration curves for the quantification of the selected natural products. Single injections were used for the calibration curves. The formulas and R\({}^{2}\) values for the calibration curves were tetracycline y = 5487981x \(-\) 112802, 0.97; cyclosporine 253601x \(-\) 11282, R\({}^{2}\) = 0.93; colchicine y = 5617051x + 438251, R\({}^{2}\) = 0.97; with y being the response and x being the injected compound quantity in \(\mu\)g. We failed to generate a proper calibration curve for gentamycin.