## 3 Results and Discussions
### Analytical Features  
Working solutions, prepared as previously mentioned in Section 2.2, were used to spike either pure water or synthetic seawater to create the calibration standards used to generate calibration curves for each compound of interest. Synthetic seawater was prepared according to the ASTM international standard practice for the preparation of substitute ocean water (method D1141-98, 2013). At least five calibration standards were used to prepare the calibration curves, either directly or after extraction with MTBE for seawater solutions. The calibration curves were drawn up by the relative responses obtained when analyzing the standards in relation to the internal standard (triphenyl phosphate) as a function of the standard concentration. Quantitation of pharmaceuticals was performed by following the same procedure used to establish the calibration curves. MS/MS transitions used for quantitation of the pharmaceuticals are described in Table 2.  
Pharmaceuticals thus detected were identified based on matching the retention times and the relative intensity ratio of transition ions with those of the standards. Quantitation was performed using the ion transition representing the highest abundance. The performance parameters of the analytical method are presented in Table 3.  
Method accuracy was validated by calculating the percent recovery using nine determinations over three concentration levels (at the LOQ, and at middle and high concentration points of the calibration curve) in seawater and in freshwater matrices representative of the planned sampling campaign. As shown in Table 3, repeatability relative standard deviation (RSD) ranged between 8% and 16%, and mean recovery covered 87% to 105% of the test concentrations. Within the studied concentration range, the calibration curves were linear (r\({}^{2}\)\(\geq\) 0.99). The limits of detection (LOD) were determined as a signal-to-noise ratio 3:1 and ranged between 2 and 7 ng L\({}^{-1}\). Limits of quantitation (LOQ) were determined based on a signal-to-noise ratio 10:1 and ranged from 7 ng L\({}^{-1}\) to 23 ng L\({}^{-1}\). These limits are in line with levels obtained in previous studies using direct injection coupled to UHPLC-MS/MS [17; 36].  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Analyte** & **RT (min)** & **Parent Ion [M + H]\({}^{+}\)** & **Cone Voltage (V)** & **Fixed CE Value (V)** & **Fragments** \\ \hline Azithromycin & 1.71 & 749.5164 & 45 & 25 & 591.422 \\ Erythromycin & 2.19 & 734.4691 & 20 & 14 & 576.374 \\ Fluoxetine & 2.47 & 310.1419 & 20 & 4 & 148.113 \\ Sotalol & 1.02 & 273.1273 & 20 & 4 & 255.117 \\ \hline \end{tabular}  
Representative chromatograms of the selected pharmaceuticals, along with their mass spectrum, is presented in Appendix B.  
\end{table}
Table 2: Pharmaceuticals retention time and MS/MS operating conditions.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Compound** & **Precision (\%)** & **Linearity (r\({}^{2}\))** & **Range (ng L\({}^{-1}\))** & **LOD (ng L\({}^{-1}\))** & **LOQ (ng L\({}^{-1}\))** & **Recovery (\%)** \\ \hline Fluoxetine & 16 & 0.994 & 1–1000 & 2 & 7 & 87 \\ Sotalol & 13 & 0.993 & 2–1000 & 2 & 6.8 & 105 \\ Azithromycincin & 14 & 0.993 & 2–1000 & 2 & 8 & 92 \\ Erythromycin & 8 & 0.990 & 5–1000 & 7 & 23 & 98 \\ \hline \end{tabular}
\end{table}
Table 3: LC-MS/MS method performance characteristics for the analysis of pharmaceuticals.