## 3 Results and Discussion  
### Absolute Recoveries of Spiked Isotope Labelled Compounds and Ion Suppression  
The eight isotope labelled compounds used in this study include positive, negative and neutral APIs, Table 1. Seven of these were spiked at the top of the extraction cell (internal standard mixture, Table S1), containing the sewage sludge, and underwent the entire extraction process, while the internal standard thiazolprid-D4 was spiked just prior to the analysis. By calculating the absolute recovery of the 7 labelled compounds the extraction efficiency of the Espresso method compared to _PHWE method_ could be revealed. The obtained extraction efficiency results also contain a component of ion suppression, which might depend on the applied extraction methodology. To get basic information about possible differences in the magnitude of ion suppression caused by the two individual extraction methods the results for thiazolprid-D4 was used. The absolute recoveries obtained for all 8 compounds are shown in Table 1. It should be noted that four of these compounds were analysed in both the _acid_ and the _basic chromatographic UPLC method_, giving information on possible differences in reduction of ion suppression by better separation of compounds from co-eluting matrix components during the two different UPLC runs. To provide additional knowledge of differences in terms of ion suppression, TIC spectra for both chromatographic methods and both extraction methods were also collected as shown in Figure 2.  
Starting with the standard, thiazolprid-D4, added post extraction, it was shown that the absolute recovery for this compound was 70% in the Espresso method and 34% in the PHWE method with basic chromatographic conditions. This difference can be explained by a heavier matrix suppression expressed in the PHWE method, which is further supported by the higher TIC numbers for PHWE compared to Espresso, Figure 2a,b. Interestingly, there are also differences in absolute recovery comparing the thiazolprid-D4 results from the basic and acid chromatographic method separately. Recovery results dropped from the above-mentioned 70% in the basic method to 37% in the acid \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Compound** & **UPLC Condition** & **Espresso (\(n=3\)) (\%)** & **RSD (\%)** & **PHWE (\(n=3\)) (\%)** & **RSD (\%)** \\ \hline _Thiazolprid-d4_ & _Basic_ & _70_ & _17_ & _34_ & _17_ \\ _Carbanzzpine-D10_ & _Basic_ & _44_ & _18_ & _28_ & \(4\) \\ _Diciofen-13C6_ & _Basic_ & _30_ & _18_ & _14_ & _24_ \\ _Sulfulfmentkuzzo-13C6_ & _Basic_ & _53_ & \(7\) & _26_ & _19_ \\ _Atenolol-d7_ & _Basic_ & _56_ & \(5\) & _45_ & _10_ \\ _Methiocarb-d3_ & _Basic_ & _20_ & _12_ & _17_ & \(5\) \\ _Metoprolol-d7_ & _Basic_ & _36_ & _15_ & _34_ & \(9\) \\ _Estrone-d4_ & _Basic_ & _21_ & _29_ & _17_ & \(2\) \\ \hline _Average recovery 7 compounds_ & _Basic_ & _37_ & _24_ & & \\ \hline _Thiazolprid-d4_ & _Acid_ & _37_ & _22_ & _14_ & _18_ \\ _Carbanzzpine-d10_ & _Acid_ & _19_ & _31_ & _10_ & \(7\) \\ _Diciofen-13C6_ & _Acid_ & _20_ & _11_ & _14_ & _12_ \\ _Sulfmentkuzzo-13C6_ & _Acid_ & _34_ & _18_ & _20_ & _13_ \\ \hline _Average recovery 3 compounds_ & _Acid_ & _24_ & & _16_ & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Absolute recovery and relative standard deviation (RSD %, \(n=3\)) for the instrumental standard _thiazolprid-d4_ spiked just prior to the analysis and seven labelled standards spiked at the top of the two types of extraction cells, Espresso and PHWE, containing the sewage sludge. Absolute recovery was calculated using Equation (1). Four of the compounds were analysed at both basic and acid conditions and are marked in _italic_. method, for samples run in the Espresso method. In the acid method, the recovery dropped from 34 to 14%. One plausible explanation is less ion suppression in the basic method.