## 3 Results and Discussion
### Extraction and Clean-Up Optimisation
#### 3.2.2 Clean-Up  
The clean-up step is necessary to reduce the ME, which strongly influences recoveries [80], but can also result in loss of analytes [68]. As indicated earlier, samples were previously frozen before the actual purification, which is the most common method for direct clean-up using the QuECherSR technique. This procedure mainly allows the removal of fats from the sample [81], which in the effluent can be up to 2000-5000 mg\(\cdot\)L\({}^{-1}\), depending on the origin [82; 83; 84]. Compared to the melting point of fats (usually below 40 \({}^{\circ}\)C), the melting point of pharmaceuticals is generally high (Table 1)--it is estimated that for 78.9% of pharmaceuticals, it is in the range of 80-240 \({}^{\circ}\)C. In comparison, for more than 90%, it is higher than 50 \({}^{\circ}\)C [85; 86]. Due to the differences in melting point, the extracted pharmaceuticals are still dissolved in the organic solvent. At the same time, the frozen co-extracted fat can be easily separated from the extracts via centrifugation [81].  
To minimize ME during the analysis of pharmaceuticals in wastewater, the effectiveness of different sorption materials used to purify extracts--PSA/C18/MgSO\({}_{4}\), PSA/GCB/MgSO\({}_{4}\), chitosan, GO, and Fe\({}_{3}\)O\({}_{4}\)MNPs--was assessed. Without a clean-up step, 35% of pharmaceuticals showed signal suppression or enhancement, confirming the need for appropriate sample purification.  
GO had a negligible effect on the recovery values (Figure 3); they were in the 70-98% range when used. It also reduced RSD values to 3-13%, and provided satisfactory ME values, ranging from \(-11\%\) to \(15\%\). The usefulness of GO has previously been demonstrated in determining 52 veterinary drugs in mutton, using a reduced graphene oxide-coated melamine sponge. The material enabled ME values \(\leq\pm 13\%\) to be obtained in the 10-500 mg\(\cdot\)kg\({}^{-1}\) range [87]. GO has been applied in the clean-up of guttation liquid samples to analyze insecticides, providing negligible ME and analyte recoveries between 70% and 120% with relative standard deviations \(<20\%\) for most compounds tested [88]. [PERSON] et al. [89] described using GO to extract organophosphorus pesticides from water samples. Reduced graphene oxide-titanium dioxide (rGO-TiO\({}_{2}\)) nanomaterial has been successfully applied as a dispersive solid phase extraction (d-SPE) adsorbent to extract, enrich, and purify aflat in poultry feed [90].  
Fe\({}_{3}\)O\({}_{4}\)MNPs generally yielded comparable recovery values to the GO clean-up option but slightly reduced ramipril recovery below the acceptable value. Significantly, the ME for some pharmaceuticals was not reduced to the desired degree with either Fe\({}_{3}\)O\({}_{4}\)MNPs or PSA/C18/MgSO\({}_{4}\) and was \(-25\%\) and \(-23\%\), 32% and 38%, 25% and 22%,  
Figure 2: Matrix effects of pharmaceuticals for the modified QuECherSR procedures.  
\(-24\%\) and \(-21\%\), \(23\%\) and \(24\%\), and \(25\%\) and \(26\%\), respectively. Clean-up with classical sorbents--PSA/C18/MgSO\({}_{4}\) and PSA/GCB/MgSO\({}_{4}\)--negatively affected the recoveries of some compounds, including ramipril and venlafaxine. Like GO, chitosan significantly reduced the ME and recovery values below the acceptable limit for almost half of the compounds (Figure 4). The application of PSA/C18/MgSO\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\)MNPs resulted in moderate signal enhancement for indapamide, levonorgestrel, and two metabolites of venlafaxine, while suppression was observed for doxycycline and ramipril.