## 3 Results and Discussion  
### LC-MS/MS Optimisation  
A review of relevant literature and the authors' own experience indicated that liquid chromatography coupled with tandem quadrupole mass spectrometry (LC-MS/MS) in the multiple reaction monitoring (MRM) mode is well-suited for the determination of pharmaceutical residues with diverse physicochemical properties [60; 61; 62; 63; 64; 65; 66]. In the initial phase of the study, detection parameters were selected and optimized. For this purpose, standard solutions of individual pharmaceuticals at a concentration of 0.1 \(\upmu\)g mL\({}^{-1}\) were introduced into the mass spectrometer to establish appropriate operating conditions. The first aspect evaluated was the optimal ionization technique--electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI). Across all tested compounds, APCI produced a significantly lower signal intensity (in terms of peak height and area), which led to the selection of ESI due to its superior signal response. The ionization mode (positive or negative) was also examined. The positive mode consistently yielded better signal intensity for all analytes, although negative ionization was found to be effective for acidic compounds such as candesartan, valsartan, and ramipril. The addition of ammonium formate to the standard solutions proved to be a critical factor, significantly enhancing signal intensity by facilitating ion formation. The full-scan mode was initially used to identify the most intense precursor-to-product ion transitions for each compound. These transitions were then employed in the MRM mode, with two transitions selected per analyte to ensure reliable qualitative identification. In each case, protonated molecules ([M+H]+) were chosen as the precursor ions. Subsequently, fragmentation of the selected ions was optimized by adjusting detector parameters specific to each analyte, including the declustering potential, collision energy, and ion exit potential from the collision cell. Fragmentation spectra were recorded at varying collision energies. Lower energy values favored the formation of higher-mass fragments, while increased energy led to more abundant low-mass ions and the disappearance of heavier ones. For each MRM transition, the collision energy that provided the highest sensitivity was selected. Specific values of tested parameters are presented in Table 3.  
Determining the optimal parameters of ion sources, including curtain gas pressure, spray support pressure, auxiliary pressure, temperature, and voltage applied to the electrode, was an important part of the work of selecting MS/MS detection conditions. It was particularly challenging to select the aforementioned parameters in the case of the analysis of a mixture of 20 pharmaceuticals. Compounds with diverse physicochemical features (diverse lipophilicity, acid-base nature, and different types and quantities of functional groups in molecule), deciding their retentive properties, were included in the composition of the separated mixtures. The mixtures subjected to separation included compounds with varying physicochemical properties--such as differences in lipophilicity, acid-base character, and the presence of diverse functional groups--all influencing their chromatographic retention behavior. Optimal chromatographic conditions were established based on key analyte characteristics, including molecular weight, logP, and pKa values. During the optimization process, several variables were evaluated: the type of column packing, its temperature, the mobile phase composition and pH, the flow rate, and the elution program. The primary objective was to achieve high separation efficiency, sensitivity, and reproducibility within a short analysis time (under 10 min). Although full separation of co-eluting analytes is not strictly required in MRM mode, improving the resolution reduces matrix interferences and enhances both sensitivity and selectivity. Additionally, the differences in retention times helped confirm analyte identity.