## 3 Results
### Evaluation of Mesoporous Silicas as Sorbents in SPE  
In the purification step by SPE, the selection of a suitable sorbent is crucial, as it directly influences the experimental results. The affinity of the sorbent toward the target analytes is a determining factor to be considered. The target TAs have basic characteristics (pKa values of 9.43 and 7.75 for atropine and scopolamine, respectively), with a ternary amine protonated at low pH [40; 41]. Since the pH of media determines whether the analytes are in their ionic or molecular form, this variable plays a crucial role in strong cation-exchange SPE. In the functionalized silicas, their negatively charged sulfonic acid group has a strong cation-exchange capacity, so this sorbent interacts with the positively charged TAs via electrostatic (ionic) interactions that can be achieved by adding to the loading solvent HCl. However, other types of interactions can also occur, such as hydrophobic interactions between non-polar alkyl chains and the non-polar parts of the analytes, or hydrogen bonds between the free silanol groups of the silica or the sulfonic groups and suitable functional groups present in the analytes, such as hydroxyl or carbonyl groups. These multiple interactions enhance the efficiency of the sorbent material in capturing the target TAs. Figure 9 shows the types of interactions described that can occur between functionalized mesoporous silicas and target TAs. Therefore, to evaluate the recovery efficiencies (%) of the functionalized mesoporous silicas, cartridges were packed with 50 mg of MCM-41-SO3H or MSU-2-SO3H and the following protocol was followed: (1) conditioning the cartridge with 3 mL of H2O; (2) equilibration with 3 mL of the acidified loading solvent; (3) loading with 10 mL of a 0.1 \(\upmu\)g/L acidified solution (pH 1) of the target analytes in EtOH, MeOH, H2O or MeOH:H2O (50:50, \(v\)/\(v\)); and (4) elution with 3 mL of 5% ammonia solution in MeOH. Figure 10 shows the atropine and scopolamine recoveries obtained for the two different sorbent materials  
Figure 9: Different types of interactions that can occur between TAs with the MCM-41-SO3H sorbent used for solid phase extraction. and four loading solvents. As can be seen, in general, the best recoveries were obtained with acidified H\({}_{2}\)O as loading solvent, with recoveries >85% for both analytes and both materials, followed by EtOH, (recoveries >70%), while with MeOH and the MeOH:H\({}_{2}\)O (50:50, \(v/v\)) the recoveries were generally below 50%. The affinity of analytes such as scopolamine and atropine for mesoporous materials (MCM-41 and MSU-2) functionalized with sulfonic groups in different media can be explained by several reasons related to the properties of the solvent and the interaction of the analytes with the functional groups of the material (sulfonic groups). Water is a highly polar solvent that can form an extensive network of hydrogen bonds. This ability to form hydrogen bonds facilitates the interaction between the sulfonic groups of the mesoporous material and the analytes, thus increasing the affinity in an aqueous medium at pH 1 [42]. In contrast, although MeOH and EtOH are also polar solvents, their ability to form hydrogen bonds is lower and their structure is less extensive. This can reduce the interaction with the sulfonic groups of the mesoporous material, resulting in a lower affinity compared to water. Additionally, at pH 1, both in water and in alcohols, analytes such as scopolamine and atropine will be predominantly in their protonated form. However, the ionization of these analytes may be more favored in an aqueous medium due to the higher dielectric constant of water (79.99), significantly higher than that of MeOH (33.30) and EtOH (24.55), which allows for better stabilization of charged species [43; 44]. This greater stabilization in water can result in a higher affinity of the analytes for the sulfonic groups of the mesoporous material.