Evaluation of polymeric methacrylate-based monoliths in capillary electrochromatography for their potential to...

Polymeric methacrylate-based monoliths are evaluated in capillary electrochromatography (CEC) and pressurized capillary electrochromatography (p-CEC) for their potential in pharmaceutical analysis. Using a given polymerization mixture as a basis for the monolith synthesis, different mobile phase pH at constant organic modifier concentrations are tested in both CEC and p-CEC. The test set consists of basic, acidic, amphoteric, and neutral compounds, which are mainly pharmaceuticals. Because of the mainly hydrophobic character of the stationary phase, the interactions are largest when the compounds appear in an uncharged state, but some ion-exchange phenomena with negatively charged compounds can also be observed. In CEC, acidic substances are most retained at low pH. For amphoteric and neutral compounds, no preference regarding analyzing pH can be derived from these experiments. For basics, a high pH is chosen, but a reduced solvent strength is needed to enhance the retention of these compounds. The retention mechanism in p-CEC can also be assigned to both hydrophobic and ionic interactions. For acidic, amphoteric, and neutral compounds, acceptable retention is seen. For the basic compounds, the retention with a mobile phase containing 50% organic modifier is low, as in CEC. However, when the organic modifier content in the mobile phase is decreased, retention increases and the selectivity of the stationary phase is more pronounced. This mode of operation presents a possibility for separating some test mixtures, thus some potential for pharmaceutical analysis is seen. More efforts are needed to obtain higher efficiencies and better peak shapes, which might be solved by a further optimization of both the stationary phase synthesis and the mobile phase composition.