Polymers for selectivity control in the capillary electrophoretic separation of small ions
Polymers were used to control selectivity and electroosmotic flow (EOF) in the capillary electrophoretic separation of small ions. Polymers used included cationic and anionic polyelectrolytes, polyethylene glycols, and oligomeric saccharides (cyclodextrins). In the presence of cationic polymers changes in migration order were possible due to ion-exchange interactions with anions. Enhancement of ion-exchange interactions was observed with large electrolyte anions, which could be more easily displaced by analyte species. Although increases in electrolyte concentration and pH led to improvements in resolution their effects on selectivity were minor. In the presence of organic solvents, ion-pairing interactions and changes in analyte/polylectrolyte solvation resulted in different migration orders, which depended on the concentration and solvating properties of the organic solvent. In the presence of cyclodextrins, different migration patterns were observed due to inclusion of relatively hydrophobic anions in the cyclodextrin cavity. Inhibition of such interactions, with an effect on selectivity, was observed in the presence of hydrophobic electrolyte anions. Depending on polythylene glycol molecular weight and concentration decreases in metal ion/cation migration rates were observed. Larger decreases were observed for species having a large affinity for the polyethylene glycols. Ion-exchange interactions between metal ions and anionic polyelectrolytes were relatively weak and capable of minor analyte differentiation. In the presence of anionic polysaccharides, which can complex metal ions, changes in migration order were observed in agreement with the relative complexing affinity of the metal ions for the dextran moieties. Adjustment of EOF direction and magnitude was possible with different types of polymers. Adsorption of cationic or anionic polyelectrolytes provided for anodal or cathodal EOF respectively, which was stable over a wide range of polyelectrolyte concentration and pH. Large decreases in EOF magnitude were possible depending on polyethylene glycol concentration and molecular weight. Application of the developed methodologies to the separation and quantitation of small ions in real or simulated samples of industrial and environmental significance were also shown.