Effect of an anionic polymer on the separation of cationic molecules by capillary electrophoresis with conductivity detection

Abstract

Separations of quaternary amines tetramethylammonium (TMA+), 2-(chloroethyl)trimethylammonium (CIETMA+), tetraethylammonium (TEA+), tetrapropylammonium (TPA+), and tetrabutylammonium (TBA+), alkali metals potassium and sodium in the presence of choline, and alkyl-substituted ethylenediamine polyamines are examined using capillary electrophoresis at +25 kV with conductivity detection in 50 mM CHES electrolyte with poly(sodium 4-styrenesulfonate), PSSS, as a buffer additive. It is shown that the concentration of PSSS polyelectrolyte from 0.03 to 0.08% has a strong interaction with quaternary ammonium compounds, inducing a decrease in analyte electrophoretic mobility and improved resolution. For TPA+ and TBA+, using 0.03% PSSS, these changes are striking at about 33 and 60%, respectively. At the higher percentage of 0.08% PSSS, the TBA+ peak actually migrated after the neutral marker. The change in the electropherograms due to PSSS is not due simply to viscosity but to an actual electrostatic and/or hydrophobic effect. The influence of the additives such as acetonitrile and sodium dodecyl sulfate (SDS) as well as PSSS in the buffer system on the electrophoretic behavior of alkali metals and choline is examined. Interaction of PSSS with Na+ is more pronounced than that for choline causing a decrease in peak spacing. Although a slight increase in all three analyte mobilities is noted, peak resolution is actually improved using SDS. Separation of structurally related polyamines is problematic with and without PSSS at acidic pH. Pre-rinsing the capillary with poly-l-glutamic acid (PGA) to ensure adsorption of PGA onto the capillary resulted in resolution improvement as well as sharper peaks for the polyamines.