Cation-exchange chromatography in non-aqueous solvents

Abstract

Much of the selectivity of organic ions in conventional ion chromatography comes from hydrophobic interaction between the carbon chain of the analyte ions and the polymer matrix of the ion exchanger. By operating in organic solvents containing little, if any, water, the true ion-exchange selectivity of various organic ions can be measured. Retention factors (capacity factors, k′) for a series of protonated amine cations were measured in methanol, ethanol, 2-propanol and acetonitrile using a polymeric cation-exchange resin, conductivity detection, and eluents of methanesulfonic acid in the same organic solvent. Plots of log t′ R vs. log methanesulfonic acid concentration were linear with slopes close to the theoretical slope of −1.0 except for acetonitrile where the slopes averaged −0.82. The retention factor showed little change with increasing carbon chain length in n-alkylamine cations, but increased substantially at fixed eluent concentration in going from methanol, to ethanol, to 2-propanol and especially to acetonitrile. Several practical separations of organic amine cations were demonstrated in non-aqueous media. One advantage of this technique is that neutral organic analytes elute very quickly in non-aqueous media and thus do not interfere with chromatographic separations of the ions.