ELUENT MODIFIERS FOR THE LIQUID-CHROMATOGRAPHIC SEPARATION OF CARBOXYLIC-ACIDS USING CONDUCTIVITY DETECTION

It has long been held that chromatographic separation of carboxylic acids requires (a) the use of a gel-type column of fairly large dimensions and (b) an acidic aqueous eluent to repress ionization of the analytes and thereby give sharp peaks. The present work shows that neither of these conditions is necessary. Excellent separations of aliphatic carboxylic acids were obtained on macroporous resin columns using a predominantly aqueous eluent with no added acid. Addition of an alcohol, such as 1-butanol, to an aqueous mobile phase has a dramatic effect on the chromatographic separation of carboxylic acids. The alcohol is believed to coat the surface of a macroporous polystyrene-divinylbenzene resin and establish a dynamic equilibrium between the mobile and stationary phases. This increases the hydrophilicity of the resin surface and reduces the hydrophobic attraction of the analytes for the resin phase. The eluent systems have a very low background conductance, thus giving a very sensitive conductometric detection. The presence of carbonic acid (from atmospheric carbon dioxide) in the eluent was found to play a role in the retention mechanism of the carboxylic acid analytes.