HPLC of homologous series of simple organic anions and cations on an anion exchange column
The retention of phenyl-substituted alkylcarboxylate anions and phenyl-substituted alkylammonium cations on a styrene-divinylbenzene copolymeric anion exchange column with acetonitrile--aqueous solution mobile phase was studied.
The existence of reversed phase retention was investigated by injecting a homologous series of alkylcarboxylate anions. The plots of capacity factor of organic anions vs. reciprocal of eluent ion concentration showed good linearity. From the slope and y-intercept data the retention mechanism of organic ion was interpreted as ion exchange and reversed phase interactions. The effects of temperature, acetonitrile concentration of the mobile phase, type and concentration of the eluent ion, and pH of the mobile phase on the retention and selectivity were also investigated.
The effect of temperature on a homologous series of organic anions in ion exchange chromatography was complex but identical for all the members of the series. The effect of temperature on the competition between the eluite and eluent ion for ionic sites on the stationary phase could account for part of this complexity.
The selectivity between two members of a homologous series of organic anions was not affected by the nature or concentration of eluent ion. However, the selectivity was affected by the acetonitrile concentration of the mobile phase. The study of the effect of pH of the mobile phase proved the existence of reversed phase interaction in ion exchange chromatography.
The retention of phenyl-substituted alkylammonium cations on an anion exchange column has been interpreted as being the results of a combination of ion exclusion and solvophobic interactions. As the eluent ion concentration increased,the eluite cation's retention was increased. The extent of retention depended on the concentration and kind of inorganic eluent ion. However, the selectivity between two homologous cations was not affected by the nature or concentration of eluent ion.
When simple organic anions are injected in ion exchange HPLC, the peak height and shape depends on the relative strengths of the injection solvent and the mobile phase. A study of the effect of injection solvent on the peak height of phenylacetate ion was performed on an anion exchange column. Injection solvent strength was varied by changing the eluent ion strength or organic composition of the solvent. When the injection solvent was stronger than the mobile phase the peak was broadened, while a taller peak was obtained when a weaker injection solvent was used. These effects were more significant as the injection volume increased and as the difference in solvent strength between the mobile phase and the injection solvent was greater.