Manipulation of Retention of Acids in Ion Interaction Chromatography

The parameters which affect the adsorption of the IIR onto the stationary phase and hence the retention of solutes include the nature of the stationary phase, the lipophilicity of the IIR, the concentration of the IIR in the mobile phase, the ionic strength of the mobile phase, the nature and concentration of any competing ion added to the mobile phase, and the mobile-phase pH.

The first four of these factors will determine the surface concentration of the IIR on the stationary phase, and hence the surface charge density and the effective ion exchange capacity. The higher the surface concentration of IIR, the greater the solute retention. Thus, retention times will increase as the lipophilicity of the IIR is increased and as the percentage of modifier in the mobile phase is decreased. Solute retention generally increases with the concentration of IIR in the mobile phase, but there is a threshold concentration above which solute retention decreases with further increase in the concentration of IIR. The stationary phase becomes saturated with IIR and any further addition to the mobile phase results in decreased retention because of the increased concentration of the IIR counter-ion.

The nature and concentration of any competing ion added to the mobile phase will determine the retention times and elution order for solute ions. Increases in the concentration of the mobile phase competing ion will result in decreased solute retention, in the same manner as observed for ion exchange separations. Finally, the mobile phase pH may influence the charges on the competing ion and the solutes. An example of this effect is the influence of pH in an ion interaction chromatographic system using tet-rabutylammonium as the IIR and phthalate as the competing anion. Increases in mobile-phase pH over the range 4.0-6.0 cause a decrease in the solute retention as a result of increased ionization of phthalate, leading to the formation of a strong, divalent competing anion.

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