Mechanism of peak fronting in ion exclusion chromatography

Ion exclusion chromatography (IEC) is a distinctive separation technique for weakly acidic analytes that can use pure water as the eluent. Under these conditions, analytes are eluted with a fronted peak shape. A thorough investigation of the conversion of IEC columns (fully functionalised 8% crosslinked styrene-divinylbenzene copolymer substrate) from the aqueous phase to the appropriate analyte form and back to the water form revealed that the IEC column capacity is dynamic. Specifically, a certain quantity of analyte is retained by the stationary phase only when a specific amount of the same analyte is present in the mobile phase. This characteristic is referred to as the dynamic column capacity. Moreover, dynamic column capacity is influenced by both the type and concentration of the analyte. The studied stationary phase shows linear concentration-dependent dynamic capacity, which was found to be significantly greater for aromatic and heteroaromatic organic acids in comparison to those of aliphatic organic acids. Through rigorous experimentation, it has been determined that the dynamic column capacity is the primary factor governing the amount of dynamically retained fraction of the analyte by the stationary phase as the injected sample progresses along the column. Consequently, characteristic peak fronting is generated, with the peak initiating at the column void volume. The resultant peak fronting curves (for differing concentrations of the selected analyte) follow the same fronting curve up to the inflection point where the individual concentration reach steady-state level. As the IEC column exhibits a dynamic column capacity specific to each analyte, the profile of the analyte in the eluent at the column exit is determined by the characteristics of each active component, as exemplified here by a single organic acid. Additionally, it was observed that owing to this phenomenon, an identical mass of injected analyte (generated by using increased volumes with decreased concentration of the analyte) results in identical chromatographic peaks.