Mechanism of formation of system peaks in ion-exclusion chromatography.

Single or multiple system peaks can be observed in ion-exclusion chromatography (IEC) based on whether the eluent is composed of single or multiple active eluent constituents. It was confirmed experimentally that the number of system peaks is always equal to or lower than (when co-elution occurs) the number of active eluent components. Positive and negative system peaks can be recorded in the IEC systems. Negative system peaks can be expected for each eluent component that is also present in the injected sample plug at a lower concentration than in the eluent. In the opposite case, where the eluent components in the injected sample plug are present at a higher concentration than in the eluent, positive system peaks will be recorded. The retention times of individual system peaks are not dependent on each other, but rather depend on the capacity of the column toward the individual eluent component. This capacity depends on the nature of the eluent component and can be concentration-dependent (e.g. for 2,6-pyridinedicarboxylic acid) or concentration-independent (e.g. for acetic acid). The higher the column capacity, the longer the retention time of the corresponding system peak produced by that eluent component. The positive and negative system peaks are the result of column re-equilibration to an injected sample containing a higher or lower concentration of the eluent component, respectively. In general, positive system peaks have longer retention times than negative system peaks. The larger the concentration-dependent capacity of the IEC stationary phase toward an individual eluent component, the larger the retention time difference between positive and negative system peaks.