Indirect detection in liquid chromatography I. Response models for reversed-phase ion-pairing systems

Summary: Ionic compounds without inherent detectable properties show good response in reversed phase ion-pairing systems containing a detectable ionic compound (probe) in the mobile phase. The basis for the indirect detection is the common interaction effect for solutes and probe caused by the limited binding capacity of the adsorbent. Quantitative models for solutes and probe with the same or opposite charges show a simple dependence of the response on the retention of the solute and the coverage of the binding surface by the probe. Applications of the models have shown good agreement between calculated and found response. The binding to sites of different kinds on the adsorbent and possibly ion-pair formation in the mobile phase have an influence on the indirect response. Uncharged solutes give low response in such systems owing to low binding to the competitive sites.