Behavior of weak acid anions in response to changes in mobile phase pH in indirect photometric chromatography

Indirect photometric chromatography involves the separation of analytes via ion-exchange chromatography and indirect photometric detection. Weak organic acids are typically used as eluting ions. Changes in mobile phase pH affect the speciation of the analytes and thus affect the retention and detection characteristics of analytes. The situation is complicated in the case of weak acid eluents whose speciation is also affected by the pH changes. In this study, the retention and detection characteristics of several weak and strong acids were studied as a function of mobile phase pH with a phthalate-based eluent. Trends in both the detection and retention properties of the weak acids reflected the changing speciation and provided estimates of the dissociation constants of the acids.