(33a) pH Transitions in Chromatography Columns
pH transitions, defined as temporary excursions from a steady pH value even when initial and feed pH are constant, can occur in chromatography columns that contain weak acid or weak base groups even when the solutions used are buffered by non-adsorbed buffering species. Such transitions can lead to very long equilibration times, altered chromatographic separation performance, damage to the stationary phase and/or to the species being separated, and other operational problems. In this paper, we review how the local equilibrium theory of coherent waves can be used to understand the nature of these pH transitions, predict their magnitude and duration, and devise ways to either avoid them, when undesirable, or use them to an advantage for pH-based chromatographic resolution. As illustrative examples, we consider pH transitions in weak acid or weak base ion exchange columns and in hydroxyapatite columns caused by constant pH buffer steps as well as pH transitions in mixed beds of weak and strong ion exchangers generated by pH steps designed for the chromatographic resolution of protein charge variants.