pH transitions in cation exchange chromatographic columns containing weak acid groups

Complex pH transitions occur in cation exchange columns used for protein chromatography during equilibration and salt elution steps when the stationary phase contains weak acid groups even if the mobile phase is buffered and the buffering species do not interact with the stationary phase. In this work, we present a local equilibrium model to predict the magnitude and duration of these pH transients. The model equations are solved by the method of characteristics and by numerical simulations using an equilibrium-dispersive model. By incorporating an explicit description of the dissociation of the weakly ionogenic groups in the resin, we show that counterion binding in the column can be predicted for different buffer systems based on a single experimental resin titration curve without having to resort to empirically defined adsorption equilibrium constants. Model predictions based on these assumptions are found to be in excellent agreement with experimental results obtained for three different resins containing varying concentrations of weak acid groups. Four common buffer systems, acetate, citrate, MES, and phosphate are considered with both step and gradient changes in salt concentration at pH 5.5. Each buffer yields a different pH excursion behavior. We demonstrate that when the counterion concentration is kept constant in each of these buffers, which is needed to attain identical protein adsorption behavior, the magnitude of the pH transitions occurring during salt steps is nearly independent of the buffer system. On the other hand, the duration of the pH transitions is smallest for MES suggesting that this buffer system is preferable where pH variations are to be prevented.