Understanding and mitigating conductivity transitions in weak cation exchange chromatography

Large conductivity fluctuations were observed during a high pH wash step in a weak cation exchange chromatography process. These conductivity transitions resulted in a conductivity drop during pH increase and a conductivity rise during pH decrease. In some cases, the absolute conductivity change was greater than 6 mS/cm which was sufficient to affect target protein retention on the column. Further investigation revealed that wash buffer concentration, resin ligand density, and resin ligand pK have a profound effect on the magnitude of the conductivity transitions and the shape of corresponding pH traces. A potentiometric electrode selective for sodium ions was used to measure effluent counterion concentrations from two preparative resins during high pH washes, and the number of exchangeable counterions was compared to predictions made using ion exchange equilibrium theory. Results from this analysis show that conductivity transitions can be effectively mitigated without compromising process performance by optimizing the trade-off between wash buffer concentration and wash phase duration.