Practice question

Ion exchange selectivity depends on net surface charge of analyte at operating pH relative to matrix charge. For proteins, net charge is determined by relationship between buffer pH and isoelectric point pI. At pH above pI, protein is negatively charged and binds anion exchanger; below pI, it binds cation exchanger. Therefore stationary phase choice and pH optimization require consideration of pI, ensuring target molecule bears appropriate charge for binding while contaminants differ. Molecular mass influences gel filtration but not ion exchange, while solubility governs partition methods. Understanding pI-matrix relationship is essential for rational purification strategy and gradient elution design.