Ion exchange (IEX) ligands can either be negatively or positively charged. Negatively charged ion exchangers adsorb cations while positively charged ones adsorb anions. Hence they are named cation exchangers (CIEX) and anion exchangers (AIEX) respectively.
Depending on the pKa value of the charged ligand, the ion exchangers are further divided into strong and weak.
Strong ion exchangers are fully charged over the total pH range normally applicable to proteins and peptides. With weak ion exchangers, on the other hand, the charge displayed is a function of the eluent pH.
For protein and peptide purification, the weak ion exchangers are nowadays less frequently used since they provide no essential advantage over the strong ones.
Note that strong or weak in this sense does not refer to the binding strength provided but to the IEX ligand as a strong or weak acid or base.
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