Biochemical Techniques and Instrumentation

2.3.4.4. Choice of ion exchanger

The selection of an ion exchanger is based on molecule stability, molecular weight and effect of pH on charge. Many biological molecules, especially proteins are stable within only a fairly narrow pH range. The ion exchanger selected must therefore operate within this range. The nature of resins, their optimum pH range and applications are given in the Table 2.3.2.

Nature of resin	pH range	Applications
Anion exchangers
Strong	2 - 11	Nucleotides
Intermediate	2 - 7	Organic acids
Weak	3 - 6	Proteins
Cation exchangers
Strong	2 - 11	Amino acids
Intermediate	6 - 10	Peptides
Weak	7 - 10	Proteins

If the molecule is stable over a wide range of pH, either an anion or a cation exchanger can be used. For example, the weak electrolytes that require very low or high pH for ionization are separated only by strong exchangers, as they operate over a wide pH range. In case of the strong electrolytes, weak exchangers are advantageous because they reduce protein denaturation, inability to bind weakly charged impurities and enhance elution characteristics.