Ion Exchange Chromatography
Ion-exchange phases separate solutes on the basis of ionic charge. Retention in ion-exchange chromatography is determined by the pH of the eluent, the nature and ionic strength of the buffer and temperature. Column efficiencies are lower than in reversed-phase HPLC. Eluents are normally aqueous but can contain some organic component.
Both silica-based and polymer-based ion-exchangers are available. For the former, ionic species are attached to the silica surface, whereas for the latter the ion-exchange groups are distributed throughout the matrix. Silica based materials maintain a mechanical strength and higher efficiency advantage whereas the polymer based materials have greater pH stability.
Ion-exchange is used for the analysis of small ions but its key application area is in the separation of biomolecules such as proteins and nucleic acids. Weak ion-exchangers are used for the analysis of inorganic ions, a technique more specifically termed ion-chromatography.
The exchange capacity of an ion-exchanger is an important measure of its retentivity (typically measured in milliequivalents per gram material). For any one column the packing density of the phase must also be taken into account. Wide pore materials will typically have lower ion-exchange