Ion Chromatography - Introduction
Chromatography, in general, is a separation technique whereby substances are separated from one another by distributing them between two phases, a stationary phase and a moving phase. Those substances that are distributed more favorably in the mobile phase pass through and from the system more rapidly than those that are more favorably distributed in the stationary phase. As a result, the individual substances in the original mixture are eluted from the distribution system in order of their increasing distribution coefficients with respect to the stationary phase, thus, affecting a separation.
The distribution system in normal analytical liquid chromatography takes the form of a column packed with the stationary phase if a solid, or bonded to or coated on an inert support if not. The two phases must always be completely immiscible. Alternatively, for some types of chromatography the column can take the form of an open tube and the stationary phase coated or bonded to the tube's internal surface. The different substances are distributed between the phases due to the intermolecular forces between the solute molecules and the two phases differing either in frequency or magnitude, or both. There are three basic types of interaction that can occur between molecules and they are dispersive, polar and ionic.
Ion chromatography is a chromatographic separation technique where the dominant interactive mechanism (but not the only type of mechanism), is ionic.
Early activity in ion chromatography largely involved the separation of simple metal cations and inorganic anions but as the technique developed it has been extended to complex macromolecules of biological origin. Such materials involved interactive mechanisms other than ionic in the separation of the sample contents. As a consequence, the different forms of molecular interactions that are active in chromatography need to be be considered.
The Different Types of Molecular Interaction
There are two requirements for a separation to be achieved in chromatography, Firstly the peaks must move apart in the column and secondly, the distribution system must be so designed such that the solute bands, having been moved apart, are constrained from spreading so that the solutes, having been separated, can be eluted discretely and measured.
The capacity of the chromatographic system to move the solute bands apart is called column selectivity and depends on the differential interactions of the solutes between the two phases. The capacity of the distribution system to restrain band spreading is determined by the exchange kinetics of the column system and is defined as the column efficiency. The greater the efficiency the more the band spreading is constrained and the better the separation. It follows that to understand solute retention and selectivity it is necessary to understand the different types of molecular interactions that can occur and how they can affect retention and selectivity.