Theoretical Considerations

Abstract

Chromatography is a dynamic separation system consisting of two media, a stationary phase and a mobile phase. The stationary phase consists of small solid particles (usually less than 150 μm in diameter) with a microporous surface, which may be packed into a column or coated on to a plate. These particles are sometimes coated with an inert chemical agent to modify the surface properties. The mobile phase may be a gas or a liquid and serves to carry the sample component molecules through the chromatographic system. During this procedure the component or solute molecules in the mobile phase come into contact with the stationary phase. There is now competition between the two phases for the solute molecules which depends on their physical properties and affinity for the stationary phase. This process is termed partition with each component (A, B, C, etc) distributed between the stationary phase (s) and mobile phase (m) as they pass through the system. At any stage there is an equilibrium established with [A_s] and [A_m] the concentrations in the stationary and mobile phases respectively;

where D _A is the distribution coefficient for component A. The larger the value of D the greater the affinity of the component for the stationary phase. Since D varies for different molecules the affinity will vary and so each component will proceed through the system at differing speeds as shown in Fig. 2.1(a), where D _A< D _B<D _C (D _C has greater affinity for the stationary phase than D _B than D _A).