Consequently, each solute will be sequentially eluted from the column in the reverse order of the magnitude of the interacting forces between each solute and the stationary phase. Secondly, the spreading of each solute band (that is its dispersion) must be constrained so that each solute is eluted discreetly. The first function of the column is achieved by choosing the appropriate phase system (the optimum stationary phase in GC and the optimum combination of mobile phase and stationary phase in LC) to separate the solutes. The second function is achieved by selecting the optimum physical properties of the column (column dimensions, particle diameter, mobile phase velocity etc.) to ensure that band dispersion is adequately constrained. As all chromatographic separations are carried out using a mobile and a stationary phase, the primary classification of chromatography is based on the physical nature of the mobile phase. The mobile phase can be a gas or a liquid which gives rise to the two basic forms of chromatography, namely, gas chromatography (GC) and liquid chromatography (LC).

Table 1 The Classification of Chromatography

chromatography systems;

The stationary phase can also take two forms, solid and liquid, which provides two subgroups of GC and LC, namely; gas–solid chromatography (GSC) and gas–liquid chromatography (GLC), together with liquid solid chromatography (LSC) and liquid chromatography (LLC). The different forms of chromatography are summarized in Table 1. Most thin layer chromatography techniques are considered liquid-solid systems although the solute normally interacts with a liquid-like surface coating on the adsorbent or support or, in some cases an actual liquid coating.