Principles and Practice of Chromatography - Introduction > Page 3
In practice, the distribution system, (that part of the chromatographic apparatus where the solutes are distributed between the phases) can take the form of a column such as a tube packed with particulate matter on which the stationary phase is bonded or coated. The mobile phase (which may be a gas or a liquid) passes under pressure through the column to elute the sample. The column form may also be a long, small-diameter open tube that has the stationary phase coated or bonded to the internal surface. Alternatively, the chromatographic system may take the form of a plate (usually glass) the surface of which is loaded with particulate matter to which the stationary phase is coated or bonded. The mobile phase (a liquid) is arranged to percolate up the plate (usually by surface tension forces) to elute the sample.
The sample is injected into the mobile phase stream just before the front of the columns. The column is designed to allow two processes to take place that will produce the separation. Firstly, as a result of different forces between each molecular type and the stationary phase, each solute is retained to a different extent and, thus, the more weakly held will elute first and the more strongly held elute last. The process is diagramatically depicted in figure 1. 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.