An introduction to the process

The process of a chromatographic separation takes place within a chromatography column. This column, made of glass or metal, is either a packed bed or open tubular column. A packed bed column contains particles which make up the stationary phase. Open tubular columns are lined with a thin film stationary phase. The center of the column is hollow.

The mobile phase is typically a solvent moving through the column which carries the mixture to be separated. This can either be a liquid or a gas, depending on the type of process. The stationary phase is usually a viscous liquid coated on the surface of solid particles which are packed into the column as discussed above, although the solid particles can also be taken as the stationary phase. In any case, the partitioning of solutes between the stationary and mobile phases lead to the desired separations.

A mixture which enters a column is called the eluent and the product outflow of the column is called the eluate. The process in which liquids or gases flow through a chromatography column is termed elution.

The figure below shows a typical setup for a chromatographic separation. A column is contained within an oven for temperature control. Feed from a process is mixed into a solvent which flows down the column. The eluate is then analyzed by a detector called the chromatograph, usually by the refractive index or ultra-violet absorbance.

The figure below shows a simple separation by chromatography. A continuous flow of solvent carries a solution of solutes A and B down a column. (a) As the solvent carries the two solutes down the column, we begin to see some separation of the solution. (b) At some later point in time, it can be seen that solute B is moving at a much faster rate than A. (c) In (d), solute B emerges first, while solute A finally emerges in (e). Thus, solute A has a greater affinity for the stationary phase than solute B. By varying the pH of the solvent or temperature of the column, the output of the column can be significantly altered, such as the timing of when individual species emerge.

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© 1997 Kevin Yip - Introduction to Biochemical Engineering