Chromatography is the technique for the separation, purification, and testing of compounds. The term “chromatography” is derived from Greek, chroma meaning, “colour,” and graphein meaning “to write.”
In this process, we apply the mixture to be separated on a stationary phase (solid or liquid) and a pure solvent such as water or any gas is allowed to move slowly over the stationary phase, carrying the components separately as per their solubility in the pure solvent.
The process of chromatography is classified into different categories:
In the process of adsorption chromatography, different compounds are adsorbed on the adsorbent to different degrees based on the absorptivity of the component. Here also, a mobile phase is made to move over a stationary phase, thus carrying the components with higher absorptivity to a lower distance than that with lower absorptivity. The main types of chromatographic techniques that are used in industries are given as under.
In the process of thin layer chromatography (TLC), the mixture of substances is separated into its components with the help of a glass plate coated with a very thin layer of adsorbent, such as silica gel and alumina, as shown in the figure below.
The plate used for this process is known as chrome plate. The solution of the mixture to be separated is applied as a small spot at a distance of 2 cm above one end of the plate. The plate is then placed in a closed jar containing a fluid termed as an eluant, which then rises up the plate carrying different components of the mixture to different heights.
Thin Layer Chromatography
Column chromatography is the technique used to separate the components of a mixture using a column of suitable adsorbent packed in a glass tube, as shown in the figure below. The mixture is placed on the top of the column, and appropriate eluant is made to flow down the column slowly.
Depending upon the degree of adsorption of the components on the wall adsorbent column, the separation of the components takes place. The component with the highest absorptivity is retained at the top, while the other flow down to different heights accordingly.
In this process, a continuous differential partitioning of components of a mixture into a stationary phase and mobile phase takes place. The example of partition chromatography can be seen in paper chromatography. In this process, chromatography paper is used as a stationary phase which is suspended in a mixture of solvents which act as a mobile phase.
Here, we put a spot at the base of the chromatographic paper with the mixture to be separated and as the solvent rises up this paper, the components are carried to different degrees depending upon their retention on the paper. The components are thus separated at different heights.
Differential extraction is the method of separation of any organic component present in an aqueous solution. In this process, we use an organic solvent for which the solubility of the desired compound is more as compared to that in water. Also, the organic solvent is chosen such that it is immiscible with the aqueous solution so that it can form layers and can be separated easily using a separating funnel.
The organic compound is later recovered by the process of distillation or evaporation. The process of continuous extraction is used in cases when the solubility of the compound is less in the organic solvent.