Chromatography is a technique based on (a) Solubilities of solute (b) Adsorption of solute (c) Chemical adsorption followed by dispersion (d) Differential adsorption of different constituents of a mixture

One of the most fundamental objectives in chemistry is isolating individual components from a mixture. Separtion techniques in chemistry are various methods used to achieve this separation, each based on different physical and chemical properties of the substances involved.

• Distillation exploits differences in boiling points.
• Filtration separates solids from liquids through a porous barrier.
• Crystallization isolates solids based on their solubility in a solvent.
• Centrifugation separates components by density using centrifugal force.
• Chromatography, meanwhile, uses the differing affinities of compounds to separate them.

Chromatography stands out amongst these techniques due to its sophistication and ability to separate components that are otherwise very similar in chemical properties. It can be applied to purify, analyze, and quantify substances within complex mixtures.

At the heart of chromatography lies the principle of adsorption, which is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. In chromatography, adsorption is responsible for retarding the flow of components through the stationary phase.

During the operation, some compounds in the mixture adhere to the stationary phase more strongly than others due to adsorption. These compounds will move slower than those that do not adsorb as strongly. Gradually, this differential adsorption results in the separation of compounds as they travel at different speeds along the stationary phase, eventually leading to their individual isolation.

The practical execution of chromatography requires two critical components: the stationary phase and the mobile phase. The stationary phase is typically a solid or a liquid supported on a solid, and it remains fixed in place. The mobile phase, on the other hand, is a fluid that moves through or around the stationary phase.

• In paper chromatography, the stationary phase is a strip of paper while the mobile phase is a solvent that travels up the paper.
• In column chromatography, the stationary phase could be silica gel or alumina packed in a column, and the mobile phase could be a solvent passing through the column.

Compounds interact with these two phases depending on their chemical properties, and due to their varying degrees of affinity to the stationary phase, they will migrate at different rates when carried by the mobile phase, leading to their eventual separation.

The objective in chromatography is to achieve the separation of compounds based on differential adsorption. As the mixture is introduced to the system, various interactions between the compounds and the stationary phase—such as ionic, hydrogen bonding, and van der Waals forces—influence how quickly a compound moves along with the mobile phase.

A compound with a strong affinity to the stationary phase will adsorb more and move slower, while one with less affinity will travel faster. Incrementally, this leads to components spreading apart and forming distinct bands or spots. These can be collected separately or detected within the system if the intention is analysis rather than collection. The precision of compound separation makes chromatography a vital tool in analytical chemistry, biochemistry, and even forensic science.