What is chromatography?

We explain what chromatography is, how it is used to separate mixtures, what are its phases, what types exist and examples.

1. What is chromatography?

Chromatography is a method of separating complex mixtures , which is widely used in various branches of science . For this, it uses the principle of selective retention , which consists of the different behavior of the components of a mixture on a specific support (such as paper, gas, a neutral liquid, etc.).

Thus, chromatography uses various techniques that take advantage of the differences in the adsorption rate of each component, identifying them, in addition, by taking into account the color spectrum that the mixture throws over time. This method allows each component to be separated in a high state of purity or, by identifying them, determining their exact proportion.

For this, adsorption (different from absorption ) is key, a concept that refers to the coefficient of adhesion that the mixture manifests with respect to the support. Thus, according to the difference in reaction rates of the components of the mixture, its concentration percentage can be measured, when not separated from the whole.

Chromatography separation involves two distinct phases:

• Static phase . It starts when the mixture is applied to its specific support and prepares for the mobile application.
• Mobile phase . Another substance is moved on the support, thus allowing its reaction with the components of the mixture, so that the difference in reaction rate serves as a separation criterion.

Depending on their nature, some substances will tend to move and others to remain on the support, according to how many aesthetic and mobile phases of different conditions (liquid, solid and gaseous) are carried out.

2. Types of chromatography

Depending on the nature of the support (static phase) and the mobile substance (mobile phase), the following types of chromatography can be distinguished:

• Solid-liquid chromatography , in which the static phase is solid and the mobile phase is liquid.
• Liquid-liquid chromatography , in which both phases are liquid .
• Liquid-gas chromatography , in which the static phase is liquid and the mobile is gaseous .
• Solid-gas chromatography , in which the static phase is a solid and the mobile is gaseous.

On the other hand, according to the type of interaction between the stationary and mobile phases, we have the following types of chromatography:

• Adsorption chromatography , when the stationary phase is a solid and is capable of absorbing each element of the mixture by polar type interactions.
• Partition chromatography , when the separation of the mixture is caused by differences in the solubility of its components, both phases being liquid.
• Ion exchange chromatography , when the stationary phase is solid and has ionizable functional groups, capable of exchanging its charge with the mobile phase.

3. Chromatography Examples

Some everyday examples of the application of chromatography are:

• Spilled wine on a white tablecloth . An accident at dinner time will allow us to observe, when the wine dries by contact with the air, the various substances that compose it. Each one will dye the fabric white with a different shade or color, thus being able to identify them separately, which would normally be impossible.
• Blood tests . Chromatography of blood samples is often carried out to identify the substances contained in it, normally imperceptible given that it is a very complex mixture. For this, the color that the blood reflects on a support or subjected to a specific light is observed.
• Urine tests . Like blood, urine is a mixture of various compounds, some solids and other liquids, whose presence or absence can reveal details about the functioning of the body. So a chromatographic separation can be carried out to fish for unusual residues, such as blood, salts, glucose or illegal substances.
• Review of a crime scene . Something we often see in the movies: researchers take fabrics, fibers, fabrics or other supports and observe the separation by adhesion of the different substances spilled on them, such as semen or blood, even when the naked eye could pass unnoticed
• Sanitary food checks . Starting from the fact that food specialists know the reaction of the components of the food when subjected to a chromatographic spectrum, this technique can be used to detail in a sample if there is any type of undue substance in it, product of microbial agents or of any type of contamination, before the product goes on the market and puts people’s health at risk.