May 30, 2014

Chromatography

Alexis St-Gelais - Popularization

Chromatography refers to a wide range of separation techniques, frequently used in the field of analytical chemistry . The principles of chromatography allow PhytoChemia to achieve several of its analyzes.

Chromatography is, for every chemist working in organic chemistry, a valuable tool. It brings together all techniques used for separating mixtures of different molecules (eg, a plant extract). It is used both to isolate some of them, for dosing chemicals of interest, or to generate a general profile for a mixture, such as in the case of essential oils. Let us examine how this works.

An old principle in chemistry says that "like dissolves like." This means that a molecule will be easier to combine with other molecules that are similar in some respects. In contrast, very different molecules are incompatible and will easily tend to separate from each other. Thus, oil and water repel each other, while the water and alcohol mix easily.

Chromatography uses this phenomenon to generate affinity-based separations, making it possible for molecules to associate with two different media concurrently. Typically, one of these media is stationary (the stationary phase) and the second flows through or in contact with the first (the mobile phase). For example, molecules will be introduced into a column containing solid silica, wherein a liquid solvent is flowing. At the microscopic level, we find in the column a lot of small solid particles of silica around which a liquid is flowing.

When a molecule is placed at the entrance of the column, two phenomena occur:

A) The molecule interacts with the solid and immobile silica. The more affinity these two substances have, the stronger the interaction is, and as the molecule adhere to the stationary phase, it in turn becomes far less movable. This is known as retention or adsorption of the molecule.

B) The molecule interacts with the liquid and mobile solvent. Again, the more affinity they have, the faster the solvent pushes the molecule forward, allowing it to ultimately cross the entire column. This is called elution or migration of the molecule.

Adsorption and elution occur concurrently and simultaneously, depending on the affinity of the molecule for each of the phases. In the end, this results in the molecule progressing through the column at a speed of its own, which depends on its chemical structure.

Figure 1. Type chromatogram obtained by  high pressure liquid chromatography (HPLC). Each peak corresponds to a molecule. The later the molecule appears, the more it interacted with the stationary phase.
If several different molecules enter the column simultaneously, they do not pass through at the same speed and thus come out of the column more or less separated from each other. This is the power of chromatography, put to good use to isolate chemicals or to examine them separately in a mixture for an assay, for example. Most analyzes offered by PhytoChemia are possible thanks to chromatography, which can therefore be of considerable use to you too.

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