Types of Chromatography

1. Paper Chromatography:
Paper chromatography is a useful technique for separating and identifying pigments and other molecules from cell extracts that contain a complex mixture of molecules. The solvent moves up the paper by capillary action, which occurs as a result of the attraction of solvent molecules to the paper and the attraction of solvent molecules to one another. As the solvent moves up the paper, it carries along any substances dissolved in it. The pigments are carried along the different rates because they are not equally soluble in the solvent and because they are attracted, to different degrees, to the fibers in the paper through the formation of intermolecular bonds, such as hydrogen bonds.

2. Thin Layer Chromatography:
Thin layer chromatography (TLC) is a widely used chromatography technique used to separate chemical compounds. It involves a stationary phase consisting of a thin layer of adsorbent material, in this case, silica gel immobilized onto a flat, inert carrier sheet. It can be used to determine the pigments a plant contains, to detect pesticides or insecticides in food, in forensics to analyze the dye composition of fibers, or to identify compounds present in a given substance, among other uses.

3. Gas Chromatography:
 A Gas Chromatography is used to detect the components based on the selective affinity of components towards the adsorbent materials. The sample is introduced in the liquid/gas form with the help of GC syringe into the injection port, it gets vaporized at injection port then passes through column with the help of continuously flowing carrier stream.

Chromatography Theory

Retention Theory:

The retention is a measure of the speed at which a substance moves in a chromatographic system. In interrupted development systems like the retention is measured as the retention factor R_f, the run length of the compound divided by the run length of the solvent front:

Rf Value:

The distance moved by a particular spot from the origin line is measured and related to the position of the solvent front. The ratio of those distances is called the Rf value. This value is used to identify the substance.
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Rf value

 

Principle of Chromatography

The technique of chromatography is based upon the fact that the different constituents of a mixture are absorbed to different extents by adsorbent.

Stationary Phase: is a thin layer of adsorbent coated on a plate
Mobile Phase: is a developing liquid which travels up the stationary phase, carrying the samples with it.

Solvent is a substance, especially a liquid that can dissolve another substance.

Common Solvent: Water, Ethyl alcohol, acetone, etc........

Adsorbent is a material which can easily absorb liquids.

Common Adsorbent: Filter paper(cellulose), Magnesium oxide, Silica gel, Alumina.

History of Chromatography

Mikhail Tswett, Russian, 1872-1919 Botanist.
In 1907 Tswett used chromatography to separate plant pigments. He called the new technique chromatography because the result of the analysis was 'written in color' along the length of the adsorbent column.
Chroma means "color" and graphein means to "write". Chromatography is the process of separation of different dissloved components of a mixture by their adsorption over an appropriate adosrbent material.
The name chromatography owes its origin to the fact that the technique was first used for the separation of colored substances from plants (Greek, chormes, meaning color). However, these days it can be applied to colorless substances.