What is Chromatography?
Chromatography is basically a process for separating components of a mixture. To start off with, the mixture is dissolved in a substance called the mobile phase. Following this, the mixture passes through a second substance called the stationary phase.
While passing through the stationary phase, different components of the mixture travel at different speeds, causing them to separate from one another. The speed of the components and their separation are determined by the nature of the particular mobile and stationary phases.The different travel times are referred to as ‘retention time’.
Chromatography derived its name from a technique which was used in the late 19th century for separating pigments in a complex mixture.
If a container filled with water or alcohol in which a complex pigment is dissolved comes in contact with a sheet of paper or cloth, the mixture will be carried up the paper or cloth as a result of the capillary action. However the components of the pigment will not be traveling at the same rate.
While the smallest ones race ahead, the largest molecules of the mixture will travel more slowly causing the stationary phase to develop discrete bands of color representing each component of the mixture. This is how the technique is named “chromatography” or “writing color.”
From Art to Science
Chromatography was initially utilized by artists, artisans and color theorists. Later on it developed as a unique branch of chemistry, which deals with purification of mixtures.
Modern laboratories make use of the same old principles barring the color aspect. Once we dissolve a mixture of interest in a mobile phase and transport it through a stationary phase, the components of the mixture will be separated from one another depending on their different speeds of travel.
There are a variety of chromatographic methods available. They have been created by altering the mobile phase, the stationary phase or the factor determining speed of travel. Each chromatographic method serves a different purpose.
Common forms of Chromatography
The mixture of interest is vaporized and carried through a stationary phase in gas chromatography. The stationary phase will be mostly a metal or glass separation column with an inert gas, such as nitrogen or helium. It takes longer for the larger molecules in the mixture to pass through the column and finally reach the detector.
In this case, the mixture of interest is dissolved in a liquid and moved through a solid stationary phase. ‘Silica material’ is often used as the stationary phase. There are several varieties of liquid chromatography. It depends on the relative polarities of the mobile and stationary phases.
Thin-layer Chromatography (TLC)
It is relatively easy to digitize the output of TLC. While the stationary phase is usually a thin layer of solid material (mostly silica-based), the liquid in which the mixture of interest is dissolved acts as the mobile phase.
Ion exchange chromatography
The positively (cations) or negatively (anions) charged ions are separated with different stationary phases and different pH mobile phases. In Ion exchange chromatography, the components of a mixture are separated on the basis of their charge, in addition to/instead of their size.
Chromatography is an analytical tool, the output is passed on to a detector which reads the contents of the mixture. It is used as a purification tool as well.
In solid-phase extraction, different mobile phases are used in sequence for separating out different components of a mixture trapped in the solid phase.
Gel electrophoresis is all about separating nucleic acids and proteins based on their size, by drawing them through the gel via an electric field. This technique is similar to chromatography. On the other hand, distillation separates the components of a mixture according to their boiling and condensation points.
In two-dimensional chromatography, the pyrolysis gas chromatography is used as part of mass spectrometry. On the other hand, chiral chromatography is used for separating stereoisomers.
Chromatography is a simple and flexible procedure. It can have several variations and applications in the future.
Read more:Indian Council of Agricultural Research
Check your knowledge
Answer) Chromatography is basically a process for separating components of a mixture.
Answer) Gas Chromatography, Liquid Chromatography, Thin-layer Chromatography (TLC), and Ion exchange Chromatography.
Answer) Pyrolysis gas chromatography and Chiral chromatography.