Chemical Analysis: Chromatography

Understanding Chromatography

Chromatography is a powerful method used to separate components of a mixture based on their molecular properties.
It works on the principle that different substances move at different speeds through a medium, depending on their affinity to the mobile phase (liquid/gas) and stationary phase (solid/liquid).
The mobile phase moves over the stationary phase, carrying the mixture with it.
Components of the mixture then get separated due to differences in their partitioning behaviour between the mobile phase and the stationary phase.

There are various types of chromatography, including paper chromatography, thin layer chromatography (TLC), gas chromatography, and liquid chromatography.
Paper chromatography is a type of chromatography that uses a strip of absorbent paper as the stationary phase and a liquid solvent as the mobile phase.
Thin-layer chromatography (TLC) is similar to paper chromatography, but uses a thin piece of plastic or glass coated with a layer of silica gel or alumina as the stationary phase.
Gas chromatography uses a gas as the mobile phase and a liquid or solid stationary phase, often held in a metal or glass column.
Liquid chromatography utilises a liquid mobile phase to transport the sample mixtures through a column filled with a material that acts as the stationary phase.

A chromatogram is a visible record showing the result of separation of the components of a mixture by chromatography.
In chromatograms, each band or spot represents a substance, and its intensity can often relate to the amount of that substance in the mixture.
Vertical distance moved by a component (Rf value) relative to the solvent’s movement can be a good indicator of identity of the substance.
The Rf value or retention factor is a ratio of the distance travelled by a specific component to the distance travelled by the solvent.
Similar substances have similar Rf values under identical conditions, which can be used to identify unknown compounds by comparing their Rf values.

Chromatography is used in various fields including pharmaceuticals, environmental monitoring, forensics, and food industries.
In forensics, it can be used to analyse ink samples or other evidence to establish links to crime scenes.
In the food industry, it can be employed to detect harmful contaminants, or to ascertain the authenticity of products by checking for presence/absence of certain compounds.
In pharmaceuticals, it’s used to separate and analyse complex mixtures, ensuring drug purity.