Chromatography

Chromatography is a technique used to separate the substances found in a mixture. It can be used to help identify a substance by comparing it to known references, or to compare the constituents of two substances. Different techniques can be used to help identify such things as drugs, sugars, hydrocarbons, pollutants and pesticides. The simplest method is the use of a chromatography paper and solvent to carry the mixture. This can easily be used to compare the different colours used to make an ink.

The mobile phase (methylated spirits and water) sweeps up the components of the ink into the paper (stationary phase). As this occurs, they continually undergo a process of adsorption onto the solid phase (paper) and desorption back into the liquid mobile phase. The rate of movement depends on how strongly a component is adsorbed and dissolved; as this occurs at different rates in each component then the components will separate.

For comparison, the degree of movement of compounds is measured using R_f values. This is the ratio of movement of a component in relation to the solvent front. This provides a useful qualitative comparison.

For example in the chromatogram shown above, the R_f of the yellow component is 0.2, while the R_f of the blue component is 0.8.