Chromatography

Chromatography is a method for analyzing complex mixtures (such as ink) by separating them into the chemicals from which they are made. Chromatography is used to separate and identify all sorts of substances in police work. Drugs from narcotics to aspirin can be identified in urine and blood samples, often with the aid of chromatography.

It works because some of the coloured substances dissolve in the liquid (like ink) better than others, so they travel further up the paper.

In all chromatography there is a "mobile phase" and a "stationary phase". The mobile phase moves through the stationary phase picking up the compounds to be tested. As the mobile phase continues to travel it takes compounds with it. At different points the different components are going to be absorbed and will stop moving. This is how the results of chromatography are gotten, from the point at which the different components of the compound stop moving and separate.