In paper chromatography, a test sample is dotted onto a piece of paper, placed in a liquid that mobilizes its components separately and inspected to determine the component's final positions. Depending on the type of liquid used, the underlying mechanism for paper chromatography may involve the test sample's preference for dissolving in the solvent or the thin layer of water bound to the cellulose fibers in the paper.
Though the basics of paper chromatography are simple, and can be used by elementary school students in science class, the technique is robust enough for use in criminal investigations. Because of its ability to separate trace amounts of substances from mixtures, paper chromatography is used by some forensic investigators to detect minute levels of narcotics, prescription drugs or over-the-counter medications in blood and urine samples. Others use it to identify the inks used in ransom notes, forgeries and even counterfeit currency.
Though versatile, this technique is not without difficulties. For example, for analysis of the result of paper chromatography, the paper must be visually inspected. This is impossible when colorless samples are tested, except when the sample is known to contain a substance that can undergo a color-change reaction. In this case, paper chromatography can still be employed, and after the test is concluded, the paper can be sprayed with the reagent that causes the color change, and then subsequently inspected.