Thin-layer Chromatography
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Thin-layer chromatography (TLC) is a simple form of chromatography used extensively in forensic drug analysis, toxicology, explosives analysis, and analysis of dyes and inks. TLC separates mixtures by exploiting solvents traveling over a solid support phase and the resulting chemical interactions between the solvent, solid support, and molecules of interest. As shown in the figure, tiny spots of dissolved sample are placed in a line across the bottom of a plate or paper. The plate is coated with a thin layer of a silica or related powdery material. The line is called the “origin.” The plate is then placed in a shallow solvent bath so the level of the solvent is below that of the origin. The solvent can be as simple as water (used for ink analysis), or it may consist of two or more organic solvents such as benzene, ethanol, or acetonitrile.
Capillary action draws solvent up the plate in the same way water is drawn up into a paper towel. As the solvent encounters the sample, some or all of it dissolves and begins moving along with the solvent “front” as it creeps up the plate. Some components in the mixture will interact with the silica material, causing it to fall behind components that interact less. Eventually, all separable components will be spread out in spots across the plate. Any components that are not soluble in the solvent will remain at the origin.
In the case of ink analysis, spots are clearly visible. However, in many cases a developer must be sprayed or otherwise applied to visualize the spots. Control samples are routinely applied to the plate as well for comparative purposes. For example, TLC is a key component of the identification of marijuana. After initial presumptive tests indicate that a sample of plant matter could be marijuana, a small amount is dissolved in a solvent such as petroleum ether. The extract is spotted on a thin layer plate along with extracts of known marijuana samples. After the runs are completed, the plates are sprayed with a developer such as “Fast Blue B,” which reacts with the components to produce colored spots. If the sample is marijuana, the pattern of the questioned sample should be similar to that of the known marijuana.
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