Thin Layer Chromatography

Thin Layer Chromatography

A central part of many forensic investigations is the analysis of materials that are recovered from the scene of the investigation. A mainstay technique used to separate and identify individual components in a mixture of compounds is chromatography .

One type of chromatography that is relevant in forensic science is thin layer chromatography (TLC). TLC is a type of liquid chromatography that can separate chemical compounds of differing structure based on the rate at which they move through a support under defined conditions.

TLC is useful in detecting chemicals of forensic concern, including chemical weapons, explosives , and illicit drugs . Advances in TLC technology, largely driven by the efforts to quell terrorism, have benefited forensic science. As one example, the Forensic Service Center of Lawrence Livermore National Laboratory has designed a computerized and portable TLC machine that can be taken to the field, and which has the ability to analyze 20 samples at a time. Analysis can be completed within 30 minutes. This allows an analysis that previously required a dedicated laboratory to be done at the scene.

The current TLC technology was introduced by Justus Kirchner in 1951. From its beginning, the technique was an inexpensive, reliable, fast, and easy to perform means of distinguishing different compounds from each other. The method was qualitative—it showed the presence of a compound but not how much of the compound was present. In the late 1960s, TLC was refined so that it could reliably measure the amounts of compounds. In other words, the technique became quantitative. Further refinement reduced the thickness of the support material and increased the amount of the separating material that could be packed into the support. In High Performance TLC (HPTLC) the resolution of chemically similar compounds is better than with conventional TLC, and less sample is required. HPTLC requires specialized analysis equipment, and so is still not as popular or widespread as conventional TLC.

In TLC a solution of the sample is added to a layer of support material (i.e., grains of silica or alumina) that has been spread out and dried on a sheet of material such as glass. The support is known as the plate. The sample is added as a spot at one end of the plate. The plate is then put into a sealed chamber that contains a shallow pool of chemicals (the solvent), which is just enough to wet the bottom of the plate. As the solvent moves up through the plate support layer by capillary action, the sample is dragged along. The different chemical constituents of the sample do not move at the same speed, however, and will become physically separated from one another. The positions of the various sample constituents and their chemical identities are determined by physical methods (i.e., ultraviolet light) or by the addition of other chemical sprays that react with the sample constituents.

see also Analytical instrumentation; Toxicology.