Monday, May 14, 2012

Thin Layer Chromatography


A central part of many forensic investigations is the analysis of materials that are recovered from the scene of the investigation.
Chromatography is the main technique used to separate and identify individual components in a mixture of compounds. Thin layer chromatography (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. Chromatography involves separation using two phases; a mobile phase and a stationary phase. Thin layer chromatography, the stationary phase is a thin layer (alumina or silica) on a plate. The mobile phase depends on the nature of the sample you want to separate.
In thin layer chromatography, 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 material 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 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 and will become physically separated from one another. The solvent is allowed to move up the plate but must be removed before the solvent reaches the top.
The point at which the solvent has reached on the plate is then marked and is the solvent front which is important in calculating the retardation factor or the RF value.
The distance each part of the sample has travelled up the plate is also marked and this distance divided by the solvent front gives the RF value. This can be compared with known RF values to try and identify a compound, although a direct comparison with a standard and unknown on the same plate would produce the most accurate results.
The position of the various sample constituents and their chemical identities are determined by physical methods e.g. ultraviolet light or by the addition of other chemical sprays that react with the sample constituents.
While the very similar technique of paper chromatography is only carried out on coloured samples, the use of visualization reagents means that a wider array of samples can be tested with TLC.
Advances in thin layer chromatography technology largely driven by the efforts to quell terrorism have benefited forensic science.
Thin layer chromatography is useful in detecting chemicals of forensic concern including a wider aspect in Narcotics, Toxicology and Chemistry i.e.:
1.Chemical weapons
4.Explosives and
5.Illicit drugs.

In identification and comparison of drugs, explosives, inks and dyes the samples need to be soluble for the technique to be accurate. This is a particular issue when investigating pen inks since the development of gel pens, the ink of which is insoluble. Gel inks cannot be analysed via thin layer chromatography.
However liquid dye samples and ink within pens can be applied directly to TLC plate.
Thin layer chromatography of drugs and explosives is used as a presumptive test, a means to provide an indication of the nature of the sample rather than being used for positive identification of a sample.

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