Thin Layer Chromatography

• Introduction

The thin layer chromatography (TLC) is a kind of chromatography where the stationary phase is a thin layer of material deposited on a support, this set is called a plate. The mobile phase is a solvent mixture, call eluent, that it moves for the stationary phase by means of capillarity. Generally, the plates are formed by a layer of silica gel or another material with adsorbent properties.

• Obtaining the chromatogram

The sample solves in few quantity of a volatile solvent. If it’s going to be in use patrons of some substance it’s prepared of the same way. Later it’s drawn a base line with a pencil and a few points of reference where the samples will be puncture with a capillary.

It’s necessary to prepare the container in which the chromatography is going to be realized. This container is a simple glass container where is added the eluent that is going to be in use, always in a level lower than the base line that it has drawn in the plate. In the container there gets a strip of filter paper in order that the eluent vapor is for all the container. In this moment the plate gets carefully in the container and it wraps up, hoping that the level of the eluent ascends for the plate.

When the level is near to top, the plate is extracted and a new line is drawn in this level. The volatile solvent is being evaporates in a little time leaving the plate dry. Now it’s necessary find a method to visualize the result of the chromatogram and see the separation of components punctured in the plate. In case that the substance is colored there would be no problem since the components would be seen. For other compounds, the commercial plates are in the habit of taking a fluorescent substance that if it’s radiated by UV light, the components can see as dark circular spots. These spots must be marked on the plate with a pencil. Another method can be make react the components with some substance that colour these compounds to give a visual sign.

• Factors that influence in the elution

The factor that more influences in the separation of the components in this kind of chromatography is the polarity of these components. The plate is formed by a layer of gel of silica, a very polar substance, and therefore, it will attract in major measurement to more polar components.
On having fulfilled the chromatogram, the most polar substances will adsorbed more with the support and its displacement speed will be smaller, this means that on having revealed the plate of chromatography, the most polar component will be in a place nearer to base line that we have drawn, and on the contrary, the least polar component will move more rapidly and will be more far from the base line.

The eluent polarity is a factor that get that the components move more rapid or slower in a chromatography. That is to say, for an eluent with greater polarity, this one will be more attracted by the support and the components will be freer to move better, for that, on having the plate revealed, the components will go out more far from the base line. For the same reason, with a less polar eluent, this one almost doesn’t adsorbed for the polar support, and the compounds will have a greater interaction with the support, for that its speed will be smaller and on having finished the chromatography, it will have moved not much from the base line. For these factors, there is advisable the election of a good eluent for the compounds separation. For every experience the kind of eluent will be different, depending on the components to separate, therefore, the best thing will be to realize several tests with different kind of eluent, this way consider the most ideal eluent for our chromatography.

• Applications

Thin layer chromatography is very useful, fast and cheap method that can be used principally for:

• Identification of some substance known in a mixture for its comparison with a patron of pure substance.

• Measures the purity degree of a sample, if several spots exist in the chromatogram, it’s a sign that exist several components in the sample.

• Obtain a semiquantitative measurement of some component. Puncturing the same quantity in a plate, as much bigger is the spot obtained in the chromatogram, the concentration of this substance will be bigger.

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