Thin layer chromatography (TLC)

It stands for Thin Layer Chromatography.

It’s a simple technique used by chemists in separation of Mixtures in addition to supporting the identification of the separated compounds by comparing the “Retention Factor” of the separated compound with that of a known compound.

The TLC plate on which the Thin Layer Chromatography is performed is usually a sheet of glass, aluminum or plastic. It’s has a coat of adsorbent material ( known as stationary phase ) such as silica gel, alumina or cellulose.

First step is application of the sample to the plate, then in a capillary action- the chemist draws the solvent up the plate. Separation occurs due to the different ascendance rates of analytes on the plate.

Advantages of TLC over other types of chromatography

1. Paper Chromatography
• TLC is faster than paper chromatography.
• It’s more sensitive to many substances.
• Its preparation is usually sharper.
• It requires smaller quantity of sample.
• Potential application of different types of reagents without causing damage to the plate.
2. Column chromatography
• TLC has smaller apparatus
• It has more speed
• Previous points leads to cheaper over all procedure
• It provides easier measurement of the “Retention Factor”.
• The above advantages makes it more suitable for analytical purposes rather than separation purposes.

Preparation of TLC plate

To prepare the TLC plate, first we must mix a small quantity of an inert substance such as “calcium sulfate” with adsorbent substance like “Silica gel” and water. The second step is spreading this mixture on sheet of unreactive substance such as “glass” or “plastic”. The third step is activation of the plate that results from the previous procedures by heating it in an oven at 110oC for nearly half an hour.

The adsorbent layer’s thickness depends on the purpose of TLC as in case of analytical purposes, it ranges from 0.1 to 0.25 mm. In case of preparative TLC it ranges from 0.5 to 2.0 mm.

Technique

A small amount of the sample’s solution is poured on the plate to cover about one centimeter high from the base. Then the plate is soaked in a convenient solvent like “Hexane” and put in a container that’s well sealed. Using capillary action, the solvent goes up the plate to meet the sample’s mixture which it helps to move up the plate and dissolves as well. Differences between rates of the compounds in the mixture are because each compound has different affinity to the adsorbent layer (stationary phase) from the other compounds and also each compound has different solubility in the mixture. If the solvent is changed, the compounds can be separated (using Retention Factor’s value).