Thin-layer chromatography, TLC 78-79

Question 1

What are the uses of TLC?

Answer 1

TLC is a simple and quick method used to monitor the extent of a chemical reaction or to check purity of compounds. It is particular useful in organic chemistry.

Question 2

What is the stationary and mobile phase in TLC?

Answer 2

• The stationary phase is a thin layer of an dsorbent such as silica gel (SiO₂) or aluminia (Al₂O₃) coated on a flat, inert support, usually a sheet of glass or plastic - this is called a TLC plate.
• The mobile phase is a liquid solvent, which moves vertically up the TLC plate.

Question 3

Explain the steps in producing a chromatogram.

Answer 3

• A small sample of the mixture is dissovled.
• A small spot of the sample solution is placed on the TLC plate a short distance from one end of the plate. It is useful to first draw a penci line as a guide for the sample.
• The setup is placed in a jar. The jar is then sealed to saturate the space in the jar with solvent vapour. This slows down the evaporation of solvent from the TLC plate and prevents solvent loss.
• The solvent rises and it sweeps up the sample. Seperation occurs when as different components bind to the adsorbent more strongly.
• To achieve maximum separation of the component , the solvent is left to rise until it nearly reaches the top of the TLC plate. The plate is then taken out of the solvent. The position of the solvent front is marked with a pencil line and solvent is then allowed to evaporate.

Question 4

What is done in TLC if the separated components are not coloured?

Answer 4

If the components are colourless you may need to use a locating agent to ‘develop’ the chromatogram. Alternatively, UV radiation is often used to show up each component by fluorescence.

Question 5

What is an R_f value?

Answer 5

An R_f value shows how far a component has moved compared with the solvent front:

R_f = (distance moved by component)/(distance moved by sovlent front.)

Question 6

How can R_f values be used to identify samples?

Answer 6

R_f values of components can be compared with those of pure compounds to aid in the identification of an unknown substance. A chromatogram can be run with samples of known substances alongside the mixture. It is then easy to identify a component by comparing the distances travelled by the component and the known substance.

Question 7

What are the limitations of thin-layer chromatography?

Answer 7

• Similar compounds often have similar R_f values.
• Unknown compounds have no referance R_f for comparison.
• It may be difficult to find a solvent that separates all the components in a mixture. If the components are very soluble in the solvent, they will just be washed up the TLC plate with the solvent front. If the components have little solubility, they will hardly move. Trial and error may be required before a suitable solvent, or mixture of solvents, is discovered.