Question

Past paper question R, values for photosynthetic pigments may be determined using the technique of thin-layer chromatography (TLC). TLC plate Origin Solvent a. Outline what happens when spinach extract is spotted on a TLC plate and placed into a container of solvent. [2 marks] b. Explain what the Rf values represent in chromatography. [3 marks) c. State two photosynthetic pigments that could be identified using chromatography. [1 mark]

Answer 1

a) Thin layer chromatography is an adsorption chromatography.It involves separation of substances of a mixture over a thin layer of an adsorbent coated on glass plate.A thin layer ( .2mm thick )of an adsorbent like silica gel is spread over a glass plate of suitable size.The solution of the mixture to be separated is applied as a small spot about 2 cm above one end of TLC plate.The glass plate is then placed in a closed jar containing the eluent.As the eluent rises up the plate the componens of the mixture move up along with the eluent to different distances depending on their degree of adsorption and separation occur.

Spinach leaf extract contain chlorophyl a,b,carotene and xanthophylls.When spinach extract is spotted on TLC plate and placed into a container of solvent ( eluent) the solvent will rises up and the component of spinach extract ( the pigments) move up along with solvent.

b)The relative adsorption of each component of the mixture is expressed in terms of its Rf value ie retardation factor.

Rf=distance moved by substance from base line divided by distance moved by solvent from base line

c)chlorophyl and carotene pigments can be identified using TLC