Alevel chemistry: chromatography and spectroscopy

Chromatography and spectroscopy

Chromatorgraphy is used to seperate individual chemicals in a mixture. All forms of chromatography use a stationary phase and a mobile phase. 

stationary phase:normally a solid or a liquid supported in a solid that does not move.

mobile pahse: normally a liquid or gas, does move.

Thin layer chromatorgraphy (TLC)

TLC uses a plate made of plastic or glass that is coated in an adsorbant substance such as sillica which is the staionary phase, the substance being analysed  is dotted onto the plate and the plate placed in a beaker of a suitible solvent. The plate is placed with the bottom submerged into the solvent and the height that the different componenets in the mixture travel up to is recorded. 

Adsorbtion is the process by which the solid holds the different substances in the mixture onto its surface. Different substances have different adsorbtion to the substance on the TLC plate and so the chemicals in the mixture will be seperated. 

Interpretation of a TLC plate: This type of chromatogram is analysed by calculating the retention factor also called the R_f value for each component seperated. 

R_f = distance moved by component / distance moved by solvent front

R_f values canbe looked up on a data base of known values to determine the identities of a substance.

Important things to keep in mind when carrying out TLC chromatography:

Make sure to draw the base line in pencil around one centimetre from the edge of the plate. This is important as pen could dissolve and change the results. Make sure to dot the…