Thin layer chromatography is a technique used to analyse mixtures and separate them into their component parts

The video below shows one technique for thin layer chromatography (TLC) using a beaker and a watch glass lid. Sometimes a bit of filter paper on the bottom of the beaker can help to prevent the TLC plate from slipping.

The following video from Capilano University offers a more in-depth overview of TLC and uses a developing tank rather than a beaker. Jump to 5:36 to see a visualisation of the TLC plate under a UV-lamp and 7:18 for Rf calculations. You will notice that the ‘starting’ line where the compounds are spotted is called the origin or base line.

You could ask the following questions to probe your students understanding:

  • Why is it important that the beaker’s atmosphere is saturated with solvent vapour?
  • What steps can you take to minimise contamination on the TLC plate?
  • Why is it important that the solvent line is below the origin or base line?
  • Why do some compounds travel further up the plate than others?      

This video from Eastern Kentucky University is a further demonstration of using TLC and Rf values to compare the identity of an unknown against a range of known compounds.

Visualising the spots can be achieved in a number of ways, and you will most commonly see UV light being used. Alternative techniques include using stains – such as potassium permanganate – that react with certain functional groups to produce a colour change or exposing the TLC plate to iodine vapour. The following short video from  ChemistryConnected may not look the most modern, but it helpfully highlights a range of visualisation techniques.

Questions to probe your students understanding:

  • What functional groups might be visualised by exposure to iodine vapours?
  • What functional groups might be visualised using potassium permanganate stain?

The concepts behind TLC are just as applicable to understanding column chromatography and gas chromatography, despite the more complex technology, as shown in the following two videos.

The following questions will help probe your students understanding:

  • What are the mobile phases in column and gas chromatography respectively?
  • What are the stationary phases in column and gas chromatography respectively?

In this final video from Eastern Kentucky University, b-carotene is extracted from spinach. Note that TLC is still needed to identify the b-carotene in the extracted fractions. This is a helpful demonstration that two chromatography techniques can work together in a complementary fashion. It’s important for students to take note that a pure product will give a single spot on the TLC plate.

Also check out…

  • Practical chromatography – this CPD article includes ideas for enhancing practical chromatography and a curriculum overview.
  • TLC and aspirin analysis – the classroom organic synthesis of aspirin is an experiment that brings together many required practical skills, and TLC is a useful tool for analysing the purity of the final product. A pure substance will give one ‘spot’ on the TLC plate.
  • Paracetamol ­– this experiment encourages students to monitor the progress of a reaction using TLC.