RP 12 - Thin-Layer Chromatography (TLC)

Question 1

What are the uses of thin layer or paper chromatography?

Answer 1

To separate a mixture into its constituent components for analysis. This allows identification by the calculation and comparison of Rf values

Question 2

How do you calculate an Rf value?

Answer 2

= Distance travelled by component / Distance travelled by solvent

Question 3

How do you carry out TLC?

Answer 3

1. Add solvent to a jar. Deal to create a saturated environment
2. Draw a pencil line on TLC plate. Spot samples along this line using a capillary tube
3. Add TLC plate to jar. Make sure solvent is below pencil line
4. The solvent rises up the TLC plate, bringing components of the samples with it
5. Draw a pencil line where the solvent finishes (don’t allow it to travel to the top of the plate). Allow to dry

Question 4

Why must any lines drawn on the TLC plate be in pencil?

Answer 4

Because if drawn in ink, this will dissolve in the solvent and run up the plate with the other components, contaminating the plate

Question 5

Why should the solvent be below the pencil line in TLC?

Answer 5

If above the pencil line, the solvent will dissolve all samples and they won’t run up the plate

Question 6

What happens if your sample is too concentrated?

Answer 6

The spots overlap

Question 7

In what scenario would spots overlap in TLC?

Answer 7

If the sample is too concentrated

Question 8

In TLC, what happens if you use less solvent and have a high baseline?

Answer 8

There would be large spots

Question 9

How do you view the spots on a TLC plate?

Answer 9

• Some spots may be visible, others may only be visible under a UV lamp/light
• Sometimes the plate is sprayed with a chemical/locating agent to show ‘invisible’ spots

Question 10

What does a diagram of TLC look like?

Answer 10

*There are 2 diagrams. The diagram on the left is a ‘TLC plate’ with a dotted line across the plate, 1-2 cm from the bottom end of the plate, labelled ‘Start line drawn in pencil’. There’s also a dotted line across the plate near the top of the plate labelled ‘Solvent Front’. There are also multiple dots between the 2 dotted lines labelled ‘Spots of substance’. The right diagram has this ‘TLC plate’ (but with all the dots on the start line) inside of a ‘Beaker’. There’s a liquid inside of this beaker and the plate is just about dipped into it; except the start line is still above the liquid. There’s also a ‘Watch glass’ that covers the top of the beaker