O2: 33 Chromatography

Question 1

What can chromatography be used for?

Answer 1

To separate and identify the components in a mixture.

Question 2

Describe thin-layer chromatography.

In general

Answer 2

A plate is coated with a solid and a solvent moves up the plate.

Question 3

Describe column chromatography.

In general

Answer 3

A column is packed with a solid and a solvent moves down the column.

Question 4

Describe gas chromatography.

In general

Answer 4

A column is packed with a solid or with a solid coated by a liquid, and a gas is passed through the column under pressure at high temperature.

Question 5

What does separation in chromatography depend on?

Answer 5

The balance between solubility in the moving phase and retention by the stationary phase.

Question 6

What are retention times and Rf values used for?

Answer 6

To identify different substances.

Question 7

What can mass spectrometry be used for in chromatography?

Answer 7

To analyse the components separated by gas chromatography.

Question 8

What is the process of thin-layer chromatography?

Answer 8

1. Draw a pencil line near the bottom of the TLC plate.
2. Put a small drop of each mixture on the line.
3. Place the plate in a beaker with solvent inside. The solvent level must be below the pencil line, so it doesn’t dissolve the samples.
4. Leave the plate until the solvent has almost moved to the top of the plate.
5. Remove the plate and, before the solvent evaporates, draw a pencil line at the solvent front.
6. Place the plate in a fume cupboard to dry.
7. Use UV light to make colourless compounds visible.

Question 9

What is the equation for Rf value?

Answer 9

Rf = distance travelled by spot / distance travelled by solvent