Chromatography

Question 1

Outline the process of thin layer chromatography

Answer 1

Take a sheet/plate of glass, plastic or metal.

Coat it with a thin layer of silica gel or alumina.

Draw a pencil line near to the bottom.

Place a small drop of the sample on this line.

Place the plate in a beaker containing solvent, but not enough to reach the pencil line.

Cover the beaker.

Question 2

What is the moving phase in chromatography?

Answer 2

Whatever moves the components e.g. the solvent

Question 3

What is the stationary phase in chromatography?

Answer 3

Whatever the solvent has to move through e.g. silica gel

Question 4

What happens if a component and the stationary phase e.g. silica gel are both polar?

Answer 4

The component adsorbs strongly to the stationary phase.

As a result, retention of this component by the stationary phase is high.

Question 5

What happens if a component e.g. ethanol forms hydrogen bonds with the stationary phase e.g. silica gel?

Answer 5

Ethanol adsorbs strongly to the stationary phase.

As a result, retention of ethanol by the stationary phase is high.

Question 6

What 2 factors affect movement up the plate in TLC?

Answer 6

1) retention by stationary phase
2) solubility in solvent (moving phase)

Question 7

Rf equation

Answer 7

Distance moved by substance / Distance moved by solvent

Question 8

Name two methods which can be used to see components on chromatogram which don’t show up under normal circumstances.

Answer 8

1) Developing agent
2) UV light

Question 9

How can we use TLC to monitor the progress of reactions?

Answer 9

• Take samples of reaction mixture at different times.
• Run TLC.
• Analyse how number and position of the dots changes over time.

Question 10

What is the moving phase in gas chromatography?

Answer 10

Moving phase is an unreactive gas e.g. nitrogen. Substances do not dissolve in gas.

Question 11

What is the stationary phase in gas chromatography?

Answer 11

Stationary phase is powder/powder coated in oil which coats inside of tube.

Question 12

Outline process of gas chromatography

Answer 12

• Sample injected into long, thin tube.
• Tube in oven, which is heated to a specific temperature.
• Sample moves through tube.
• Detector detects when each component reaches the end of the tube - creates gas chromatagram.
• Retention time values compared to table of known data.

Question 13

What are the X and Y axis in a gas chromagram?

Answer 13

X axis = time
Y axis = amount of sample detected by detector at any one time

Question 14

If there were 4 peaks in a gas chromatagram, how many components were in the mixture?

Answer 14

4

Question 15

How can you tell which component is most abundant in gas chromatagram?

Answer 15

Tallest/thickest peak.
Greatest area.

Question 16

What do we measure instead of Rf value in gas chromatography?

Answer 16

Time it took each component to travel from start to end of tube.
(How long each component is held back by stationary phase)

Question 17

If we had a non-polar stationary phase, would a non-polar or polar substance have a longer retention time?

Answer 17

non-polar substance

Question 18

Why can’t we measure Rf values in gas chromatography?

Answer 18

All components reach end of the tube.

Question 19

What is GCMS?

Answer 19

Gas chromatography mass spectrometry.
Instead of just finding out a substance’s retention time, the sample passes out of the tube and into a mass spectrometer.
Also shows fragmentation pattern.