3.3.16 Chromatography

Question 1

What is chromatography used for?

Answer 1

Separating and identifying components in a mixture

Question 2

What does separation depend on?

Answer 2

The balance between solubility in the mobile phase and retention in the stationary phase

Question 3

What is the stationary phase in TLC?

Answer 3

Thin layer of silica or alumina fixed to glass or metal plate

Question 4

What is the mobile phase in TLC?

Answer 4

Liquid solvent such as ethanol

Question 5

How is a mixture separated using TLC? (5)

Answer 5

Draw pencil line near bottom of plate
Put a small drop of each mixture on the base line
Place the plate in a beaker containing the solvent (lower than base line) and cover the top with a lid
When the solvent has almost moved to the top of the plate, remove it from the beaker and mark how far the solvent moved
Leave it to dry in a fume cupboard to prevent fumes escaping into the room

Question 6

How can colourless chemicals be seen on a chromatogram? (2)

Answer 6

Use a fluorescent dye and UV light to find the spots

Expose the plate to iodine vapour which will stick to the chemicals and show up as purple spots

Question 7

What is the Rf value?

Answer 7

Distance travelled by spot / distance travelled by solvent

Question 8

What is the Rf value dependent on? (3)

Answer 8

Composition of TLC plate
Solvent
Temperature

Question 9

What is the stationary phase in column chromatography?

Answer 9

Solid absorbent material (slurry of aluminium oxide)

Question 10

What is the mobile phase in column chromatography?

Answer 10

Solvent

Question 11

What is column chromatography used for?

Answer 11

Purifying organic products

Question 12

How does column chromatography work?

Answer 12

A glass column is packed with the stationary phase
The mixture is added to the column, followed by the mobile phase which is run continuously through the column
The components of the mixture separate according to how soluble they are in the mobile phase and how strongly adsorbed onto the stationary phase they are
The more soluble a component is in the mobile phase, the faster it passes through the column
As a component reaches the bottom of the column it is collected

Question 13

What is retention time?

Answer 13

The time taken for a substance to pass through a chromatography column

Question 14

What is gas chromatography used for?

Answer 14

Separating a mixture of volatile liquids

Question 15

What is the stationary phase in gas chromatography?

Answer 15

A solid or a solid coated with a viscous liquid such as oil packed into a column

Question 16

What is the mobile phase in gas chromatography?

Answer 16

An unreactive carrier gas such as nitrogen

Question 17

How does gas chromatography work? (3)

Answer 17

The sample is vaporised and passes through a temperature-controlled oven as a gas
Each component takes a different amount of time to pass through the column
Requires pressure and high temperatures

Question 18

How is a substance identified from gas chromatography? (3)

Answer 18

A chromatogram shows peaks (components) at different retention times
The area under the peak corresponds to the amount of that component in the mixture
Under standard conditions each component has a unique retention time

Question 19

What is GC-MS? (3)

Answer 19

Gas chromatography-mass spectrometry
Separated components from chromatography are passed through a mass spectrometer to produce a mass spectrum for each peak
This method is better than just using retention times as similar compounds often have very similar retention times but will have a different mass spectrum