Chromatography

Question 1

Suggest why it was necessary to use two different solvents. [1]

Answer 1

Some of the amino acids didn’t dissolve with the first solvent

Question 2

How can you suggest which compounds move more quickly through the column?

Answer 2

• If moving phase is non-polar e.g hexane: non-polar compounds pass through more quickly
• If moving phase is polar e.g ethanol: polar compounds pass through quickly
• It’s all about solubility and IMF.

Question 3

TLC: Why do we dry in a fume cupboard?

Answer 3

Toxic solvent

Question 4

TLC: Why do we care about the depth of solvent?

Answer 4

if solvent is too deep, it dissolves the sample spots from the plate

Question 5

TLC: Why do we use a lid?

Answer 5

Prevents evaporation of toxic solvent

Question 6

TLC: Why do we wear gloves?

Answer 6

Prevent contamination

Question 7

TLC: Why do we use a pencil line?

Answer 7

will not dissolve in the solvent

Question 8

Recall how to carry out TLC.

Answer 8

• Draw pencil line
• Use capillary tube to add a tiny drop of each
• Add solvent to a chamber
• Place TLC into chamber; ensuring solvent is below pencil line
• Leave for time; then remove and mark solvent front with pencil
• Dry in a fume cupboard

Question 9

What two properties can be measured to identify substances in chromatography?

Answer 9

• Retention times: how long each component takes from being injected to recorded
• Rf values: Distance moved by sample / distance moved by solvent
• In both instances, they are compared with standards from a data book

Question 10

Recall Gas Chromatography (GC).

Answer 10

• Used to separate volatile liquids
• SP: Column packed with a solid or viscious liquid
• MP: Inert gas e.g N2passed through under pressure at high temperature
• Retention time measured

Question 11

Recall Thin Layer Chromatography (TLC).

Answer 11

• Used to separate and identify components in a mixture
• Stationary Phase: Plate coated with a solid
• Mobile Phase: Solvent moving up the plate

Question 12

Recall Column Chromatography (CC).

Answer 12

• Used to separate an organic product from impurities.
• Column is packed with a solid (Al2O3) and a solvent moves down the column
• More soluble components move down column faster.

Question 13

What does the movement of a substance in chromatography depend on?

Answer 13

A balance between:
- Solubility in solvent
- Retention in the stationary phase

Question 14

What are the two phases in a chromatography set up? What do they normally consist of?

Answer 14

• Stationary phase - solid/liquid held in a solid
• Mobile phase - liquid or gas

Question 15

State in general terms what determines the distance travelled by a spot in TLC. [1]

Answer 15

Balance between solubility in moving phase and retention by stationary phase

Question 16

What is the use of chromatography?

Answer 16

To separate and identify the components in a mixture