Chromatography

Question 1

What is chromatography used for?

Answer 1

To separate and identify the components in a mixture

Question 2

How is a mixture separated in chromatography, in terms of differences in affinities to the stationary phase?

Answer 2

If a component of the mixture has a high affinity to the stationary phase it will travel slowly as it will ‘stick to it’

If a component of the mixture has a low affinity to the stationary phase it will travel quickly

Question 3

How is a mixture separated in chromatography, in terms of differences in solubility in the mobile phase?

Answer 3

If a component of the mixture is highly soluble in the mobile phase it will travel along quickly with the solvent ( in a polar mobile phase a polar molecule will dissolve well in the solvent and travel further up)

If a component of the mixture has low solubility in the mobile phase it will travel slowly with the solvent

Question 4

What are the three types of chromatography?

Answer 4

Thin layer chromatography
Column chromatography
Gas chromatography

Question 5

Describe thin-layer chromatography (TLC)

Answer 5

A thin piece of aluminium or glass is coated in silica gel or aluminia which acts as the stationary phase. This is known as the TLC plate.
Samples of the mixture to be separated are dropped on a line on the bottom of the TLC plate.
The TLC plate is suspended in a small amount of solvent which soaks up the plate. The solvent is the mobile phase.
Once dry, the plate is examined under ultra-violet light, or developed using chemical reagents to make the spots visible.
The number of spots on the finished chromatogram gives the number of components present in the mixture

Question 6

What are Rf values and what can they be used for

Answer 6

Retention factor
Used to identify components

Question 7

Equation for Rf values

Answer 7

distance moved by component / distance moved by solvent front

Question 8

State 3 advantages of TLC

Answer 8

Faster than paper chromatography
Will work on very small samples
Can be used to determine when a chemical reaction is complete

Question 9

State 5 disadvantages of TLC

Answer 9

Similar compounds may have similar Rf values.
Conditions must be kept the same in order to fairly compare Rf values.
New and unknown compounds have no reference Rf values.
It can be difficult to find a solvent which separates all components in a mixture.
Cannot be used to separate large quantities.

Question 10

Describe column chromatography

Answer 10

Column chromatography uses a narrow glass tube with a spout and tap. This is called the column.
The column is packed with a powder such as silica or aluminia to act as a stationary phase.
The mixture to be separated is loaded onto the top of the column. A solvent is poured onto the top of the column and soaks down through the stationary phase. This solvent is the mobile phase.
The different components of the mixture travel down through the solid phase at different rates. One will reach the bottom and can be collected into a beaker first, and the next can then be collected into a separate beaker some time later.

Question 11

Advantage of column chromatography

Answer 11

Larger quantities can be separated

Question 12

What is gas-liquid chromatography used for?

Answer 12

To separate complex mixtures of volatile components

Question 13

Describe gas liquid chromatography

Answer 13

The mobile phase in GC is a gas, known as a carrier gas.
This carrier gas flows through a long and very thin tube. This is known as a capillary column.
The column has a thin coating of a liquid or a solid which acts as the stationary phase.
The sample to be separated is injected into the column under pressure at high temperature and the carrier gas transports the components of the mixture along the column at different rates causing them to separate out.
The components move into the detector at different times, an electrical signal is produces and a retention time is recorded for each component.
Components can be identified by comparing their retention times with known compounds.
This area of a peak in a gas chromatogram is proportional to the amount of a component in a mixture.

Question 14

What is a typical liquid stationary phase in GC

Answer 14

A high molecular mass alkane with a high boiling point
Silicon polymers can be used as a solid stationary phase

Question 15

What is the retention time?

Answer 15

Time taken from the point of injection for the component to reach the detector

Question 16

What are 3 limitation of GC?

Answer 16

Several chemicals have similar retention times, peak shapes and detector responses, so compounds cannot be positively identified as they held in a similar way to the stationary phase.

Not all substances in a sample may be separated and detected/peaks may be ‘hidden’ behind others.

Unknown compounds have no reference retention times.

Question 17

What is the advantage of linking a GC with a mass spectrometer(MS)?

Answer 17

GC can separate components in a mixture but cannot identify them conclusively
MS cannot separate components in a mixture but can identify them

Question 18

What does separation depend on (2)

Answer 18

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