Chromatography

Question 1

Key Principles

Answer 1

• There is a solid substance called the ‘stationary phase’ which the mixture passes over in order to be separated.
• There mixture is carried over the solid material by a ‘mobile phase’. This is either a liquid or a gas.
• If a component of the mixture has a high affinity to the stationary phase it will travel slowly.
• If a component of the mixture has a low affinity to the stationary phase it will travel quickly.
• If a component of the mixture is highly soluble in the mobile phase it will travel along quickly with the solvent.
• If a component of the mixture has low solubility in the mobile phase it will travel slowly with the solvent.

Question 2

Thin-Layer Chromatography (TLC)

Answer 2

A thin piece of aluminium or glass is coated in silica gel or aluminia which acts as the stationary phase. (TLC 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 3

Calculating Rf values

Answer 3

Rf = distance moved by component / distance moved by solvent front

Question 4

Advantages of TLC

Answer 4

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

Question 5

Limitations of TLC

Answer 5

• Similar compounds may have similar Rf values.
• Conditions must be kept the came 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 6

Column Chromatography

Answer 6

Column chromatography uses a narrow glass tube with a spout and tap (like a burette). This is called the column.

The column is packed with a powder such as silica or aluminia to act as a 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.

The advantage of column chromatography is that larger quantities can be separated.

Question 7

The Polarity of the Stationary Phase and Mobile Phase

Answer 7

With mobile phase: ‘Like dissolves like’
With stationary phase: ‘Like sticks with like’

Question 8

5. Gas-liquid chromatography (GC)

Answer 8

used to separate complex mixtures of volatile components.

The mobile phase in GC is a gas, known as a carrier gas. (nitrogen or helium)

This carrier gas flows through a long and very thin tube. This is known as a capillary column which can be as long as 100m coiled up. The temp can be varied

The column is made of glass or metal, and has a thin coating of a liquid or a solid which acts as the stationary phase.

A typical liquid stationary phase in GC is a high molecular mass alkane with a high boiling point.
Silicone polymers can be used as a solid stationary phase in GC.

You can only explain separation in terms of affinity to the stationary phase.

record the retention time. This is the time it takes from the point of injection for the component to the detector.

When the components leave the column and reach the detector, an electrical signal is produced.

Question 9

GC retention times

Answer 9

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.

relative amounts of components in a mixture to be determined by comparing peak areas.

Question 10

Limitations of GC

Answer 10

• Potentially 1000’s of 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 11

GC-MS

Answer 11

linking a gas chromatograph (GC) with a mass spectrometer (MS)

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

Question 12

What does separation depend on?

Answer 12

Separation depends on the balance between solubility in the moving phase and retention by the stationary phase