Gas chromatography, GC 80-81

Question 1

What is gas chromatography in a sentence?

Answer 1

Gas chromatography is a technique used to separate volatile components in a mixture.

Question 2

What is GC particularly useful for?

Answer 2

It is useful for many organic compounds that have a low boiling point and evaporate easily.

Question 3

Describe the setup of the gas chromatograph.

Answer 3

• The chromatograph consists of some capillary tubing. The tubing may be up to 30m long and is wound into a coil so that it fits inside a thermostatically controlled oven.
• The liquid for the stationary phase is often a long-chain alkane with a high boiling point. Suitable solid stationary phases include silicone polymers.
• The mobile phase is a carrier gas which moves through the column. An inert or unreactive gas such as helium or nitrogen is usually used.

Question 4

Explain the steps involved in producing a gas chromatogram.

Answer 4

• The mixture is injected into the gas chromatograph, where it is vaporised.
• The mobile carrier gas flushes the mixture through the column.
• As the mixture moves through the column, the components slow down as they interact with the stationary phase lining of the column. The greater the solubility or adsorption, the more the components are slowed down.
• Each component leaves the column at a different time and is detected as it leaves the column. A computer processes the results to display a gas chromatogram.

Question 5

What is the retention time?

Answer 5

The time for a component to pass from the column inlet to the detector.

Question 6

What does a typical gas chromatogram show?

Answer 6

• The x-axis show the retention time.
• The y-axis shows the absorption of the components
• The area under the peaks is proportional to the amount of a compound in the sample.

Question 7

What are the limitations of gas chromatography?

Answer 7

• Potentially, thousands of chemicals may have the same retention time, peaks shape and detector response.
• Not all substances in a sample will necessarily be separated and detected. A small amount of a substance can ‘hide’ beneath another substance that has a higher concentration and the same retention time.
• Unknown compounds have no reference retention times for comparisons. To make results meaningful, analysts need to know which components are to be expected.