Gas Chromatography (Oxford AQA International A Level Chemistry)

Gas Chromatography

• Gas-Liquid Chromatography (GLC) is used for analysing:

  • Gases

  • Volatile liquids

  • Solids in their vapour form

• The stationary phase:

  • This method uses a long coiled column for the stationary phase

  • The column is typically packed with a solid or a solid coated with a non-volatile liquid

• The Mobile phase

  • An inert carrier gas (e.g. helium, nitrogen) moves the sample molecules through the stationary phase

• The sample is injected into the column through a self-sealing disc and the vapour formed is carried through the stationary phase using the inert-gas mobile phase

Retention times

• Once sample molecules reach the detector, their retention times are recorded

  • This is the time taken for a component to travel through the column

  • It depends upon the attraction between the solute and the stationary and mobile phases as well as the volatility and nature of the solute

• The retention times are recorded on a chromatogram where each peak represents a volatile compound in the analysed sample

  • The relative sizes (i.e. areas) of the peaks are related to how much of each compound is present in the mixture

• Retention times are then compared with data book values to identify unknown molecules

 A gas chromatogram of a volatile sample compound has six peaks. Depending on each molecule’s interaction with the stationary phase, each peak has its own retention time

Gas chromarographt and mass spectrometry (GC-MS)

• Gas chromatography is used to separate small amounts of components from a mixture

• It is often used to:

  • Provide forensic evidence

  • Drug testing, particularly in sports

  • Analysis of environmental pollution

  • Detecting explosives in baggage

• However, it is not very useful at identifying specific compounds because: 

  • Different compounds may have the same retention time

  • It can be difficult to manage the conditions, e.g. temperature and pressure

  • Some unknown compounds may not have a reference for comparison in the databases

• For this reason, gas chromatography is often coupled with mass spectrometry

  • This results in GC-MS

  • This means that components can be separated from mixtures and then analysed all within one machine