6.9.2 Gas Chromatography, GC

• 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
  • Normally a non-volatile liquid is the stationary phase in GLC

• 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

Worked Example

Answers:

i) D (the larger the relative size of the peak, the greater the quantity of that substance present)

ii) B and C (the peak sizes are equal)

iii) D (the larger the retention time, the greater the interaction of that component with the stationary phase)