Gas Chromatography, GC (OCR A Level Chemistry)

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Interpreting Gas Chromatograms

  • 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

Analytical Techniques - GLC Chromatogram, downloadable AS & A Level Chemistry revision notes

 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

Analysis of a compound by GLC shows the presence of four components, A, B, C and D.GLC Worked example chromatogram, downloadable AS & A Level Chemistry revision notes

i) Which compound is present in the greatest quantity?

ii) Which compounds were present in equal amounts?

iii) Which compound had the strongest interaction with the stationary phase?

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)

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Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.