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9701 · 37.2

Gas/liquid chromatography — practice questions

Practice and worked examples for 9701 Gas/liquid chromatography. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

A mixture containing several straight-chain alkanes is analysed by GLC. The chromatogram shows three major peaks with retention times of 2.8 min, 4.5 min, and 6.9 min. Separate analyses of pure hexane, heptane, and octane under identical conditions gave retention times of 4.5 min, 6.9 min, and 9.8 min, respectively. Identify the components present in the mixture.

Show solution outline
  1. Compare retention times: The retention time of a compound is a characteristic property under specific GLC conditions.
  2. Identify Peak 1 (2.8 min): This retention time does not match any of the provided standards (hexane, heptane, octane). It is an unknown component, likely an alkane with fewer than 6 carbon atoms (e.g., pentane), as it has a shorter retention time.
  3. Identify Peak 2 (4.5 min): This retention time matches that of the pure hexane standard (4.5 min). Therefore, the mixture contains hexane.
  4. Identify Peak 3 (6.9 min): This retention time matches that of the pure heptane standard (6.9 min). Therefore, the mixture contains heptane.
  5. Conclusion: The mixture contains hexane, heptane, and at least one other unknown, more volatile component. Octane is not present in the mixture as there is no peak at 9.8 min.

Worked example 2

The concentration of caffeine in a soft drink was determined using GLC with an internal standard. A series of standard caffeine solutions were prepared, each containing a constant concentration of an internal standard. The ratio of the caffeine peak area to the internal standard peak area was calculated. The results were:

  • 10 mg dm^{-3} caffeine: Peak Area Ratio = 0.52
  • 20 mg dm^{-3} caffeine: Peak Area Ratio = 1.05
  • 30 mg dm^{-3} caffeine: Peak Area Ratio = 1.56
  • 40 mg dm^{-3} caffeine: Peak Area Ratio = 2.10

The soft drink sample, with the same amount of internal standard added, gave a Peak Area Ratio of 1.35. Determine the concentration of caffeine in the drink.

Show solution outline
  1. Plot a calibration curve: Plot Peak Area Ratio (y-axis) against Caffeine Concentration (mg dm^{-3}) (x-axis).
  2. Draw the line of best fit: The points should form a straight line that passes through the origin (as a concentration of 0 should give a ratio of 0).
  3. Determine the concentration of the unknown: Locate the Peak Area Ratio of 1.35 on the y-axis of the graph. Draw a horizontal line to the calibration line, and then a vertical line down to the x-axis to read the concentration.
  4. Calculation (optional, for checking): The gradient of the line is approximately (2.10 - 0.52) / (40 - 10) = 1.58 / 30 ≈ 0.0526 (mg dm^{-3})^{-1}. Concentration = Peak Area Ratio / Gradient = 1.35 / 0.0526 ≈ 25.66 mg dm^{-3}.
  5. Final Answer: From the graph or calculation, the concentration of caffeine in the soft drink is approximately 26 mg dm^{-3} (to 2 significant figures).
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