Study Guide

Gas and liquid chromatography

ChemistryΒ· 9701 syllabus section 29.2Β· 12 min read

1. Core Partitioning Principles of Separationβ˜…β˜…β˜†β˜†β˜†β± 10 min

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πŸ“˜ Definition

Partition Coefficient

KK

The ratio of the concentration of a solute in the stationary phase to its concentration in the mobile phase at dynamic equilibrium.

K=[Solute in stationary phase][Solute in mobile phase]K = \frac{[\text{Solute in stationary phase}]}{[\text{Solute in mobile phase}]}
πŸ“ Worked Example

Predict the relative elution order of a mixture of ethanol (polar, low boiling point) and octane (non-polar, high boiling point) on a non-polar stationary phase.

  1. 1

    First identify stationary phase properties: non-polar phases interact most strongly with non-polar solutes via London dispersion forces.

  2. 2

    Compare the two solutes: octane is non-polar, so it will have a higher partition coefficient K for the stationary phase than polar ethanol.

  3. 3

    Solute with lower K elutes first: ethanol will travel faster through the column, followed by octane.

  4. 4

    Final elution order: 1. Ethanol, 2. Octane

2. Gas Chromatography (GC) Operationβ˜…β˜…β˜…β˜†β˜†β± 12 min

Gas chromatography uses an inert carrier gas (usually helium or nitrogen) as the mobile phase. The stationary phase is a high-boiling point viscous liquid coated onto the inside of a long, narrow capillary column, or packed onto small solid particles. The column is housed in a temperature-controlled oven that can be heated gradually to adjust solute volatility.

  • Sample is injected as a liquid, and immediately vaporised in the heated injection port

  • Vapourised sample is carried into the column by the inert mobile phase gas

  • Solute components separate as they interact with the liquid stationary phase

  • Eluted components pass through a flame ionisation detector that generates a peak on the chromatogram

βœ“ Quick check

Test your understanding of GC operating parameters:

  1. Which of the following is a valid mobile phase for GC?

    • A) Liquid ethanol

    • B) Helium gas

    • C) Aqueous buffer solution

    • D) Silica gel

    Reveal answer
    B β€”

    Helium is inert, low molecular weight, and will not react with or interact with solute molecules during separation.

πŸ“ Worked Example

A GC column is run at 250 Β°C, and a mixture of hexane, decane and dodecane is injected. Rank the three alkanes by their expected retention time, from shortest to longest.

  1. 1

    All three alkanes are non-polar, so their boiling point increases with carbon chain length: hexane (6C) < decane (10C) < dodecane (12C).

  2. 2

    More volatile (lower boiling point) compounds spend more time in the gaseous mobile phase, so they elute faster.

  3. 3

    Longer chain alkanes have higher boiling points, so they spend more time adsorbed on the non-polar stationary phase.

  4. 4

    Final order (shortest to longest t_R): Hexane < Decane < Dodecane

3. High Performance Liquid Chromatography (HPLC)β˜…β˜…β˜…β˜†β˜†β± 12 min

HPLC uses a liquid mobile phase that is pumped at very high pressure through a tightly packed column of fine solid particles coated with the stationary phase. Unlike GC, the entire system operates at room temperature, making it ideal for thermally fragile compounds that would decompose when heated to vaporisation temperatures.

πŸ“˜ Definition

Reversed-phase HPLC

The most common HPLC separation mode, where the stationary phase is non-polar (C18 hydrocarbon coated silica) and the mobile phase is a polar organic solvent mixture.

  • Reversed-phase HPLC mobile phase is usually a mixture of methanol and water

  • Polar solutes elute first, as they prefer the polar mobile phase

  • Non-polar solutes bind strongly to the C18 stationary phase, so they elute later

πŸ“ Worked Example

A reversed-phase HPLC system uses a C18 stationary phase and methanol-water mobile phase. A mixture of glucose (very polar), benzene (non-polar) and ethanol (moderately polar) is injected. What is the elution order?

  1. 1

    Reversed phase stationary phase is non-polar, mobile phase is polar.

  2. 2

    Rank solutes by polarity: Glucose > Ethanol > Benzene.

  3. 3

    Most polar solute favours the polar mobile phase most, so elutes first.

  4. 4

    Final elution order: Glucose, Ethanol, Benzene.

4. Chromatogram Interpretation and Quantitationβ˜…β˜…β˜…β˜…β˜†β± 10 min

A chromatogram plots detector response (y-axis) against retention time (x-axis). Each peak corresponds to one separated component. The area under each peak is directly proportional to the mass or concentration of that component in the original injected sample.

πŸ”¬ Derivation
Goal:

Calculate unknown solute concentration from peak area

Starting from:

Peak area ∝ mass of solute injected

  1. 1
    1. Run a standard reference sample of known concentration of the target solute, measure its peak area.
  2. 2
    1. Calculate the response factor: response factor = peak area of standard / concentration of standard.
  3. 3
    1. Inject the unknown sample, measure the peak area for the target solute.
  4. 4
    1. Rearrange to find unknown concentration: concentration_unknown = peak_area_unknown / response_factor.
Result:

This calibration method eliminates errors from injection volume variation and detector drift.

πŸ“ Worked Example

A 0.1 mol dm⁻³ standard of ethanol gives a GC peak area of 420 arbitrary units. An unknown ethanol sample gives a peak area of 1050 units. Calculate the concentration of ethanol in the unknown.

  1. 1

    Calculate response factor: 420 / 0.1 = 4200 units per mol dm⁻³.

  2. 2

    Divide unknown peak area by response factor: 1050 / 4200 = 0.25.

  3. 3

    Final concentration: 0.25 mol dm⁻³.

5. Common Pitfalls

Wrong move:

Stating that Rf values are used to identify components in GC

Why:

Rf requires a measurable solvent front, which does not exist in closed GC columns

Correct move:

Use published or experimentally matched retention times t_R to identify GC components

Wrong move:

Saying the mobile phase in GC is a liquid

Why:

GC mobile phase must be a gas to carry vapourised solutes through the column

Correct move:

Name helium or nitrogen as the inert gaseous mobile phase for GC

Wrong move:

Assuming non-polar solutes elute first in reversed-phase HPLC

Why:

Non-polar solutes bind strongly to the non-polar C18 stationary phase, so they move slowly

Correct move:

Polar solutes elute first in reversed-phase HPLC

Wrong move:

Forgetting to mention that peak area, not peak height, is proportional to solute concentration

Why:

Wider peaks have the same height but represent far more solute mass

Correct move:

Explicitly reference peak area when doing quantitative chromatogram calculations

Wrong move:

Claiming GC can be used to separate large, thermally unstable proteins

Why:

GC requires heating samples to vaporisation, which will decompose heat-sensitive molecules

Correct move:

Use HPLC for thermally fragile, high molecular weight compounds

6. Quick Reference Cheatsheet

Parameter

Gas Chromatography

Reversed-Phase HPLC

Mobile phase

Inert He/Nβ‚‚ gas

Polar methanol/water liquid

Stationary phase

Non-polar high-boiling liquid

Non-polar C18 silica

Best for

Volatile, thermally stable small molecules

Polar, non-volatile, heat-sensitive molecules

Identification metric

Retention time t_R

Retention time t_R

Quantitation

Peak area proportional to mass

Peak area proportional to mass

7. Frequently Asked

Can I use Rf values directly for gas chromatography?

No. Rf values are designed for planar chromatography (TLC, paper) where a visible solvent front travels across a flat surface. For GC and HPLC, you use retention time (t_R) instead, as the mobile phase flows through a closed column with no observable front.

Why is HPLC preferred over GC for large biological molecules?

Large, polar or thermally unstable molecules (like proteins and prescription pharmaceuticals) decompose at the high temperatures required to vaporise samples for GC. HPLC operates at room temperature with liquid mobile phases, so these compounds remain intact during separation.

When this came up on past exams

AI-estimated based on syllabus patterns β€” cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2023 Β· 42

    Explain HPLC separation principles

  • 2022 Β· 41

    Compare GC mobile phase properties

  • 2021 Β· 43

    Interpret GC chromatogram for mixture analysis

Going deeper

  • videoCIE A-Level Chromatography WalkthroughStep-by-step demo of GC and HPLC lab setup
  • worksheetChromatogram Interpretation Practice10 exam-style questions with marking schemes

What's Next

Mastering GC and HPLC is critical for scoring full marks on the analytical techniques section of CIE A-Level Paper 4, which often carries 10-15% of total exam marks. You will next build on these separation principles to explore mass spectrometry coupling, where eluted components from GC or HPLC are fed directly into a mass spectrometer for definitive structural identification of unknown compounds. You will also practice full chromatogram analysis workflows, including calibration curve construction and error analysis for real-world experimental data, to ensure you can answer any exam question on this topic with confidence.