# Gas and liquid chromatography

> Chemistry · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9701-u29-gas-and-liquid-chromatography/

This module covers core operating principles of GC and HPLC, phase properties, chromatogram interpretation, and exam-standard calculation workflows for CIE A-Level Chemistry Paper 4.

**Prerequisites:** [Basic principles of chromatographic separation](https://www.owlsprep.com/study/cie-9701-u29-intro-to-analytical-techniques/); [Kinetic and molecular theory of gases](https://www.owlsprep.com/study/cie-9701-u11-gas-laws-and-kinetic-theory/)

## Learning objectives

- Distinguish between stationary and mobile phases used in gas chromatography (GC) and high-performance liquid chromatography (HPLC)
- Interpret chromatograms to calculate retention time and use peak area for quantitative analysis of unknown mixtures
- Explain how differences in partition coefficients drive solute separation for both techniques
- Select the appropriate chromatography method for a given sample based on volatility and thermal stability

## Core Partitioning Principles of Separation

All forms of chromatography separate mixture components based on differences in how strongly each solute interacts with the stationary phase, relative to its solubility in the mobile phase. Solutes that bind more strongly to the stationary phase move more slowly through the system, while solutes that favour the mobile phase elute first.

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

*Notation:* K

$$K = \frac{[\text{Solute in stationary phase}]}{[\text{Solute in mobile phase}]}$$

> **tip**
>
> A larger K value means the solute spends more time bound to the stationary phase, so its retention time will be longer.

**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. First identify stationary phase properties: non-polar phases interact most strongly with non-polar solutes via London dispersion forces.
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. Solute with lower K elutes first: ethanol will travel faster through the column, followed by octane.
4. Final elution order: 1. Ethanol, 2. Octane

**Exam command terms**

CIE exam questions use specific command terms with strict marking requirements for this topic:

- **Explain the separation** — You must reference partition coefficient differences between solutes, not just state one moves faster

- **Compare GC and HPLC** — You must explicitly state one similarity and one difference to earn full marks

## Gas Chromatography (GC) Operation

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

**Check your understanding**

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

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

**Worked example:** A GC column is run at 250 °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. All three alkanes are non-polar, so their boiling point increases with carbon chain length: hexane (6C) < decane (10C) < dodecane (12C).
2. More volatile (lower boiling point) compounds spend more time in the gaseous mobile phase, so they elute faster.
3. Longer chain alkanes have higher boiling points, so they spend more time adsorbed on the non-polar stationary phase.
4. Final order (shortest to longest t_R): Hexane < Decane < Dodecane

## High Performance Liquid Chromatography (HPLC)

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.

**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.

> **mnemonic**
>
> Remember: RP = Non-Polar Stationary, so Polar solutes Run Past the stationary phase first to elute earliest.

- 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

**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. Reversed phase stationary phase is non-polar, mobile phase is polar.
2. Rank solutes by polarity: Glucose > Ethanol > Benzene.
3. Most polar solute favours the polar mobile phase most, so elutes first.
4. Final elution order: Glucose, Ethanol, Benzene.

## Chromatogram Interpretation and Quantitation

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.

**Derivation:** Calculate unknown solute concentration from peak area

*Starting from:* Peak area ∝ mass of solute injected

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

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

**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. Calculate response factor: 420 / 0.1 = 4200 units per mol dm⁻³.
2. Divide unknown peak area by response factor: 1050 / 4200 = 0.25.
3. Final concentration: 0.25 mol dm⁻³.

## Common pitfalls

- **Wrong:** Stating that Rf values are used to identify components in GC
  - Why it fails: Rf requires a measurable solvent front, which does not exist in closed GC columns
  - Correct: Use published or experimentally matched retention times t_R to identify GC components
- **Wrong:** Saying the mobile phase in GC is a liquid
  - Why it fails: GC mobile phase must be a gas to carry vapourised solutes through the column
  - Correct: Name helium or nitrogen as the inert gaseous mobile phase for GC
- **Wrong:** Assuming non-polar solutes elute first in reversed-phase HPLC
  - Why it fails: Non-polar solutes bind strongly to the non-polar C18 stationary phase, so they move slowly
  - Correct: Polar solutes elute first in reversed-phase HPLC
- **Wrong:** Forgetting to mention that peak area, not peak height, is proportional to solute concentration
  - Why it fails: Wider peaks have the same height but represent far more solute mass
  - Correct: Explicitly reference peak area when doing quantitative chromatogram calculations
- **Wrong:** Claiming GC can be used to separate large, thermally unstable proteins
  - Why it fails: GC requires heating samples to vaporisation, which will decompose heat-sensitive molecules
  - Correct: Use HPLC for thermally fragile, high molecular weight compounds

## Cheatsheet

| Parameter | Gas Chromatography | Reversed-Phase HPLC |
| --- | --- | --- |
| Mobile phase | Inert He/N₂ 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 |

## 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.

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