37.2 Gas/Liquid Chromatography

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe and understand the terms: stationary phase, mobile phase, and retention time
  2. Interpret gas/liquid chromatograms in terms of the percentage composition of a mixture
  3. Explain retention times in terms of interaction with the stationary phase

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What is Gas/Liquid Chromatography?

Gas/liquid chromatography (GLC) is a technique used to separate and identify the different substances in a mixture. It is especially useful when a mixture contains several components that are hard to tell apart by eye.

Think of it like a race — different substances in a mixture travel through a special tube at different speeds. Because they travel at different speeds, they arrive at the end of the tube at different times. This allows us to tell them apart and find out how much of each substance is present.

The Key Parts of the GLC System

To understand GLC, you need to know about two essential components: the stationary phase and the mobile phase.

Stationary Phase

The stationary phase is the phase that does not move. In GLC, the stationary phase is a high boiling point, non-polar liquid that is coated onto a solid support material inside a long, coiled tube called the column.

  • High boiling point — this means the liquid stays as a liquid (it does not evaporate) even when the column is heated to high temperatures during the experiment.
  • Non-polar — the liquid has no significant electrical charges across it, which affects how strongly different substances stick to it (more on this in the retention time section).
  • On a solid support — the liquid cannot just float around; it is held in place by being coated onto tiny solid particles packed inside the column. This solid support does not take part in any chemical reaction — it just holds the liquid in place.

Mobile Phase

The mobile phase is the phase that does move. In GLC, the mobile phase is an unreactive (inert) gas, such as nitrogen or helium.

  • Unreactive — the gas does not chemically react with the sample mixture or with the stationary phase. If it reacted, it would change the mixture being tested, giving false results.
  • This carrier gas is pumped continuously through the column at a steady flow rate.
  • The gas picks up the vaporised sample and carries it through the column.

How the System Works — A Simple Summary

  1. A small amount of the liquid mixture is injected into the machine at the injection port.
  2. The mixture is instantly vaporised (turned into gas) by the high temperature inside the machine.
  3. The carrier gas (mobile phase) sweeps the vaporised sample into and through the column.
  4. Inside the column, the different substances in the mixture interact with the stationary phase liquid to different extents.
  5. Substances that interact more strongly with the stationary phase move more slowly through the column.
  6. Substances that interact less strongly move more quickly.
  7. Each substance exits the column at a different time and is detected by a detector at the end.
  8. The detector sends a signal to a computer, which draws a chromatogram — a graph showing the results.

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