Gas Chromatography

Simple Gas Chromatography :)

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  • Created by: Rosie
  • Created on: 11-01-12 17:55

What is Gas Chromatography?

What is Gas Chromatography?

  • Seperates volatile componants in a mixture.
  • Useful for organic compounds with a low boiling point & evapourate easily.
  • Takes place in a gas chromatograph.
  • Measures retention time.
  • Takes place in a controlled oven.

Retention time: the time for a component to pass from the column inlet to the detector

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Stationary and Mobile Phases:

Stationary and Mobile Phases:

Stationary Phase:

  • Thin layer of liquid or solid coated inside the capillary tubing.
  • The liquid used in gas chromatography is usually a long-chained alkane because they have a high boiling point and wont evaporate.
  • The solid used in gas chromatography include silicone polymers.
  • The stationary phase depends on what compound is being tested.

Mobile Phase:

  • A carrier gas which moves through the column.
  • The gas is unreactive such as helium or nitrogen so the componants of the mixture being tested in gas chromatography wont react with the mobile phase.
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How is the Chromatogram Produced?

How is the Chromatogram Produced?

  • The testing mixture is injected into the gas chromatogram (it is vapourised here)
  • mobile carrier (remember helium or nitrogen gas) flushes the mixture through the column.
  • As the mixture moves through the column it slows down as it interacts with the stationary phase.
  • If the stationary phase is solid, the components may adsorb onto the surface.
  • If the stationary phase is liquid, the components may dissolve.
  • The greater the solubility(liquid) or adsorption(solid) the more the individual components are slowed down.
  • Because different components of a compound have different solubility or adsorptions, they are seperated.
  • Each component therefore leaves the column at different times and once they leave they're detected.
  • A computer processes the results to display a gas chromatogram.
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Adsorption : the process by which a solid holds molecules of gas or liquid or solute as a thin film of a solid.

Must use aDsorption otherwise marks could be lost for using aBsorption.

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Limitations of Gas Chromatography


  • ALOT of chemicals (e.g. 1000's) have the same retention time, peak shape and detector results :( therefore GC cannot positively identify most components.
  • Not all substances will be seperated and detected. A small amount of substance could "hide" behind one with a higher concentration and the same retention time.
  • Unknown compounds have no retention times for comparisons.
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