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

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.

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.

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.

LIMITATIONS

• 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.