Gas chromatography
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Explainer Video
Summary
Gas chromatography
In a nutshell
Gas chromatography is a technique which can be used to identify unknown chemicals in a sample. The retention time of chemicals can be compared to retention times on databases to identify chemicals.
Uses
Gas chromatography is a technique which can be used to separate complex mixtures, identify unknown chemicals and determine how much of a chemical is present.
Example
Gas chromatography is used in sports; it is used to detect prohibited substances in urine samples.
Substances which are not already in the gas phase need to be vaporised.
Gas chromatography is made up of a mobile phase which is an inert gas and a stationary phase which is an inert liquid.
Examples
Nitrogen can be used as a mobile phase because it is unreactive.
Oil can be used as a stationary phase.
Technique
procedure
| 1. | The gas chromatography column is coated with the stationary phase. |
| 2. | The unknown gas/vaporised sample is injected into the column along with the mobile phase. |
| 3. | The unknown sample and the mobile phase pass through the column together. |
| 4. | Different substances will travel down the tube at different speeds depending on how well they dissolve in the mobile phase and how much they interact with the stationary phase. Substances which interact with the stationary phase more take more time to pass through the column and reach the detector. |
| 5. | The amount of time it takes for a given substance to travel through the tube and reach the detector is the retention time. Each substance has a different retention time, which makes it possible to identify unknown chemicals. |
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Chromatograms
Gas chromatography produces a chromatogram in the form of a graph.
Each peak on the graph shows a different chemical with a different retention time. If there is only a single peak on the graph a pure substance is present.
On the -axis of the graph is the retention time. The retention time of the peak can be compared with standard reference data to find out what the unknown substance is.
On the -axis of the graph is the strength of the response. The areas under the peaks show the relative amounts of each chemical present in the sample.
Example
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In this sample, substance A has the smallest peak therefore there is relatively little present.
Equal amounts of substances B and C were present in the sample.
Substance D has the highest peak therefore there is a relatively large quantity present in the sample.
Substance F has the longest retention time which means that it reached the detector last. The substance with the longest retention time has the highest affinity to the stationary phase and the lowest affinity to the mobile phase.
FAQs - Frequently Asked Questions
In gas chromatography, what does the height of the peaks in the chromatogram show?
On the y-axis of the graph is the strength of the response . The areas under the peaks show the relative amounts of each chemical present in the sample.
How does gas chromatography separate chemicals in a mixture?
Different substances will travel down the tube at different speeds depending on how well they dissolve in the mobile phase and how they interact with the stationary phase. Substances that interact with the stationary phase take more time to pass through the column and reach the detector.
Why is gas chromatography useful?
Gas chromatography is a technique which can be used to separate complex mixtures, identify unknown chemicals and determine how much of a chemical is present.