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By now, you will probably have a general idea of what chromatography is. You may have already encountered its origins and read about different chromatography techniques such as thin-layer chromatography and column chromatography.
This article is about another chromatography technique, gas chromatography, and how it can be used to separate samples in the gas phase.
Gas chromatography (GC) is a sensitive technique and is used for compounds that vaporise (turn from liquid to vapor) on heating without decomposing.
Gas chromatography, GC, is an analytical technique that analyses components of a sample in the gas phase.
Gas chromatography is also known as Gas-Liquid Partition Chromatography (GLPC).
This type of chromatography not only separates the chemicals in a sample but also gives a measure of how much of each is present. Therefore, it is useful in allowing chemists to analyse complex mixtures, both qualitatively and quantitatively.
How does gas chromatography work?
In gas chromatography, a column is packed with a solid or a solid coated with a viscous liquid. This is the stationary phase. The analyte solution is then vaporised and injected into the column. An unreactive gas such as helium acts as the mobile phase. It is passed through the column under pressure at a high temperature.
Vaporisation: the phenomenon of a liquid turning into vapour.
Analyte solution: the solution that we want to analyse.
Mobile phase: the fluid (liquid or gas) that flows through a chromatography system. It can also be called carrier gas.
The diagram is a visual representation of how gas chromatography works. Source: Wikimedia Commons
The steps for the process are as follows:
The sample injector is also known as the gas chromatography (GC) inlet.
Some gas chromatography instruments have capillary columns. This is a wire-like object on which the stationary phase is coated. It's usually made up of materials such as stainless steel, borosilicate glass, or fused silica. The column is coated with a polyamide on the outer surface. Most capillary columns are approximately 1.5 - 10 m long and 2-4 mm thick.
Capillary columns for gas chromatography, Chemeurope
Other gas chromatography columns are made of glass or steel tubes that are packed with powder. This powder is a non-reactive solid that has been coated with a thin film of a liquid with a high boiling temperature. The stationary phase in these types of columns is the liquid coating. Generally, this type of column is used in gas-liquid chromatography.
Each component in the sample absorbs into the stationary phase by a different amount. This means that each component takes a different amount of time from when it is injected to when it is recorded on the other end. This is called the 'retention time' and is used to identify the components. The separation of the components depends on the balance between solubility in the mobile phase and retention in the stationary phase.
The retention time in gas chromatography is the time taken by a compound in a mixture to pass through the chromatography column and reach the detector.
GC-MS chromatogram of AGE. GC-MS, gas chromatography-mass spectrometry; AGE, aged garlic extract. Source: www.researchgate.net
A specific property, such as the thermal conductivity of gases leaving the column (in the case of the thermal conductivity detector, TCD) is measured by a detector on the other end of the column. A recorder then gives a peak on a plot showing the retention time. The area under each peak on the plot represents the relative quantities of each component.
If we combine gas chromatography with mass spectrometry, we produce a very powerful system that allows us to identify, separate, and measure complex mixtures of chemicals.
Gas chromatography is good at separating mixtures into their components. However, it is of no use when it comes to identifying these components. On the other hand, mass spectrometry is a technique used to identify substances according to their mass/charge ratio. Unknown compounds can easily be identified thanks to mass spectrometry. Therefore, the advantages of GC and MS are combined within GC-MS in order to make a very useful analysis tool.
The key features of a GC-MS system are as follows:
Some of the main advantages of gas chromatography are listed below.
There are a number of different detectors apart from the ones mentioned in the article, which are used with gas chromatography. Some of these include:
Gas chromatography is an analytical technique that analyses components of a sample in the gas phase.
In gas chromatography, a column is packed with a solid or a solid coated with a viscous liquid. This is the stationary phase. The analyte solution is then vaporised and injected into the column. An unreactive gas such as helium acts as the mobile phase. It is passed through the column under pressure at a high temperature. The components in the mixture separate as they pass through the column. They then pass into a detector which produces a chromatogram.
The basic principle of gas chromatography is that each component in the sample absorbs into the stationary phase by a different amount. This leads to different retention times for different components. The separation of the components depends on the balance between solubility in the mobile phase and retention in the stationary phase.
Temperature affects gas chromatography since it affects the retention time of the analyses, the pressure of the column, as well as the shape of the peaks which appear on the chromatogram.
Gas chromatography separates components in a gaseous sample and gives us the quantities of each analyte present. Therefore, it is useful since it allows chemists to analyse complex mixtures, both qualitatively and quantitatively.
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