Gas Chromatography

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.

• Firstly, we will have a look at what gas chromatography is and define it.
• After that, we will explore how chromatography works and state some key facts.
• Then we will look at how gas chromatography and mass spectrometry can combine.
• Finally, we will outline some key advantages of gas chromatography.

What is gas chromatography?

Gas chromatography (GC) is a sensitive technique and is used for compounds that vaporise (turn from liquid to vapor) on heating without decomposing.

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.

The diagram is a visual representation of how gas chromatography works. Source: Wikimedia Commons

The steps for the process are as follows:

1. The sample enters the oven when it is injected into the carrier gas steam, which carries it through a coiled column coated with a viscous liquid. Volatile solids (solutes) are dissolved in a solvent before injection.
2. Once the sample is injected into the sample injector, the chemical components of the sample are vaporised - that's if they're not already in the gas phase! Therefore, the chemicals in the sample turn into gases and mix with the carrier gas.
3. The gases then pass through the column. The components in the mixture separate as they pass through the column.
4. They pass into a detector which sends a signal to a recorder as each component appears. After a while, the chemicals release energy in the form of signals which are sent to the detector.
5. A chromatogram is produced through a series of peaks, one for each component in the mixture. This is then analysed by chemists.

Retention time

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.

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.

Gas chromatography (GC) and mass spectrometry (MS)

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:

• The sample is injected.
• The chemicals in the mixture separate at the GC column and exit it one by one.
• The separated components are sent into a mass spectrometer instead of a detector.
• A detailed mass spectrum is produced by the spectrometer for each component and this can be used to identify the components, as well as showing what the original sample consisted of.