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Gas Chromatography

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Chemistry

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.

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.

Gas Chromatography, The diagram is a visual representation of how gas chromatography works. Saharan Khowaja, StudySmarter

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.

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.

Gas Chromatography, Capillary columns for gas chromatography, StudySmarterCapillary 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.

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.

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.

Gas Chromatography, GC-MS chromatogram of AGE. GC-MS, gas chromatography-mass spectrometry; AGE, aged garlic extract, StudySmarter

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.

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.

Advantages of gas chromatography

Some of the main advantages of gas chromatography are listed below.

  • It has a high level of sensitivity, meaning a wider range of compounds can be detected.
  • It has a high resolution, which allows for different types of compounds to be separated.
  • Gas chromatography is an easy technique that gives accurate data and results.
  • There are different types of injectors and detectors that can be used in many different applications.

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:

  • Flame ionisation detector (FID)
  • Electron capture detector (ECD)
  • Photoionisation detector (PID)
  • Flame photometric detector (FPD)

Gas Chromatography - Key takeaways

  • Gas chromatography, GC, is an analytical technique that analyses components of a sample in the gas phase.
  • There are two phases in gas chromatography: the stationary phase, and the mobile phase.
  • The capillary column is the wire-like object on which the stationary phase is coated.
  • The retention time in gas chromatography is the time taken by a compound in a mixture in order to reach the detector by first passing through a chromatography column.
  • Combining gas chromatography and mass spectrometry gives us a very powerful system that allows us to identify, separate, and measure complex mixtures of chemicals.

Gas Chromatography

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 phaseIt 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.

Final Gas Chromatography Quiz

Question

What is gas chromatography?

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Answer

An analytical technique that analyses components of a sample in the gas phase.

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Question

What is a phase?

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Answer

It is a state such as gas, liquid, or solid.

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Question

What is the mobile phase in GC?

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Answer

The mobile phase is an inert gas which carries the analyte components along the column towards the detector.

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Question

What is the stationary phase in GC?


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Answer

This is the phase which doesn't move since it is attached to the column.

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Question

What is the retention time?

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Answer

The time taken by a compound in a mixture to pass through the chromatography column and reach the detector.

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Question

What are the key steps involved in a GC-MS system?

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Answer

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 which acts as 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.

Show question

Question

What are three advantages of gas chromatography?


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Answer

-  It has a high level of sensitivity, meaning a wider range of compounds can be detected.

- Gas chromatography is an easy technique that gives accurate data and results.

-  It has a high resolution which allows for different types of compounds to be separated.

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Question


Describe the capillary column

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Answer

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.

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Question

What acts as the mobile phase in gas chromatography?


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Answer

 An unreactive gas such as helium

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Question

What is a chromatogram?

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Answer

A chromatogram is the result of a chromatography run. It shows the results after the components of a gaseous mixture are separated. 

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Question

Where is the sample injected in GC?

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Answer

Sample injection port

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Question

What does the area under each peak in a chromatogram represent?

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Answer

The area under each peak on the chromatogram represents the relative amounts of each component.    

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Question

What type of solute is dissolved into the solvent before injection?

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Answer

Volatile solids

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Question

What does the separation of the components depend on?

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Answer

The balance between solubility in the mobile phase and retention in the stationary phase.

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Question

What types of compounds can undergo gas chromatography?

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Answer

 Compounds which vaporise on heating without decomposing.

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