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Gas Chromatography

Gas Chromatography

What happens if someone is suspected of taking drugs? They'll often be asked to carry out a urine test. This can test for either one specific compound or a whole family of drugs. If the results are positive, the urine will be analysed further using a method called gas chromatography. Also known as gas-liquid chromatography, this is a handy technique used to separate and identify the components of a gaseous mixture.

  • This article is about gas chromatography (also known as gas-liquid chromatography).
  • Firstly, we will define gas chromatography.
  • After that, we will explore its principles and delve into the method. This will involve a handy gas chromatography diagram.
  • Next, you'll learn about the importance of chromatograms and retention time. You'll also be able to try your hand at analysing chromatograms to calculate relative abundance.
  • Then we will look at how and why we combine gas chromatography with mass spectrometry. This will lead to exploring further types of gas chromatography.
  • To finish, we'll outline some key advantages, disadvantages, and uses of gas chromatography.

What is gas-liquid chromatography?

If you've clicked on this article, you are probably curious about gas chromatography. What is this technique, and how does it work?

Gas chromatography (GC) is an analytical technique that separates and analyses components of a gaseous sample. It is used for compounds that vaporise (turn from liquid to vapor) on heating without decomposing.

Gas chromatography not only separates the components within a sample but also gives a measure of the relative amount of each species present. Therefore, this technique is useful in allowing chemists to analyse complex mixtures, both qualitatively and quantitatively.

There are various types and subsets of gas chromatography, but the term is generally used to refer to gas-liquid chromatography (also known as gas-liquid partition chromatography, or GLPC). The word liquid indicates the physical state of the stationary phase used, which we'll explore later. We'll use the term gas chromatography to refer to GLPC for the rest of this article.

Principles of gas chromatography

Gas chromatography follows the same principles as all other types of chromatography. However, there are a few particular details that you should note, and we'll look into these in more detail as we come across them:

  • Like in all types of chromatography, gas chromatography uses a mobile phase to carry a sample through a stationary phase.
    • Mobile phases are often solvents. But in gas chromatography, the mobile phase is instead just an unreactive gas (such as helium).
    • In gas chromatography, the stationary phase is a viscous liquid (such as a long-chain hydrocarbon). The liquid is suspended on a fine solid (such as silica), which is packed into a long, thin capillary tube (or column). The tube is typically just a few millimetres thick, but up to 10 metres long!
  • Some components of the sample are carried through the stationary phase by the mobile phase more quickly than others. We say that components that travel faster have a greater affinity to the mobile phase, and those that travel more slowly have a greater affinity to the stationary phase.
  • The sample is separated into its components according to their relative affinities to the stationary and mobile phases. This means that the components leave the chromatography system at different times and can be individually sensed by a detector.
  • The detector produces a signal, which is used to create a chromatogram. This is a graph that tells you about the retention time of each component: the time taken for it to move through the system.
    • In gas chromatography, the chromatogram also gives you quantitive data about the percentage composition of the sample - i.e, how much of each component is found within it.

Never come across any of the various types of chromatography before? We recommend starting with the article Chromatography. It explores all the principles we've mentioned so far in much greater depth. From there, you can learn about other examples of chromatography, such as Thin-Layer Chromatography and Paper Chromatography

Gas chromatography: method and diagram

Now, let's find out about the gas chromatography method:

  1. The sample and the unreactive gas used as the mobile phase are injected into a small oven. There, they are heated and put under pressure, so that the sample vaporises and mixes with the mobile phase.
  2. The gases are then forced through a long, thin capillary tube (or column). The tube is tightly packed with the stationary phase and is also heated.
  3. The components in the mixture separate as they pass through the tube, according to their relative affinity to each phase. This means that they travel through the column at different rates.
  4. The components eventually leave the column and pass over a detector. The detector produces a signal proportional to the amount of the component present.
  5. The signal is used to produce a chromatogram, which is then analysed by chemists. The chromatogram contains a series of peaks that provide information about the retention time and relative abundance of each component.

The ovens can be heated to a range of temperatures. However, they must be hotter than the sample's boiling point so that the sample does not condense inside the capillary tube.

The following gas chromatography diagram should help you visualise the process a little more clearly:

Gas Chromatography labelled diagram showing method and apparatus set up used in gas-liquid partition chromatography StudySmarterA diagram showing the typical apparatus and set-up used in gas chromatography.StudySmarter Originals

Gas chromatography: analysing chromatograms

We've learned that gas chromatography produces a chromatogram. This is a graph that informs us about the components within our sample. It shows peaks corresponding to each component's retention time and relative amount.

Gas Chromatography a labelled chromatogram with two peaks showing retention time on the x-axis and signal on the y-axis StudySmarter An example of a chromatogram produced in gas chromatography.StudySmarter Originals

From chromatograms, we can infer two things:

  • The retention times give us clues as to the identity of the sample's components.
  • Chromatograms also tell us quantitively about the percentage abundance of each component within the sample.

Retention time

The retention time of a component within a sample is the time taken from its injection to its detection. In other words, it is the time taken for it to reach the detector.

Retention times are always the same for a particular component, provided we keep all conditions the same. This means using the same mobile phase, stationary phase, capillary tube (or column), pressure, and temperature. Although carefully controlling the conditions can be a little tricky, using standard conditions allows us to compare the retention time of a certain component to those in a database. As a result, we can identify the component.

Databases might not include all compounds. This makes identifying certain species difficult. Likewise, some compounds have very similar retention times, and are hard to distinguish. For this, we combine gas chromatography with mass spectrometry, which we'll look at later on.

Percentage abundance

Some peaks in chromatograms are much taller than others. Others are much wider. As a result, different peaks have different areas. The area under a peak is proportional to the relative amount of the component reaching the detector at a particular time. We can use this to quantitively find the percentage abundance of each component within the sample. Here's how:

  1. We first find the area under each peak. The peaks are roughly triangular, and so their area approximates 0.5 × the length of their base × their height.
  2. We then add the areas under all the peaks together to find the total area.
  3. To find the percentage abundance of a particular component, divide the area under its peak by the total area under all the peaks. Then, multiply this number by 100. This is your final answer.
  4. Repeat step 3 with the remaining peaks.

Units are irrelevant - simply assume that the length of each square on the graph equals one unit.

Calculate the percentage abundance of the component responsible for the left-hand peak in the chromatogram shown earlier. The total area under both peaks in the chromatogram equals 82.5 units squared. Assume each square has a length of one unit.

To accomplish this, we first assume that the peak is a rough triangle, and find the area under it. We first measure the height of the left-hand peak in terms of squares, along with the length of its base; the length of each square equals one unit. Here, the peak has a height of 17 units and a base length of 6 units.

Gas Chromatography a chromatogram showing retention time on the x-axis and signal on the y-axis. The left-hand peak has its height and base length measured and labelled. StudySmarter The same chromatogram as shown earlier in the article. Here, the height and base length of the left-hand peak are measured and labelled.StudySmarter Originals

The same chromatogram as shown earlier in the article. Here, the height and length of the left-most peak's base are measured and labelled.StudySmarter Originals

We then substitute these values into the formula for the area of a triangle:

$$\text{area} =0.5 \times \text{base length} \times \text{height}$$

$$\text{area} =0.5\times 6\times 17=51 \text{units squared}$$

We then divide this area by the total area of all the peaks in the chromatogram and multiply by 100. Luckily, the question gives us the total area of the peaks:

$$\text{percentage abundance} =\frac{51}{82.5} \times 100$$

$$\text{percentage abundance} =62\% $$

Gas chromatography and mass spectrometry

Gas chromatography is good at separating mixtures into their components. However, it is only good at identifying these components if you use standard conditions. For many experiments, this isn't possible. In addition, some chromatograms can give ambiguous results if the sample's components have similar retention times. To combat these issues, we combine gas chromatography with mass spectrometry.

Mass spectrometry is a technique used to identify substances according to their mass/charge ratio.

You might have come across mass spectrometry before, used to identify a single molecule by splitting it into different fragments. The pattern of the mass/charge ratios of the fragments acts as a chemical fingerprint, allowing us to work out the molecule's structure and identity. However, mass spectrometry can also be used for a mixture of multiple different species. Combining gas chromatography with mass spectrometry creates an extremely useful analytical tool known as GC-MS. GC-MS efficiently first separates (thanks to gas chromatography) and then identifies (thanks to mass spectrometry) all the different compounds within a sample.

Visit Mass Spectrometry in Organic Chemistry to find out more about mass spectrometry, its method, and its uses.

The key features of a GC-MS system are as follows:

  • The sample is injected into the chromatography system.
  • The sample components are separated according to their relative affinity to the stationary and mobile phases, and leave the chromatography system at different times.
  • The separated components are sent into a mass spectrometer instead of a detector.
  • A detailed mass spectrum is produced by the spectrometer, which is then compared to a known database. The spectrum can be used to identify all the components within the sample.

Further types of gas chromatography

As well as gas-liquid chromatography, you can also find gas-solid chromatography (GSC). There are some key differences between the two:

  • The stationary phase in GLPC is a liquid supported on a solid.
  • However, the stationary phase in GSC is just a solid - there is no liquid involved.
  • As the name suggests, GLPC relies on partition: the separation of a solute between two immiscible solvents.
  • On the other hand, GSC uses adsorption: the adhesion of molecules to a solid.

You can make gas chromatography even more specific by changing the apparatus. For example, a number of different detectors can be used within gas chromatography. These include the flame ionisation detector (FID), the electron capture detector (ECD), and the flame photometric detector (FPD). The FID mixes the separated sample with hydrogen before burning it. It is particularly good at detecting organic hydrocarbons, but ignores highly oxidised components (like water and carbon oxides). On the other hand, the ECD and FPD sense certain elements: the ECD is good at identifying components containing halogens, whilst the FPD looks for sulfur and phosphorus.

Advantages and disadvantages of gas chromatography

Gas chromatography is just one of the many types of chromatography you need to learn about for your A-level exams. They can seem quite similar - so what makes gas chromatography stand out from the others, and what makes it subpar in certain situations? Let's consider the advantages and disadvantages of gas chromatography.

Advantages

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

  • Gas chromatography can be used for all manner of samples - the only requirement is that they are volatile and don't decompose upon heating.
  • It produces quantitive data, unlike other techniques such as thin-layer chromatography (TLC).
  • Furthermore, the data is digital, so there is a reduced risk of it becoming lost or degrading.
  • Gas chromatography also has a high level of sensitivity and a high resolution, meaning a wide range of compounds can be detected.
  • Its resolution and sensitivity are both greatly improved when joined to a mass spectrometer. Because of this, we can use gas chromatography to separate and identify extremely similar species, even if they are only present in trace amounts.
  • In addition, there are many different types of capillary tubes, columns, detectors, and stationary phases available for use in a range of applications.

Disadvantages

To avoid any scientific bias, we must also take a moment to think about the disadvantages of gas chromatography. These include:

  • Gas chromatography requires careful monitoring of the external conditions, to ensure the temperature does not drop too low.
  • Non-volatile species, as well as those that aren't thermally stable, aren't suitable for analysis by this technique. This includes many organic molecules like sugars.
  • The capillary tubes require careful handling and storing.

Uses of gas chromatography

Last - but certainly not least - it is time for the real-life applications of gas chromatography. This technique has many uses in modern society. For example:

  • Gas chromatography is used to test for doping, drug use, and the presence of performance enhancers in major sporting events.
  • Similarly, it is used in security departments at airports and in forensics.
  • Gas chromatography also plays a role in the food industry. This technique is used to assess the safety and palatability of food products by not only analysing levels of additives and contaminants, but also ensuring there are enough particular flavourful or aromatic molecules.
  • We also check for environmental pollutants in water systems and natural habitats using gas chromatography.
  • A fairly recent field of study in science involves looking at the risks and dangers of volatile organic chemicals (VOCs). Gas chromatography can be used to identify VOCs released by common household items.

Gas Chromatography - Key takeaways

  • Gas chromatography, GC, is an analytical technique used to analyse a gaseous sample by separating it into its individual components. The term typically refers to gas-liquid partition chromatography (GLPC).
  • In gas chromatography, the mobile phase is an unreactive gas, whilst the stationary phase is a liquid suspended on a fine solid.
  • Gas chromatography produces chromatograms with peaks showing retention time.
    • We use retention time to identify the components within a sample.
    • We use the area under the peaks to work out the relative abundance of each component.
  • Combining gas chromatography and mass spectrometry gives us a powerful system that can identify, separate, and measure complex mixtures of chemicals. The combined technique has a high resolution and sensitivity.
  • Gas chromatography can be used for a range of samples, is highly sensitive, and produces quantitive digital data. On the other hand, the apparatus requires careful handling, and the technique can't be used to analyse non-volatile species.
  • We use gas chromatography to test for drugs, in forensics, in the food industry, and to measure pollution.

Frequently Asked Questions about 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 sample is then vaporised and injected into a capillary tube or column. An unreactive gas such as helium acts as the mobile phaseThe sample mixes with the mobile phase and is passed through the column under pressure at a high temperature. The components in the sample separate as they pass through the column according to their relative affinity to each stage. They then pass into a detector, which produces a signal proportional to the relative amount of each component. The signal is used to make a chromatogram.

Each component within the sample has a different relative affinity to the stationary and mobile phase. This leads to the components travelling through the capillary tube/column at different rates, and thus gives them different retention times. As a result, the components are separated. We can use retention times to identify the components, or connect the chromatography system to a mass spectrometer for more detailed analysis.

Temperature affects the retention time of the components of the sample, 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

  1. What does GLPC stand for?
  2. What does the letter L refer to in GLPC?

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Answer

  1. Gas-liquid partition chromatography - the specific type of chromatography typically referred to by the general term gas chromatography.
  2. The physical state of the stationary phase used in GLPC.

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Question

What is the mobile phase in GC?

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Answer

An inert gas (such as helium).

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Question

What is the stationary phase in GC?


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Answer

A viscous liquid (such as a long-chain hydrocarbon), suspended on a fine solid (such as silica).

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Question

What is 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 into the chromatography system.
  • The sample components are separated according to their relative affinity to each phase and leave the chromatography system at different times. 
  • The separated components are sent into a mass spectrometer. 
  • A detailed mass spectrum is produced by the spectrometer and compared to a known database. The spectrum is used to identify the components within the sample.

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Question

What are three advantages of gas chromatography?


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Answer

E.g:


  • Gas chromatography can be used for all volatile samples that don't decompose upon heating.
  • It produces quantitive data.
  • The data is digital.
  • It has a high level of sensitivity and a high resolution.
  • Its resolution and sensitivity are both greatly improved in GL-MS. This means that we can use gas chromatography to identify a wide range of similar species, even if they are only present in trace amounts.
  • There are many different types of capillary tubes, columns, detectors, and stationary phases available for use.

Show question

Question

Give three uses of gas chromatography.

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Answer

E.g:


  • Doping and drug use in major sporting events.
  • In security departments at airports and in forensics.
  • Testing for additives, contaminants, and quality in the food industry.
  • Checking for environmental pollution.
  • Analysing VOCs.

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Question

What conditions are used in gas chromatography? Justify their use.


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Answer

High temperature and pressure. This ensures that the sample vaporises and doesn't condense inside the capillary tube.

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Question

What information do GLPC chromatograms give us?

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Answer

  • The identity of the components in the sample.
  • The relative amounts of each component.

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Question

Outline the method of GC.

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Answer

  1. The sample and the mobile phase are injected into a small oven, then heated and put under pressure.
  2. The gases are forced through a capillary tube tightly packed with the stationary phase. The tube is also heated.
  3. The components in the mixture separate as they pass through the tube, according to their relative affinity to the different phases. This means that they leave the tube at different rates.
  4. As the components leave the tube, they pass over a detector. This produces a signal proportional to the amount of the component present.
  5. The signal is used to produce a chromatogram, which provides information about the retention time and relative abundance of each component.

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Question

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

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Answer

The relative amount of each component.    

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Question

The area under the peak caused by component A is 25 units squared. The total area under all of the peaks on the chromatogram is 90 units squared. Calculate the percentage abundance of component A to 2 significant figures.

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Answer

28%

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Question

What does the separation of the components in the sample depend on?

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Answer

The balance between the relative affinity to the mobile phase and the relative affinity to 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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Question

Which technique is often combined with gas chromatography?

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Answer

Mass spectrometry

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Question

How can components be identified in gas chromatography?

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Answer

Provided standard conditions are used, the retention times of components can be compared to those in a database. This is because a species always has the same retention time under the same conditions.

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Question

Components with a shorter retention time have a greater affinity to the _____ phase.

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Answer

Mobile

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