• 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 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.

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

Fig. 1: A diagram showing the typical apparatus and set-up used in gas chromatography.Vaia 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.

Fig. 2: An example of a chromatogram produced in gas chromatography.Vaia 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

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.

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.

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.

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.

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.

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.