Gas chromatography (GC) is an analytical technique used to separate and identify components in a volatile sample. It is most commonly used on compounds that are past boiling point at room temperature (i.e., they are a gas), or have a molecular weight of less than 1250 Da.
There are two main types of gas chromatography: Gas-Liquid Chromatography (GLC) and Gas-Solid Chromatography (GSC).
Gas-Liquid Chromatography involves mixing a small sample size of a volatile compound with a gaseous mobile phase to be passed through a non-volatile liquid stationary phase.
Gas-Solid Chromatography is when the stationary phase is solid. This method separates the compound using adsorption so has a much longer retention time than GLC.
Gas chromatography involves mixing a small amount of the sample with the mobile phase and passing it through a column filled with the stationary phase.
The mobile phase used is typically an unreactive or inert gas that is unlikely to absorb other substances. Commonly used mobile phases of this type are helium and nitrogen. Helium has been the preferred gas for many years but due to the rising cost, laboratories are looking for alternatives. Nitrogen is a lot less expensive, but the chromatography is extremely slow.
Hydrogen is now being much more commonly used as a mobile phase for gas chromatography because it is cheap to produce and has very similar chromatographic properties to helium. Laboratory hydrogen generators are now widely available and because they generate hydrogen on demand and do not store it under pressure they can be located in the laboratory environment safely.
The stationary phase used in gas chromatography is usually silicone or other chemicals that selectively attract certain molecules. The stationary phase used will depend on what compounds are suspected to be in the sample.
As the sample is passed through the column, the compounds will separate based on their interactions with the stationary phase and the mobile phase.
Less volatile molecules will interact more with the stationary phase, making them move slower through the column. While more volatile molecules will interact more with the mobile phase, meaning they move faster through the column and have a faster flow rate and shorter retention time.
To begin with, the gaseous mobile phase is passed through a molecular sieve to remove any contaminants like hydrocarbons, water vapor or oxygen. If these are present, it can skew the data and give an inaccurate analysis of the compound in question.
Next the compound sample is mixed with a volatile solvent like heptane, acetone or methanol and is injected into the column. At this stage, the sample is kept 20°C – 50°C hotter than the column to increase volatility before it passes through.
The sample is then passed through the column by the gas stream and separation will occur. This is also known as the components eluting.
The column temperature in gas chromatography is very high, at 150°C – 300°C, to encourage volatility and are relatively long compared to other types of chromatography, such as liquid chromatography.
The two key types of columns are packed columns and capillary columns.
Packed columns are made of glass or metal and are roughly 1-3m long and 2-4mm in diameter. They are filled with small particles coated in a thin layer of high molecular weight polymers, like diatomaceous earths, fluorocarbons, graphitised carbon black and glass beads.
Capillary columns are made of glass or fused silica and are much longer, between 10m and 100m long, with a small internal diameter of 0.1-1mm. The inner column is coated with a thin layer of the stationary phase so the molecules can come into contact with the walls of the tubing.
Next the sample will exit the column and reach the detector. This is where the components of the sample are identified based on how long it took for each component to pass through
One of the most common types of detector is a Flame Ionisation Detector (FID). This uses a flame to ionise the sample as it elutes, which then releases electrodes that cause an electric current. The detector then measures the current to identify the molecules present. In this type of detection, the chromatograph will show peaks where current increases, with the area underneath the peak indicating the concentration of the molecules present.
Another commonly used detector is a Thermal Conductivity Detector (TCD). This responds to changes in thermal conductivity and specific heat. When a molecule passes through the detector with the carrier gas, there is a change in thermal conductivity, which cases a peak in the gas chromatographs.
Gas chromatography is important as it is key for analysing volatile compounds to find out what they are made of. Special conditions are required to measure gases, so it’s imperative a different method is used.
It is typically used to identify unknown organic compounds and for quality control in a wide range of industries.
Here are just some of the ways gas chromatography is used:
The key advantage of gas chromatography is it helps you to identify unknown substances in a volatile compound. This helps us to have a better understanding of the compound and its properties.
As a scientific process, gas chromatography is very versatile, with lots of ways to measure hundreds of different compounds. This means is can be used for a wide range of applications such as those mentioned above.
It is also a very robust technique that can be used alongside other techniques like mass spectrometry. This further increases the ways it can be used and the applications.
Finally, gas chromatography is a very reliable method of identifying substances. As it is based on the fundamental properties of the substances in question, the results can be easily replicated.
The disadvantages of gas chromatography are due to the reliance on volatile compounds. It is not a reliable method for non-volatile substances.
The reliance on volatility also means it must be performed at high temperatures. This means compounds that degrade with heat can’t be analysed through gas chromatography.
Finally, if the equipment is not maintained properly, it can lead to analytes getting stuck in the column or gas leaks that affect the carrier gases. Both of these issues will skew the results and not give an accurate analysis. However, these issues can be rectified with regular servicing.
If your gas chromatography analyses are inaccurate, or you think there is an issue with your equipment, LC Services can help. We supply high-quality gas chromatography parts to get your equipment back up and running.