Gas Chromatography (AQA A-Level Chemistry): Revision Notes
7.11.4 Gas Chromatography
Gas chromatography is used to separate a mixture of volatile
- Stationary phase – A long capillary tube is packed with the stationary phase. The stationary phase is a solid or a powder that has been coated with a liquid such as oil (usually a viscous liquid is used).
- Mobile phase – The mobile phase is an unreactive gas, usually nitrogen or helium is used. This method is used to separate mixtures of volatile liquids.
The sample is introduced into a long capillary tube, which is housed within an oven. The oven heats the sample to a high temperature, converting it into a gas.
The gaseous sample then travels through the tube, carried by an inert mobile gas under pressure.
- A continuous flow of the carrier gas passes through the column.
- The sample is injected into the column.
- The substances are separated as they travel through the column depending on their affinity (attraction) for the stationary and mobile phases.
- The chromatogram shows the time it takes for each substance to pass through the column
- The size of the signal is proportional to the amount of that substance in the mixture. It is very common for a mass spectroscopy machine to be connected to a GLC machine. As each separated substance reaches the end of the column, it can be analysed by mass spectroscopy.
Gas Chromatography – Mass Spectrometry (GC-MS)
This technique combines the separation power of gas chromatography (GC) with the identification capabilities of mass spectrometry (MS).
First, the gas chromatography component is used to separate the components of a mixture of volatile liquids. Each separated component is then directed into the mass spectrometer, where it can be analysed individually.
The mass spectrometer generates a mass spectrum for each component, using either its fragmentation pattern or precise mass measurement to identify the specific molecules present in the mixture.