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.

1. A continuous flow of the carrier gas passes through the column.
2. The sample is injected into the column.
3. The substances are separated as they travel through the column depending on their affinity (attraction) for the stationary and mobile phases.
4. The chromatogram shows the time it takes for each substance to pass through the column
5. 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 analyzed 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.