Understanding Gas Chromatography (GC)

Gas Chromatography (GC) is a common technique used in analytical chemistry for separating and analysing compounds that can be vaporised without decomposition.
It is used to test the purity of substances, to separate and identify different components in a mixture, or to prepare pure compounds from a mixture.
It’s a crucial part of practical chemical analysis, as it enables chemists to identify the amounts and types of compounds in a mixture.

The Process of Gas Chromatography

GC is performed in a gas chromatograph that uses a variety of gases to carry a sample through the chromatograph.
The sample is vaporised, often by heat, and carried through a column by the carrier gas.
Different compounds in the sample will interact differently with the walls of the column and take different lengths of time to pass through, this is known as the retention time.
The gaseous compounds exiting the GC column go through a detector that transmits data to a computer for analysis and output.

The column is the part of the gas chromatograph where the separation of compounds occurs.
There are two types of columns, packed and capillary.
The inside of the column is coated with a stationary phase, which can be a liquid or a polymer on a solid support.
The properties of the stationary phase and its interaction with the sample components significantly affect the separation efficiency of the GC process.

The detector generates a signal representing the amount of sample component reaching it.
The two commonly used detectors in GC are the Flame Ionisation Detector (FID) and the Thermal Conductivity Detector (TCD).
The choice of detector depends on the specific requirements of the analysis - such as the nature of the compounds to be detected or the detection sensitivity required.

Each compound in a sample will produce a peak on the GC chromatogram, the position and size of which can provide useful information.
The retention time can provide information about the identity of a compound, based on comparison with known standards.
The size of the peak, usually measured as the area under the peak, can indicate the quantity of a particular compound in the sample.
Correct interpretation of GC results requires understanding of basic principles of chromatography and experience with the specific type of analysis being conducted.

While highly useful, GC cannot separate all types of compounds and not all can be vaporised for analysis.
Certain samples may require pre-treatment or derivatisation to make them suitable for GC analysis.
The choice of column, temperature and detector types can all greatly affect the analysis results, requiring careful consideration for each sample type.
Overlapping peaks in the chromatogram can make interpretation of results challenging.
Like any analytical technique, GC also requires ensuring proper calibration, maintenance and optimisation of equipment for accurate results.