Gas Chromatography (GC)

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

  • 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

  • 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.

Interpreting GC Results

  • 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.

Limitations and Challenges of Gas Chromatography

  • 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.