Understanding of Gas Chromatography

Gas chromatography is the process of separating compounds in a mixture by injecting a gaseous or liquid sample into a mobile phase, typically called the carrier gas, and passing the gas through a stationary phase. The mobile phase is usually an inert gas or an unreactive gas such as helium, argon, nitrogen or hydrogen.

What is gas chromatography?
  • Gas chromatography (GC) is an analytical technique used to separate the chemical components of a sample mixture and then detect them to determine their presence or absence and/or how much is present. 
  • These chemical components are usually organic molecules or gases. For GC to be successful in their analysis, these components need to be volatile, usually with a molecular weight below 1250 Da, and thermally stable so they don’t degrade in the GC system.  
  • GC is a widely used technique across most industries: for quality control in the manufacture of many products from cars to chemicals to pharmaceuticals; for research purposes from the analysis of meteorites to natural products; and for safety from environmental to food to forensics. Gas chromatographs are frequently hyphenated to mass spectrometers (GC-MS) to enable the identification of the chemical components.

How does gas chromatography work?
  • As the name implies, GC uses a carrier gas in the separation, this plays the part of the mobile phase. The carrier gas transports the sample molecules through the GC system, ideally without reacting with the sample or damaging the instrument components.

  • The sample is first introduced into the gas chromatograph (GC), either with a syringe or transferred from an autosampler that may also extract the chemical components from solid or liquid sample matrices.  
  • The sample is injected into the GC inlet through a septum which enables the injection of the sample mixture without losing the mobile phase. Connected to the inlet is the analytical column, a long (10 – 150 m), narrow (0.1 – 0.53 mm internal diameter) fused silica or metal tube which contains the stationary phase coated on the inside walls. 
  • The analytical column is held in the column oven which is heated during the analysis to elute the less volatile components. The outlet of the column is inserted into the detector which responds to the chemical components eluting from the column to produce a signal. The signal is recorded by the acquisition software on a computer to produce a chromatogram.

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