Gas Chromatography (GC) Working Principle and Applications


Gas chromatography

Gas chromatography, also known as vapor-phase chromatography (VPC), or gas-liquid partition chromatography (GLPC), is a technique used to detect and separate small molecular weight compounds from a mixture. Gas chromatography is of two types, gas-solid chromatography (GSC), and gas-liquid chromatography (GLC). The stationary phase in GLC and GSC is liquid and solid respectively. Gas-liquid chromatography method is widely used as compared to gas-solid chromatography.

  • Gas-solid chromatography

Gas-solid chromatography is a technique in which the separation of the mixture takes place through the adsorption process. It is used for the separation of low molecular gases, namely, H_{2}S, CS_{2}, CO_{2}, and CO, and is rarely used as compared to GLC.

  • Gas-liquid chromatography

Gas-liquid chromatography is a technique in which the stationary phase (liquid) is first converted into vapours. Then, the separation of mixtures is done which depends upon the relative vapour pressure and affinities for the stationary phase. The affinity of the substance towards the stationary phase can be described in chemical terms as partition coefficient, also known as distribution constant, as KC = [A]S / [A]m, where [A]s is the concentration of compound A in the stationary phase and [A]m is the concentration of compound A in the stationary phase.

Working Principle

Gas chromatography works on the principle of separation/partition of the mobile phase and the stationary phase in which the mobile phase is the carrier gas. It is a process in which a gaseous sample is injected into the injection port with the help of a GC syringe, where it gets vaporized and is further carried by a gas flow regulator (carrier gas) to pass through the stationary phase (viscous liquid). Inert gases like helium, argon, nitrogen, and hydrogen are usually used as mobile phase/carrier gas in the gas chromatography technique. This sample is then separated electronically at the detection port with the help of suitable temperature programming. Clear visualization of this process can be seen on recorders/computers in the form of peaks.


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Gas chromatography, both GSC and GLC, is mainly composed of the following components-

1. Carrier gas

A carrier gas is a high-pressure cylinder equipped with attendant pressure regulators and flow meters. Carrier gases used in GC are inert oxygen-free gases that act as a gas flow regulator/carrier for the mobile phase to pass through the stationary phase/column. Helium, Nitrogen, argon, and hydrogen gases are used as carriers in GC, depending upon the desired performance and the detector being used. Hydrogen, as a carrier gas, is highly efficient and provides the best separation. However, helium is preferred over hydrogen because of its inflammable properties, higher flow rates, and its compatibility to work with a greater number of detectors. For example, helium is preferred for thermal conductivity detectors because of its high thermal conductivity relative to most organic vapors.

2. Sample-injection system

It is essential for introducing the sample at the head of the column. The sample is injected into the injection port with the help of a micro-syringe/GC syringe. The sample gets volatilized here, and the resulting gas is then carried to the column with the help of carrier gas. Many inlet types exist including, Split/Splitless, Programmed Thermal Vaporizing (PTV), Cool-on-column (COC), etc. In the case of the COC injector, the sample is introduced as liquids to avoid thermal decomposition.


3. The separation column

The heart of gas chromatography is the column which is made up of metals, bent in a U-shaped or coiled into an open spiral. The sample mixture is separated here and is then carried to the detector. Different-sized columns are used, depending upon the requirement. For example, Open tubular columns or capillary columns and packed columns are most commonly used. Two types of columns  are used in GC, namely, packed columns and capillary columns.

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4. Column oven or thermostat chambers

These thermostat chambers help to control the temperature to conduct precise work. The temperature is controlled via two methods, isothermal programming, and temperature programming. In isothermal programming, the temperature is kept steady throughout the analysis, while in the other temperature can be increased or controlled according to the requirement.

5. Detector

Detectors provide signals, to give us the quantitative measurement of the components of mixtures, by sensing the arrival of separated components of the mixture. Various detectors used in GC are Mass Spectrometer, Flame ionization detector (FID), Electron capture detector (ECD), Thermal conductivity detector (TCD), Atomic emission detector (AED), Photoionization detector (PID), and chemiluminescence detector.

6. Recorder

The recorder is meant to record the signals that come from detectors and amplify them to generate an electronic response in the form of a graph, known as a chromatogram.

Gas chromatogram


Gas chromatography has a wide range of applications in various fields, such as-

  • It has medicinal and pharmaceutical applications
  • It is also used in environmental analysis and monitoring of pollutants like carbon monoxide, benzene, DDT, etc.
  • Doping of drugs is detected using this method
  • Miscellaneous analysis of foods like proteins, carbohydrates, lipids, fats, steroids, etc.
  • usually used to separate and measure organic molecules and gases
  • the components being analyzed must be volatile
  • Forensically used in drug analysis and toxicological analysis
  • the molecular weight of components should be less than 1250 Da
  • GC is also used in catalysis
  • Dairy product analysis- rancidity
  • Identification of hazardous compounds in waste dumps
  • useful in the isolation of RNA etc. Applications


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