Gas chromatography (GC) or gas-liquid chromatography is an analytical method for the quantitative analysis of complex compounds based on the separation of the compounds in a liquid-coated separation column.
Table of contents
- What is gas chromatography?
- Gas chromatography at Quality Analysis
- Design of a gas chromatograph
- Practical applications
- Summary: gas chromatography
What is gas chromatography?
Gas chromatography (GC) is a variant of adsorption and partition chromatography. In this type of chromatography, the stationary phase (separation column) is a liquid that, however, does not move because it is bound to a fixed carrier material. The mobile phase is gaseous. GC can only be used for components that are gaseous or that can be vaporised without decomposition (boiling range up to 400 °C).
During gas chromatography, the substance to be analysed is introduced into the separation column using an injector and transported to the detector using the carrier gas. On its way through the separation column the analyte interacts with the stationary phase. Depending on the structure and functional groups, the interactions are either only weak or are powerful. The latter cause the separation of the mixture.
The assessment of the individual substances in gas chromatography is based on the varying partial vapour pressures and the varying polarity of the substances analysed. A higher partial vapour pressure means that the substance remains in the gas phase longer, which in turn signifies a shorter retention time. Because the retention time is specific for many substances, the substances can be identified based on their peaks.
Gas chromatography at Quality Analysis
Gas chromatography is in many applications a standard method for the qualitative and quantitative analysis of complex mixtures. However, it involves high equipment costs because, depending on the specific task, completely different variants of the components (separation columns, detectors, carrier gases) must be used. Extensive user experience is also necessary for the reliable evaluation of the results.
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Design of a gas chromatograph
A gas chromatograph, in principle, always consists of the same components: the injector, the separation column with a thin liquid layer as the stationary phase and the related detector connected to a PC. There is also the carrier gas as the mobile phase that transports the analyte. There are numerous variants of these basic components, depending on which substance is to be analysed. Therefore only a few particularly important or common types are described in the following.
The injector is used to introduce the analyte into the separation column. This injection is without temperature control and is undertaken via direct feed systems or on-column injectors; the latter in particular have the advantage that they provide very sharp peaks, however they can only be used with samples mixed with a solvent with a low boiling point. On the other hand, if it is necessary to vaporise actively a sample, mostly split/splitless injectors are used that either first completely vaporise the sample and then introduce it into the separation column (splitless injection) or inject it already during vaporisation (split injectors). Temperature-sensitive samples can only be introduced into the separation column using a cold feed system with cooling.
A further option for the identification and quantification of highly volatile organic compounds is so-called headspace analysis. For this purpose only the headspace over a sample is analysed, not the sample as a whole. In the headspace above the sample an equilibrium of the volatile components between the headspace and sample becomes established at elevated temperatures, this equilibrium is dependent on the nature and concentration of the analytes. Headspace analysis is automated with the aid of heated autosamplers. The substances in the vapour are analysed with the aid of a gas chromatograph (GC-MS/GC-FID).
Separation columns and the stationary phase
Separation columns are differentiated between packed columns and capillary columns, where the latter are standard for most analyses these days because they permit significantly better separation and therefore shorter analysis times. As a rule, the length of the column is between 10 and 60 metres, but can also be more than 100 metres for specific applications. The stationary phase coats the inside of the column with a thin film of a viscous liquid of low volatility.
The separation column is in an oven (GC oven) that either ensures a constant temperature in the separation column during the entire analysis, or (less frequently) an increasing temperature. If the carrier gas with the vaporised analyte now flows through the separation column containing the stationary phase, the actual separation as described above occurs.
Gases for the mobile phase
Different gases and gas mixtures are used for the mobile phase, depending on the analyte, type of separation column and detection method used. Hydrogen or helium is often utilised in capillary separation columns, however nitrogen, carbon dioxide and argon or argon-methane mixtures are also used. In principle, only high-purity gases free of oxygen and water are allowed to be used because both would attack the stationary phase. Hydrocarbons are also not allowed to be used because they would cause incorrectly elevated base lines in the detector.
Along with the selection of a suitable gas, careful consideration must also be given to the flow rate. A slow flow rate produces good separation because the exchange between the mobile and stationary phase is very efficient. On the other hand, a slow flow rate results in a high diffusion rate, which causes very broad peaks in the chromatogram. Furthermore, a slow flow rate also, of course, means that the analysis takes longer.
Detectors for gas chromatography
Various types of detectors are used in gas chromatography, depending on the related analysis task. The commonest is the flame ionisation detector (FID); this type of detector is very suitable for the quantification of organic substances. For the assessment of permanent gases (hydrogen, oxygen, etc.), the thermal conductivity detector (TCD) is used. Along with a broad range of other detectors, it is also possible to couple the gas chromatograph to a mass spectrometer. This method is termed gas chromatography-mass spectrometry (GC-MS).
It is also possible to connect two or more detectors in series (tandem circuit). The electronic signals from the detector are depicted by a chromatography data system as a curve depending on the retention time. As such, there is a dedicated peak for each substance verified. Many substances can be identified simply by means of their specific retention time, for all others the mass spectrometer mentioned can be used and, at the same time, the structure resolved.
Gas chromatography is used in many areas, not only due to the straightforward identification of many substances simply by means of their retention time, but also due to its ability to separate reliably very complex mixtures and due to its high sensitivity that also makes it possible to detect very small quantities of substances.
GC is also used for the quantification of organic soluble residues (sum peak), as well as for the determination of the gas concentration of volatile siloxane (headspace) and the determination of purity.
Gas chromatography is also used for the quantification and identification of solvents, the testing and evaluation of polycyclic aromatic hydrocarbons (PAH) and the identification of plasticisers.
Pharmaceutics and pharmacology
In pharmaceutics, gas chromatography is used for the analysis of medicines, among other purposes. GC also has pharmacological significance in the context of blood analysis for intoxications of unclear aetiology; decomposition products from narcotics can also be verified using this method.
Gas chromatography is used widely in foodstuff analytics, for example for the identification of aromas and sugars or sweeteners, and also for authentication checks on fats and the verification of residues, for instance pesticides or hormones.
In summary: gas chromatography
Gas chromatography (GC) or gas-liquid chromatography is a chemical analytics method with the aid of which complex mixtures can be separated by means of strong interactions between the mixture and the stationary phase. It is possible to identify individual compounds in the mixture based on typical retention times.