What Is: Gas Chromatography (GC)?

Gas Chromatography is the separation of molecules from a mixture using a gaseous carrier medium.

Gas Chromatography allows an organic geochemist to identify and quantify some of the molecules present in a sample. The procedure is similar to Liquid Chromatography (LC), in that a collection of molecules is split into its constituent parts by passing them through a column of material. As the different molecules emerge from the column they are measured by a Flame Ionisation Detector (GC-FID) or identified by a Mass Spectrometer (GC-MS). A GC consists of a sample injector, a column housed within an oven, and an outlet to the detector.

Agilent GC system
Agilent GC system

The sample injector is responsible for putting a precise amount of sample into the column at the start of the sample run. A syringe is washed in solvent to remove any contamination, and then sucks up-and-down in the sample a few times to mix the sample and make sure there is no air in the syringe. Once the oven is ready for injection, the needle pierces the seal on the top of the GC and injects the sample onto the column. The sample is a liquid at this point, but will evaporate quickly and pass through the column as a gas.

GC autosampler
GC autosampler

The column is a 30m coil of glass tubing, just thicker than a hair, which is coated on the inside with an adsorbant material that slows the organic molecules as they pass through. The column is kept inside an oven which is carefully calibrated and can change its temperature during the sample run. In general, smaller, simpler molucules will pass through the column quicker, especially at lower temperature. Molecules will move easier when the temperature is hotter, so the easiest way to separate a mixture of molecules into their constituent parts is to start off at cool temperatures and slowly increase during the run (e.g. 40 to 300 °C over one hour). Simple molecules such as benzene or hexane might pass through the column in 2-3 minutes, whilst large, complex molecules might take up to 45 minutes. The slower the oven temperature increases, the further apart each molecule will be, making it easier to separate each one.

A GC column in the oven
A GC column in the oven

Once the molecules have passed through the column they exit into the detector. This can be a Mass Spectrometer (MS), which identifies the molecule, or a simpler detector that counts the concentration of molecules exiting the column but cannot identify them, such as a Flame Ionisation Detector (FID). Simple detectors, such as the FID, are most useful when the sample is less complex, such as comparing the concentration of a target molecule.

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