Gas Chromatography Capillary Columns
Gas chromatography, sometimes called gas-liquid chromatography, is a method used in chemistry to separate vaporized compounds for analysis or identification. Coiled gas chromatography (GC) capillary columns are tubes that contain the stationary phase (a liquid that coats the interior of the column) and carry the mobile phase (an inert gas) along with the vaporized compound. The different constituents of the compound pass through the column at different rates, depending on their properties and how they interact with the stationary phase. The substance that composes the stationary phase will vary depending upon the test and the compound to be analyzed.
A gas chromatograph is the instrument that conducts the separation. It has a chamber that heats the columns, and it controls the amount of sample and the flow rate, as well as the temperature. Gas chromatography requires a detector, and a variety of detectors may be used. Detectors indicate the time at which a component of a compound elutes from the column. However, not all components are detected, and detectors do not identify substances. In gas chromatography-mass spectrometry, a mass spectrometer is used as the detector, which can identify components of the compound according to their molecular weight.
Considerations when selecting GC columns include the type of sample, the properties and thickness of the stationary phase, and the column dimensions. The sample compound should match the properties of the stationary phase for efficient separation. For example, the components of a compound may have different polarities and separate with the use of a polar column. The thickness of the film (stationary phase inside the column) can be increased to slow elution time. Each column has an internal diameter, and the larger the diameter, the greater the size of the sample it can accommodate, but with decreased resolution and sensitivity. The length of the column can help determine the retention time of a test. Some columns are designed for testing volatile compounds and may have a thick film to slow elution. Other columns for high-speed analysis may have a shorter column length.