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Gas chromatography is used to analyse the purity of volatile compounds, and is usually only used to analyse samples.
GC behaves as a half-way house between chromatography and distillation; the sample to be analysed is heated until it forms a gas, and is them swept through a heated column by a stream of carrier gas, the mobile phase. The carrier gas is often nitrogen or argon, and needs to be inert. The column contains a stationary phase, often an absorbant support that is impregnated with an involatile liquid. The time that it takes for each component to elute from the column (the retention time) depends both on its boiling point and its interactions with the mobile and stationary phases.
Various types of detectors can be used at the end of the column, but the most commonly used is a flame ionisation detector. The gas elutes the column and is burnt (the carrier gas is usually a mixture of hydrogen and air with this type of detector), and the ions in the flame are detected, and recorded as an electrical current. With this type of setup, the GC is destructive to the sample, and hence only analysis is performed, not preparative separation.
A good video showing the operation of GC can be found here.
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