4.1 What use is a GC?

A gas chromatograph is an instrument used to separate mixtures. The principle is similar to extraction, except in chromatography, one of the phases moves and one is stationary (in simple extraction, both phases are stationary).

• The fundamental unit in separation theory is the ``theoretical plate.'' One theoretical plate is defined as the separating power (for a given set of substances under consideration) of one simple distillation.
• The GC in this laboratory uses capillary columns that provide many thousand theoretical plates.
• Factors that affect the number of theoretical plates, and hence separation ability, include carrier gas (the mobile phase), gas flow rate, the nature of the stationary phase, the column design, the detector, injection technique and temperature programming.

A GC is also useful as a means of sample introduction into the vacuum system of a mass spectrometer.