A gas chromatograph (GC) is a machine with a packed column in an oven coupled to a sophisticated detection system. Sometimes there is also a mass spectrometer (MS)

We use these for organic analysis, of oils, solvents, chemicals, fuels, drugs, and toxic substances.

They represent one of the most searching instruments in analysis - a GC is capable of detecting parts per billion. We can analyse drugs in hair samples, giving a history of drug use, month by month.