Separation in a GAS Chromatograph (GC)
A carrier gas move a sample through a coated column where the separation takes place. The GC output is measured by a detector and the signal is registrered versus time. The area of the peak is proportional to the concentration of a component.

Detection and identification in a Mass Spectrometer (MS)
The GC column is directly connected to a mass spectrometer (MS).
The separated molecules elute to the ion source of the MS.
In the ion source of the MS the molecules are bombarded with electrons. The masses and amounts of positive ions produced is a fingerprint of the molecule.