The combination of two powerful ananlytical techniques

SEPARATION
Gas chromatography is the physical separation of two or more compounds based on their differential distribution between two phases. The gas chromoatograph employs a carrier gas (mobile phase) to move a vaporised sample through a column coated with a stationary phase where separation takes place. A detector converts the column eluent to an electrical signal that is measured and recorded.
The output of the GC is a plot of detector signal abundance versus time. The abundance remains at a low "baseline" level except when a separated sample component elutes from the column and produces a peak in the chromatogram plot.
Chromatographic peaks can be identified from their corresponding RETENTION TIMES, measured from the time of sample injection to the time of the peak maximum. The retention time of any component peak is unaffected by the presence of other sample components. The height or area of a peak may be used to measure the concentration of a component in the sample mixture

MASS ANALYSIS
A mass spectrometer is one kind of GC detector. As the separated sample component molecules elute from the column to the inside of the MS they are bombarded with energy. This causes them to lose an electron and form ions with a positive charge. Some of the bonds holding the molecule together are broken in the process, and the resulting fragments may rearrange or break up further for form more stable fragments. Because of natural laws governing the relative strengths of chemical bonds, a given compound will ionize, fragment, and rearrange reproducibly under a given set of conditions
A mass spectrum is a recording of the masses of each of the ionized fragments representing a unique fingerprint of a molecule that can be used in identification.