Chromatography, in general, is an analytical method in which compounds, normally organic, or mixtures of compounds can be separated into their various constituents. Gas chromatography (GC) does this separation specifically upon gaseous species. A subset of GC employs a mass spectrometer to detect the ionized materials coming from the chromatograph; this is called gas chromatography with mass spectroscopy (GC/MS.)
Compounds are separated primarily on the basis of their boiling points. The material to be analyzed is heated slowly to vaporize it. As the boiling point of each compound is reached, it is evaporated and passes through the column to be recorded by a detector. Inasmuch as many compounds can have approximately the same boiling point, GC by itself only provides partial identification. When combined with MS, the material passing through the column is detected by a mass spectrometer which records the total ion chromatogram (TIC) for the ionized portion of each fraction that passes through to it. More importantly, a spectrum can be developed showing the mass/charge ratio for each ionic fragment that enters. A given compound will have a typical fragmentation pattern, that is a group of ions into which it decomposes in the field of the spectrometer. Thus by comparing patterns of known compounds with the boiling point range determined by the GC to the pattern in the unknown compound or mixture a match can usually be made to at worst a few possible compounds. With standards, this technique can yield very precise quantitative information down into the ppm, ppb or sometimes even ppt ranges.