A gas chromatograph (GC) is used to separate any quantitative compounds that are volatile; that is, materials that can be turned into gases.  Typically GC's are used for characterize very small amounts (1 or 2 micrograms) of organic compounds.  They have found routine use in drug testing, pesticide research, and many other areas of trace analysis.  We have two Agilent 6890N GC systems, both  split/splitless injectors and FID detectors.  One of these systems has an additional Agilent 5973 Quadrupolar Mass sensitive detector.
       The FID detectors (Flame Ionization Detectors) are very sensitive detectors that, essentially, burn the material as it comes off the GC column to turn it into ions and then detects those ions.  With this detector you can observe any material that can be burned, but you cannot directly identify what it is.
       The mass sensitive detector is a complete quadrupolar mass spectrometer that determines the mass spectrum of each compound instantaneously as it comes off the GC column.  The mass spectrum is then used to identify each compound. This detector is not quite as sensitive as the FID detector, but the ability to identify each compound as it comes off the column makes it a powerful analytical tool.