[kumar11]

To build on bstanley's answer, and go into a little more depth, gas chromatography can also be equipped with a mass spectrometer, which makes compound characterization more definitive, as each peak that elutes will be given a signature mass spectrum.  Although it is common to characterize a compound based on its retention time (utilizing a GC with an FID, TCD, ECD type detector etc), the main limitation is that analyte (compound in the sample) characterization will be limited only to retention time.  A gas chromatograph with a mass spectrometer detector (GC/MS), will generate a unique spectrum for each analyte, which makes this the preferred method for characterization.  This is particularly true when doing drug testing or other analysis where something needs to be proven definitively.

It is important to consider whether an analyte or mixture of analytes are appropriate to be analyzed by GC prior to doing so.  For example, compounds should be both heat stable (not easily degradable) and reasonably volatile.  Some polar molecules (such as amino acids, drug metabolites etc) by themselves are not very volatile, which is why is these compounds are often derivatized with silylation reagents (MSTFA or MTBSTFA) or acylation reagents (MBTFA, HFIP, etc) prior to analysis.