Gas Chromatography (GC) is generally used to isolate specific compounds from other similar compounds and to quantify each species individually. Separation of the compounds is accomplished by passing the sample through a column which "holds up" different chemical species for different amounts of time. The individual compounds elute to the detector as separate "peaks". The peaks are then identified by their residence time on the column.  

The most common GC methods are EPA Method 18 for speciation of volatile organic compounds (VOC) and EPA Methods 15 and 16 which are both for the determination of reduced sulfur compounds. Methods 15 and 16 are specific to a particular set of target compounds, a particular GC detector (flame photometric aka FPD), and a particular sampling system. While the methods are very similar, Method 15 is geared toward refinery testing and Method 16 is more appropriate for the pulp and paper industry (Kraft Mills in particular).
EPA Method 18, on the other hand is considerably more flexible and allows determination of a wide variety of organic species and may use nearly any type of detector, although generally flame ionization detectors (FID) or photo-ionization detectors (PID) are the most common.

Method 18 also allows flexibility in the sampling techniques including collecting the sample in vessels such as Tedlar bags or onto adsorbent media, or also by direct interface to the GC.  Method 18 analysis may be conducted on-site or off-site at a laboratory. One useful application for Method 18 is determining the Methane and/or Ethane content of a sample stream for subtraction of a total hydrocarbon (EPA Method 25A) test result in order to determine the total "volatile organic compounds" (VOC) emissions. (Methane and Ethane are not considered VOC by EPA's definition since they are not photo-reactive compounds and do not contribute to production of tropospheric ozone - i.e. don't contribute to smog formation.)  Other applications of Method 18 include determining the presence of specific Hazardous Air Pollutants (HAPS) as defined by the Clean Air Act Amendments of 1990 or to screen a sample matrix for any number of compounds (e.g. BTEX compounds). Method 18 can also be used to determine reaction efficiency, scrubber efficiency or destruction removal efficiency (DRE) for selected compounds in various processes and emissions control device configurations.