A Gas Chromatograph (GC) is an analytical instrument used for separating and analyzing compounds that can be vaporized without decomposition. In geochemistry, it is primarily used for the analysis of volatile organic compounds (VOCs) and gases in various geological samples, including fluid inclusions, petroleum, and sediments. GC can identify and quantify hydrocarbons (C1-C5, C6-C10), greenhouse gases (CO2, CH4, N2O), and other organic and inorganic volatiles. Applications include assessing source rock maturity, characterizing reservoir fluids, tracing petroleum migration, and environmental monitoring of organic pollutants in geological contexts. When coupled with Mass Spectrometry (GC-MS), it provides highly detailed molecular and isotopic information.