Gas Chromatography (GC) is a chromatographic technique used to separate, identify, and quantify volatile and semi-volatile compounds. It is widely employed in environmental monitoring, petrochemical analysis, food safety, and forensic toxicology.
Principle of Separation
The sample is vaporized and carried through a capillary column by an inert mobile phase (carrier gas), typically helium, nitrogen, or hydrogen. Compounds partition between the mobile gas phase and a stationary phase coated on the inner wall of the column. Separation occurs because each compound has a unique partition coefficient, resulting in different retention times.
Instrumentation
The GC system consists of several key components. The injector is heated to 200-300°C to rapidly vaporize the sample, and split/splitless injectors allow control over the amount of sample entering the column. A high-purity carrier gas supply with flow controllers maintains a constant linear velocity. The fused silica capillary column (typically 15-60 m length, 0.25 mm ID) is coated with a thin film of stationary phase such as 5% phenyl-95% methyl polysiloxane. A programmable oven provides temperature control from 40°C to 350°C to optimize separation across a wide boiling point range. Common detectors include the Flame Ionization Detector (FID) for carbon-containing compounds and mass spectrometers (GC-MS) for structural identification.
Temperature Programming
Isothermal runs hold a constant temperature throughout the separation. Temperature ramps start at a low temperature and increase at a defined rate (e.g., 10°C/min), improving the separation of compounds with diverse volatilities. Initial and final hold times ensure adequate separation of early- and late-eluting compounds.
Detectors
Several detectors are available for GC. The Flame Ionization Detector (FID) responds to carbon atoms and is universal for organic compounds with high sensitivity. The Thermal Conductivity Detector (TCD) is universal but less sensitive, measuring changes in thermal conductivity of the carrier gas. The Electron Capture Detector (ECD) is selective for halogenated compounds and is used for pesticide and PCB analysis. Mass spectrometry (MS) provides full spectral information for compound identification.
Applications
GC is used for analysis of fatty acid methyl esters (FAMEs) in food and biofuels, determination of residual solvents in pharmaceutical products, environmental analysis of volatile organic compounds (VOCs) in air and water, and forensic identification of accelerants in arson investigations.
