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

1. The sample is vaporized and carried through a capillary column by an inert mobile phase (carrier gas), typically helium, nitrogen, or hydrogen.
2. Compounds partition between the mobile gas phase and a stationary phase coated on the inner wall of the column.
3. Separation occurs because each compound has a unique partition coefficient, resulting in different retention times.

Instrumentation

1. Injector: Heated to 200-300°C to rapidly vaporize the sample. Split/splitless injectors allow control over the amount of sample entering the column.
2. Carrier Gas Supply: High-purity gas with flow controllers to maintain a constant linear velocity.
3. Column: Fused silica capillary column (typically 15-60 m length, 0.25 mm ID) coated with a thin film of stationary phase (e.g., 5% phenyl-95% methyl polysiloxane).
4. Oven: Programmable temperature control from 40°C to 350°C to optimize separation of compounds across a wide boiling point range.
5. Detector: Flame Ionization Detector (FID) for carbon-containing compounds, or Mass Spectrometer (GC-MS) for structural identification.

Temperature Programming

1. Isothermal runs hold a constant temperature throughout the separation.
2. Temperature ramps start at a low temperature and increase at a defined rate (e.g., 10°C/min), improving separation of compounds with diverse volatilities.
3. Initial hold times and final hold times ensure adequate separation of early- and late-eluting compounds.

Detectors

1. Flame Ionization Detector (FID): Responds to carbon atoms; universal for organic compounds with high sensitivity.
2. Thermal Conductivity Detector (TCD): Universal but less sensitive; measures changes in thermal conductivity of the carrier gas.
3. Electron Capture Detector (ECD): Selective for halogenated compounds; used for pesticide and PCB analysis.
4. Mass Spectrometry (MS): Provides full spectral information for compound identification.

Applications

1. Analysis of fatty acid methyl esters (FAMEs) in food and biofuels.
2. Determination of residual solvents in pharmaceutical products.
3. Environmental analysis of volatile organic compounds (VOCs) in air and water.
4. Forensic identification of accelerants in arson investigations.