gas chromatography
/ GAS kroh-muh-TOG-ruh-fee /
Open a bottle of mixed perfume in a warm room and the lighter, more volatile scents reach your nose first while the heavy ones linger behind. Gas chromatography rides that same fact: it heats a mixture until everything is a vapour, then sweeps the vapour through a long tube with a gentle gas, and the components that prefer to stay vapour race ahead of those that keep settling onto the wall.
Formally, gas chromatography (GC) separates volatile or vaporisable compounds by carrying them, as a gas, through a column whose stationary phase is a thin liquid film or solid coating. An inert carrier gas such as helium or hydrogen is the mobile phase. Each compound partitions between the moving gas and the stationary film; the more it dissolves in the film, the longer it is held back, so compounds emerge in a sequence set by volatility and by their affinity for the film.
It matters because GC is fast, sensitive, and superb for small, stable, volatile molecules — solvents, fuels, flavours, environmental gases — and pairs naturally with mass spectrometry for identification. Its honest limit is the volatility requirement: anything that decomposes or refuses to evaporate when heated cannot be run directly, so large, fragile, or very polar molecules usually go to liquid methods instead, sometimes after chemical derivatization.
A police lab injects a tiny volume of a driver's blood headspace into a gas chromatograph; ethanol vaporises, travels through the column ahead of heavier interferences, and registers as a clean peak whose size gives the blood-alcohol level.
Vaporise the mixture, let a carrier gas sweep it through, and read the peaks in order.
Most modern GC uses long, thin capillary columns coated on the inside wall, which separate far better than the older packed columns full of granular material.