gas chromatography
Summary
Gas chromatography (GC) separates a vaporized mixture as an inert carrier gas moves it through a column coated with a stationary phase. Components that are more volatile and interact less with the stationary phase generally elute first, so they have shorter retention times.
Detail
Meaning: A mixture is injected, vaporized, and swept by an inert carrier gas (the mobile phase, such as helium) through a heated column coated with a liquid or solid stationary phase. Each component partitions between gas and stationary phase. Compounds that are more volatile (lower boiling point, weaker intermolecular forces) and interact less with the stationary phase spend more time in the gas and leave sooner, so they have shorter retention times. A detector records a peak for each component, and peak area reflects amount.
On the MCAT: A passage shows a chromatogram (detector signal versus time) for a mixture and asks you to assign peaks. Typically the lower-boiling alkane elutes before the higher-boiling one, and raising the column temperature shortens retention times. With a polar stationary phase, polar compounds are retained longer. Samples must be volatile and stable when heated. GC is often paired with mass spectrometry to identify each peak.
Common confusion: The mobile phase here is a gas, not a liquid as in column chromatography or HPLC, and it is inert, so separation comes from volatility and stationary-phase interactions. Also, a larger peak means more compound, not longer retention.
Sources
- Principles of Instrumental Analysis
- Quantitative Chemical Analysis
- Organic Chemistry
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.