Glossary of Gas Chromatography Terms

Calibration standard - A gas mixture, produced either synthetically or by pumping natural air into a high-pressure cylinder, that has been systematically analyzed to assign a concentration value to a molecule or concentration values to several molecules of interest.

Carrier gas - An inert gas that constantly flows through the chromatography column and detector. Carrier gas mobilizes the sample by pushing it through the column. 'Inert' refers to non-reactivity of the carrier gas with the sample and a low response of the detector to the carrier gas.

Chlorofluorocarbons - A class of fully chlorinated/fluorinated carbon-based molecules of the general formula C_nCl_jF_k where j + k = 2n +2, k and j > 0. Uses have included refrigeration, aerosol propellants, foam blowing, and cleaning solvents.

Chromatography column - A length of metal or fused silica tubing filled or coated with one or more packing material that separates the compounds of interest as the sample mixture passes through.

Detector - A sensor that produces a measurable signal when certain molecules pass through it

Electron-capture detector (ECD) - A detector that is especially sensitive to halogen-containing molecules. Thermal electrons generated by the radioactive decay of ⁶³Ni create a small, steady current within the detector. This current is measurably reduced when certain molecules capture electrons as they pass through the detector.

Gas chromatograph (GC) - A type of instrument based on the injection of component mixtures onto a chromatography column, separation of the components as the mixture passes through the column, and sensing of the components of interest as they elute into a detector.
Gas sampling valve (GSV) - A multiple-port, two-position valve that changes the direction of gas flows through chromatography columns and other parts of an instrument. Used to fill a sample loop with sample then inject the loop contents onto the column.

Halocarbons - A class of carbon-based molecules having at least one halogen (fluorine, chlorine, iodine) atom.

Halons - A class of brominated halocarbons of the general formula C_nBr_iCl_jF_k where i + j + k = 2n + 2, i and k > 0. Halons are typically used as fire extinguishing agents.

Packing material - A granular, solid-phase powder, sometimes also coated with a liquid phase, that is used to fill a chromatography column. The packing material of a column is chosen for its ability to separate the molecules of interest from one another. Packing materials typically separate molecules by their size (molecular sieves), boiling points, or polarities.
Sample loop - A length of hollow metal tubing whose fixed volume is filled with sample to a repeatable pressure and its contents injected onto a chromatography column. Consistent sample loop volume, pressure and temperature ensure that the same amount of sample is injected for every analysis.

Stream selection valve (SSV) - A multiple-port, multiple-position valve that determines the source of gas flowing through sample loops. Used to alternately analyze air samples and calibration standards.