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.