Gas chromatographs separate organic chemical mixtures according to physical properties. Instruments consist of a flowing mobile phase, an injection port, a separation column with a stationary phase, and a detector. Differences in partitioning behavior between the mobile phase and the stationary phase cause the separation of compounds. Gas chromatographs use many different detection methods. Choices include: thermal conductivity detectors (TCD), flame ionization detectors (FID), atomic emission detectors (AED), electron capture detectors (ECD), photo-ionization detectors (PID), flame photometric detectors, and nitrogen phosphorous detectors. Gas chromatography that uses chemiluminescence spectroscopy is also available. Some gas chromatographs are used with agricultural chemicals and pesticides, cosmetics and fragrances, drugs and pharmaceuticals, electronics, energy and environmental applications, food and beverage processing, forensics and narcotics, industrial gases, medical and biological applications, or metals and welding. Others are used with organic and inorganic chemicals; paint, castings, and inks; paper, pulp and wood; petroleum products; plastics, polymers, and resins; or soaps and cleaners.
Oven operation, pump specifications, and injection operation are the main parameters for gas chromatographs. Devices with a column oven carry specifications such as operational method and oven temperature range. There are two operational methods for columnar ovens: isothermal or constant-temperature, and variable or temperature-programmable. Flow control and inlet pressure range are common pump specifications. In terms of injection operation, parameters include number of injection ports, injection method, and column injection. For optimum efficiency, a sample should not be too large for a column, and is best introduced as a "plug" of vapor. There are two choices for column injection method: split/split-less and packed column. With split or split-less column injection, the injector has an additional set of gas lines, providing another path for the vaporized sample. This extra line is sometimes called a split line or vent. Importantly, the amount of gas that exits the split line controls the amount of sample that enters the column. With packed column injection, all samples enter the column - including the gaseous sample, solvent, and analyte.
Specifications for gas chromatographs may include user interface options, display options, additional output options, and general features and functionality. Local interfaces use an analog front panel, a digital front panel, or a computer interface with parallel or serial communications. Analog meters, digital readouts, and video displays are common choices for display options. Additional output options include: analog voltage, frequency or modulated frequency, analog current, and switch or relay contacts. Some gas chromatographs are programmable, self-calibrating, or include a built-in chromatogram. Others provide data storage options and are designed for extreme environments. Gas chromatographs that accept liquid samples and include self-test diagnostics are also available.