gigahertz

(gĭg'ə-hûrtz', jĭg'-)
n. (Abbr. GHz)
A unit of frequency equal to one billion (10⁹) hertz. Also called gigacycle.

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One billion cycles per second. See GHz.

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The process and technique of applying an unknown mixture onto a supportive substrate, while blowing heated gas over and throughout the internal surface of a thin to finely stretched metal or glass column. Due to the nature of the affinity of the unknown compounds to the substrate and their relative boiling points, the chemicals will move (elute) through the column at differing rates according to their specific characteristics. The detector port at the other end is usually of two general types: a flame ionization port or a thermal conductivity unit. A flame ionization device burns off the product, and cannot be used to smell the exiting compounds to aid in their identification. For this purpose, a splitter could be used to divert some of the effluent for organoleptic analysis. Alternatively, the use of a thermal conductivity unit would allow someone to identify aromas as they exited the machine at the port site and not be destroyed in the flame of a flame ionization device. Since the advent of mass spectroscopy and computers, recent systems hook up computerized databases that contain mass spectrographic fragmentation patterns to systematically identify the unknowns. For the purpose of quality control and identification of purity, and perhaps authenticity of essential oils, GC can certainly be of great use. However, due to time constraints, conflicting identification, inaccuracy of sensitivity versus the human olfactory system, and a whole host of artifacts that might be present, to date much of the duplication work that is done in the flavor industry is still done by flavor experts trained in organoleptic type duplication. The two generally used columns substrates fall into the categories of a polar and non-polar column packing. Lengths of column vary for increased separation and therefore sensitivity. The polar-type columns (an example is Carbowax™) respond poorly to organic acids and water. The non-polar silicon-based packing reacts better for separation and identification of acids. Very thin columns known as capillary columns increase the degree of separation significantly. A proprietry-developed methodology that couples aroma description with NMR spectra is called SNIFF NMR. This is a more recently developed protocol for the identification of unknowns blends. See Mass Spectroscopy, Duplication, Chiral Columns, Gas Chromatography - Olfactometry (GCO).

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