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Gas Chromatography

Gas Chromatograph (GC) is a chemical analysis instrument for separating and identifying chemicals in a sample. GC uses a thin capillary fiber known as the column, through which different chemicals pass at different rates depending on various chemical and physical properties. As the chemicals exit the end of the column, they are detected and identified electronically. The function of the column is to separate and concentrate different components in order to maximize the detection signal.

In a GC analysis, a known volume of gaseous or liquid analyte is injected into the entrance of the column, usually using a microsyringe. Although the carrier gas sweeps the analyte molecules through the column, this motion is inhibited by the absorption of the analyte molecules either onto the column walls or onto packing materials in the column. The rate at which the molecules progress along the column depends on the strength of absorption, which in turn depends on the type of molecule and on the column materials. Since each type of molecule has a different rate of progression, the various components of the analyte mixture are separated as they progress along the column and reach the end of the column at different times. A detector is used to monitor the outlet stream from the column; thus, the time at which each component reaches the outlet and the amount of that component can be determined. Generally, substances are identified by the order in which they emerge from the column and by the residence time of the analyte in the column. Two types of columns are used in GC: Packed columns and Capillary columns.

Because molecular adsorption and the rate of progression along the column depend on the temperature, the column temperature is carefully controlled to within a few tenths of a degree for precise work. Reducing the temperature produces the greatest level of separation, but can result in very long elution times. For some cases temperature is ramped either continuously or in steps to provide the desired separation.

A number of detectors are used in gas chromatography. The most common one is the thermal conductivity detector (TCD), which monitors changes in the thermal conductivity of the effluent. The main advantage of the TCD is that it can detect any substance (except the carrier gas). Some of the other detectors are only sensitive to specific types of substances. Other detectors include the flame ionization detector (FID), electron capture detector (ECD), flame photometric detector (FPD), photo-ionization detector (PID), and Hall electrolytic conductivity detector.



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