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HISTORY Chromatography dates to 1903 in the work of the Russia n scientist, Mikhail Semenovich Tswett . German graduate student Fritz Prior developed Solid State Gas Chromatography in 1947. Archer John Porter Martin , who was awarded the Nobel Prize for his work in developing liquid-liquid (1941) and paper (1944) chromatography, laid the foundation for the development of gas chromatography and later produced liquid-gas chromatography (1950). GC ANALYSIS A gas chromatograph is a chemical analysis instrument for separating Chemical s in a sample. A gas chromatograph 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 micro Syringe . Although the carrier gas sweeps the analyte molecules through the column, this motion is inhibited by the Adsorption of the analyte Molecule s 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 Adsorption , 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:
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. DETECTORS 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 sensitive only to specific types of substances. Other detectors include:
Some gas chromatographs are connected to a Mass Spectrometer which acts as the detector. The combination is known as GC-MS . APPLICATIONS One example of the use of gas chromatography is in the study of the selectivity of Fischer-Tropsch Synthesis catalysts. The outlet from this process contains a number of light gases including N2, H2, CO, CO2, H2, CH4, and Ar, as well as heavier parafinic and olefinic hydrocarbons (C2-C40). In a typical experiment, a packed column is used to separate the light gases, which are then detected with a TCD. The Hydrocarbon s are separated using a capillary column and detected with an FID. GCS IN POPULAR CULTURE Movies, books and TV shows tend to misrepresent the capabilities of gas chromatography. In the U.S. TV show , for example, GCs are used to rapidly identify unknown samples. "This is Gasoline bought at a Chevron station in the past two weeks," the Analyst will say fifteen minutes after receiving the sample. GC analysis tends to take much more time than is shown. Some individual 'runs' can take over an hour, and more time is required between runs to let the Instrument return to its starting conditions. Multiply runs are also typically done to confirm results, as are multiple runs of a known standard to confirm the proper operation of the instrument. Analyzing a single sample can take hours and hours, not 15 minutes. Also, GCs do not positively identify most samples. Because of detector--and instrument--selectivity not all components in a sample will necessarily be detected. All a GC truly tells you ''at best'' is what relative time a component eluted from the column and that the detector was sensitive to it. In general, an analysts need to know what components at what concentrations to expect in a sample to do any kind of meaningful analysis. And even then there can be ambiguity in the results, since some components may elute at similar times. A GC-MS can remove much of this ambiguity, since the Mass Spectrometer will identify the component's molecular weight. But this still takes time and skill to do properly. Speaking of time and skill, most GC analyses are not push-button operations. You cannot simply drop a sample vial into an auto-sampler's tray, push a button and have a computer tell you everything you need to know about the sample. A push-button operation can exist for running similar samples repeatedly, such as in a chemical production enviroment. However, for the kind of investigative work portrayed in books, movies and TV shows, this is not the case. The proper equipment must be selected and installed. Operation conditions must be determined. Sample preparation methods must be developed. Known standards must be created or obtained and used to prove the capabilities of the instrument. The sample must be run repeatedly, and the results interpretted by a trained analyst. Gas chromatography is a valuable and widely used scientific technique. However, it's not as fast, simple and all-knowing as popular culture portrays it to be. MANUFACTURERS OF GAS CHROMATOGRAPHS
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