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Gas-liquid chromatographyGas-liquid chromatographyFrom Wikipedia, the free encyclopediaFrom Wikipedia, the free encyclopedia

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Gas-liquid chromatography (GLC)Gas-liquid chromatography (GLC)

Gas-liquid chromatography (GLC)Gas-liquid chromatography (GLC), or simply , or simply gas chromatography (GC)gas chromatography (GC), is a type of , is a type of chromatographychromatography in which the mobile phase is a in which the mobile phase is a carrier gas, usually an carrier gas, usually an inertinert gas such as gas such as heliumhelium or or nitrogennitrogen, and the stationary phase is a , and the stationary phase is a microscopic layer of liquid on an inert solid microscopic layer of liquid on an inert solid support, inside glass or metal tubing, called a support, inside glass or metal tubing, called a column. The instrument to perform gas column. The instrument to perform gas chromatographic separations is called a chromatographic separations is called a gas gas chromatographchromatograph (also: (also: aerographaerograph, , gas separatorgas separator).).

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ContentsContents

1 History1 History 2 GC analysis2 GC analysis 3 Physical components3 Physical components

3.1 Inlets3.1 Inlets 3.2 Columns3.2 Columns 3.3 Detectors3.3 Detectors

4 Methods4 Methods 4.1 Carrier Gas Selection and Flow Rates4.1 Carrier Gas Selection and Flow Rates 4.2 Inlet Types and Flow Rates4.2 Inlet Types and Flow Rates 4.3 Sample Size and Injection Technique4.3 Sample Size and Injection Technique 4.4 Column Selection4.4 Column Selection 4.5 Column Temperature and Temperature Program4.5 Column Temperature and Temperature Program

5 Data Reduction and analysis5 Data Reduction and analysis 6 Application6 Application 7 GCs in Popular Culture7 GCs in Popular Culture 8 Manufacturers of gas chromatographs, columns, and supplies8 Manufacturers of gas chromatographs, columns, and supplies

8.1 Instrument manufacturers8.1 Instrument manufacturers 8.2 Gas chromatography columns and accessories8.2 Gas chromatography columns and accessories

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HistoryHistory

Chromatography dates to Chromatography dates to 19011901 in the work of the in the work of the RussianRussian scientist, scientist, Mikhail Semenovich TswettMikhail Semenovich Tswett. . GermanGerman graduate student graduate student Fritz PriorFritz Prior developed developed solid state gas chromatography in 1947. solid state gas chromatography in 1947. Archer John Porter MartinArcher John Porter Martin, who was awarded the , who was awarded the Nobel PrizeNobel Prize for his work in developing liquid-liquid for his work in developing liquid-liquid (1941) and paper (1944) chromatography, laid the (1941) and paper (1944) chromatography, laid the foundation for the development of gas foundation for the development of gas chromatography and later produced liquid-gas chromatography and later produced liquid-gas chromatography (1950).chromatography (1950).

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Diagram of a gas chromatographDiagram of a gas chromatograph

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GC analysisGC analysis

A A gas chromatographgas chromatograph is a chemical analysis is a chemical analysis instrument for separating instrument for separating chemicals in a in a complex sample. A gas chromatograph uses a complex sample. A gas chromatograph uses a flow-through narrow tube known as the flow-through narrow tube known as the columncolumn, , through which different chemical constituents of through which different chemical constituents of a sample pass in a gas stream (carrier gas, a sample pass in a gas stream (carrier gas, mobile phasemobile phase) at different rates depending on ) at different rates depending on their various chemical and physical properties their various chemical and physical properties and their interaction with a specific column filling, and their interaction with a specific column filling, called the called the stationary phasestationary phase. As the chemicals . As the chemicals exit the end of the column, they are detected exit the end of the column, they are detected and identified and identified electronicallyelectronically. .

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The function of the stationary phase in the The function of the stationary phase in the column is to separate different components, column is to separate different components, causing each one to exit the column at a causing each one to exit the column at a different time (different time (retention timeretention time). Other parameters ). Other parameters that can be used to alter the order or time of that can be used to alter the order or time of retention are the carrier gas flow rate, and the retention are the carrier gas flow rate, and the temperature. In a GC analysis, a known volume temperature. In a GC analysis, a known volume of gaseous or liquid of gaseous or liquid analyteanalyte is injected into the is injected into the "entrance" (head) of the column, usually using a "entrance" (head) of the column, usually using a micromicrosyringesyringe (or, solid phase microextraction (or, solid phase microextraction fibers, or a gas source switching system). fibers, or a gas source switching system).

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Although the carrier gas sweeps the Although the carrier gas sweeps the analyte molecules through the column, this analyte molecules through the column, this motion is inhibited by the motion is inhibited by the adsorptionadsorption of the of the analyte analyte moleculesmolecules either onto the column either onto the column walls or onto packing materials in the walls or onto packing materials in the column. The rate at which the molecules column. The rate at which the molecules progress along the column depends on the progress along the column depends on the strength of strength of adsorptionadsorption, which in turn , which in turn depends on the type of molecule and on depends on the type of molecule and on the stationary phase materials. the stationary phase materials.

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Since each type of molecule has a different rate Since each type of molecule has a different rate of progression, the various components of the of progression, the various components of the analyte mixture are separated as they progress analyte mixture are separated as they progress along the column and reach the end of the along the column and reach the end of the column at different times (retention time). A column at different times (retention time). A detector is used to monitor the outlet stream detector is used to monitor the outlet stream from the column; thus, the time at which each from the column; thus, the time at which each component reaches the outlet and the amount of component reaches the outlet and the amount of that component can be determined. Generally, that component can be determined. Generally, substances are identified by the order in which substances are identified by the order in which they emerge (elute) from the column and by the they emerge (elute) from the column and by the retention time of the analyte in the column.retention time of the analyte in the column.

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InletsInlets

The The column inletcolumn inlet (or (or injectorinjector) provides the means to ) provides the means to introduce a sample into a continuous flow of carrier gas. introduce a sample into a continuous flow of carrier gas. The inlet is a piece of hardware attached to the The inlet is a piece of hardware attached to the column column headhead..

Common inlet types are:Common inlet types are: S/SL (Split/Splitless) injectorS/SL (Split/Splitless) injector; a sample is introduced ; a sample is introduced

into a heated small chamber via a syringe through a into a heated small chamber via a syringe through a septumseptum - the heat facilitates - the heat facilitates volatilizationvolatilization of the sample of the sample and sample matrix. The carrier gas then either sweeps and sample matrix. The carrier gas then either sweeps the entirety (the entirety (splitlesssplitless mode) or a portion ( mode) or a portion (splitsplit mode) of mode) of the sample into the column. In split mode, a part of the the sample into the column. In split mode, a part of the sample/carrier gas mixture in the injection chamber is sample/carrier gas mixture in the injection chamber is exhausted through the exhausted through the split ventsplit vent. .

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COC (cool on-column) inletCOC (cool on-column) inlet; the sample is ; the sample is here introduced in its entirety without heat. here introduced in its entirety without heat.

gas source inletgas source inlet or or gas switching valvegas switching valve; ; gaseous samples in collection bottles are here gaseous samples in collection bottles are here hooked up to what is most commonly a six-port hooked up to what is most commonly a six-port switching valveswitching valve: the carrier gas flow is not : the carrier gas flow is not interrupted while a sample can be expanded into interrupted while a sample can be expanded into a previously evacuated a previously evacuated sample loopsample loop. Upon . Upon switching, the contents of the sample loop are switching, the contents of the sample loop are switched into the carrier gas stream. switched into the carrier gas stream.

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P/T (Purge-and-Trap) systemP/T (Purge-and-Trap) system; samples ; samples requiring preconcentration or purification can be requiring preconcentration or purification can be introduced via such a system, usually hooked up introduced via such a system, usually hooked up to the S/SL port. The advantage of such a to the S/SL port. The advantage of such a system is that no or few prior system is that no or few prior enrichment/purification steps in the laboratory enrichment/purification steps in the laboratory are necessary. are necessary.

in addition, in addition, SPMESPME (solid phase microextraction) (solid phase microextraction) offers a convenient, low-cost alternative to P/T offers a convenient, low-cost alternative to P/T systems with the versatility of a syringe and systems with the versatility of a syringe and simple use of the S/SL port. simple use of the S/SL port.

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ColumnsColumns

Two types of columns are used in GC:Two types of columns are used in GC: Packed columnsPacked columns are 1.5 - 10 m in length and are 1.5 - 10 m in length and

have an internal diameter of 2 - 4 mm. The have an internal diameter of 2 - 4 mm. The tubing is usually made of stainless steel or glass tubing is usually made of stainless steel or glass and contains a and contains a packingpacking of finely divided, inert, of finely divided, inert, solid support material (eg. diatomaceous earth) solid support material (eg. diatomaceous earth) that is coated with a liquid or solid stationary that is coated with a liquid or solid stationary phase. The nature of the coating material phase. The nature of the coating material determines what type of materials will be most determines what type of materials will be most strongly adsorbed. Thus numerous columns are strongly adsorbed. Thus numerous columns are available that are designed to separate specific available that are designed to separate specific types of compounds. types of compounds.

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Capillary columnsCapillary columns have a very small internal have a very small internal diameter, on the order of a few tenths of diameter, on the order of a few tenths of millimeters, and lengths between 25-60 meters millimeters, and lengths between 25-60 meters are common. The inner column walls are coated are common. The inner column walls are coated with the active materials (WCOT columns), with the active materials (WCOT columns), some columns are quasi solid filled with many some columns are quasi solid filled with many parallel micropores (PLOT columns). Most parallel micropores (PLOT columns). Most capillary columns are made of fused-capillary columns are made of fused-silicasilica with a with a polyimidepolyimide outer coating. These columns are outer coating. These columns are flexible, so a very long column can be wound flexible, so a very long column can be wound into a small coil. into a small coil.

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New developmentsNew developments are sought where stationary phase are sought where stationary phase incompatibilities lead to geometric solutions of parallel incompatibilities lead to geometric solutions of parallel columns within one column. Among these new columns within one column. Among these new developments are: developments are: Internally heated Internally heated microFASTmicroFAST columns, where two columns, an columns, where two columns, an

internal heating wire and a temperature sensor are combined internal heating wire and a temperature sensor are combined within a common column sheeth (within a common column sheeth (microFASTmicroFAST); );

Micropacked columnsMicropacked columns (1/16" OD) are column-in-column packed (1/16" OD) are column-in-column packed columns where the outer column space has a packing different columns where the outer column space has a packing different from the inner column space, thus providing the separation from the inner column space, thus providing the separation behaviour of two columns in one. They can be easily fit to inlets behaviour of two columns in one. They can be easily fit to inlets and detectors of a capillary column instrument. and detectors of a capillary column instrument.

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The The temperaturetemperature The The temperaturetemperature-dependence-dependence of molecular adsorption of molecular adsorption

and of the rate of progression along the column and of the rate of progression along the column necessitates a careful control of the column temperature necessitates a careful control of the column temperature to within a few tenths of a degree for precise work. to within a few tenths of a degree for precise work. Reducing the temperature produces the greatest level of Reducing the temperature produces the greatest level of separation, but can result in very long elution times. For separation, but can result in very long elution times. For some cases temperature is ramped either continuously some cases temperature is ramped either continuously or in steps to provide the desired separation. This is or in steps to provide the desired separation. This is referred to as a referred to as a temperature programtemperature program. Electronic . Electronic pressure control can also be used to modify flow rate pressure control can also be used to modify flow rate during the analysis, aiding in faster run times while during the analysis, aiding in faster run times while keeping acceptable levels of separation.keeping acceptable levels of separation.

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The The choice of carrier gaschoice of carrier gas

The The choice of carrier gaschoice of carrier gas ( (mobile phasemobile phase) ) is important, with hydrogen being the most is important, with hydrogen being the most efficient and providing the best separation. efficient and providing the best separation. However, helium has a larger range of However, helium has a larger range of flowrates that are comparable to hydrogen flowrates that are comparable to hydrogen in efficiency, with the added advantage in efficiency, with the added advantage that helium is non-flammable, and works that helium is non-flammable, and works with a greater number of detectors. with a greater number of detectors. Therefore, helium is the most common Therefore, helium is the most common carrier gas used.carrier gas used.

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DetectorsDetectors A number of detectors are used in gas chromatography. A number of detectors are used in gas chromatography.

The most common are the The most common are the flame ionization detectorflame ionization detector (FID) and the (FID) and the thermal conductivity detectorthermal conductivity detector (TCD). Both (TCD). Both are sensitive to a wide range of components, and both are sensitive to a wide range of components, and both work over a wide range of concentrations. While TCDs work over a wide range of concentrations. While TCDs are essentially universal and can be used to detect any are essentially universal and can be used to detect any component other than the carrier gas (as long as their component other than the carrier gas (as long as their thermal conductivities are different than that of the thermal conductivities are different than that of the carrier gas, at detector temperature), FIDs are sensitive carrier gas, at detector temperature), FIDs are sensitive primarily to hydrocarbons, and are more sensitive to primarily to hydrocarbons, and are more sensitive to them than TCD. Both detectors are also quite robust. them than TCD. Both detectors are also quite robust. Since TCD is non-destructive, it can be operated in-Since TCD is non-destructive, it can be operated in-series before an FID (destructive), thus providing series before an FID (destructive), thus providing complementary detection of the same eluents.complementary detection of the same eluents.

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Other detectors are sensitive only to specific types of substances, or Other detectors are sensitive only to specific types of substances, or work well only in narrower ranges of concentrations. They include:work well only in narrower ranges of concentrations. They include:

discharge ionization detedischarge ionization detectorctor (DID) (DID)

electron capture detectorelectron capture detector (ECD) (ECD)

flame photometric detectoflame photometric detectorr (FPD) (FPD)

Hall electrolytic conductiviHall electrolytic conductivity detectorty detector (ElCD) (ElCD)

helium ionization detectorhelium ionization detector (HID) (HID)

nitrogen phosphorus detenitrogen phosphorus detectorctor (NPD) (NPD)

mass selective detectormass selective detector (MSD) (MSD)

photo-ionization detectorphoto-ionization detector (PID) (PID)

pulsed discharge ionizatiopulsed discharge ionization detectorn detector (PDD) (PDD)

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MethodsMethods The The methodmethod is the collection of conditions in is the collection of conditions in

which the GC operates for a given analysis. which the GC operates for a given analysis. Method developmentMethod development is the process of is the process of determining what conditions are adequate determining what conditions are adequate and/or ideal for the analysis required.and/or ideal for the analysis required.

Conditions which can be varied to accommodate Conditions which can be varied to accommodate a required analysis include inlet temperature, a required analysis include inlet temperature, detector temperature, column temperature and detector temperature, column temperature and temperature program, carrier gas and carrier gas temperature program, carrier gas and carrier gas flow rates, the column's stationary phase, flow rates, the column's stationary phase, diameter and length, inlet type and flow rates, diameter and length, inlet type and flow rates, sample size and injection technique. sample size and injection technique.

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Depending on the detector(s) (see below) Depending on the detector(s) (see below) installed on the GC, there may be a installed on the GC, there may be a number of detector conditions that can number of detector conditions that can also be varied. Some GCs also include also be varied. Some GCs also include valves which can change the route of valves which can change the route of sample and carrier flow, and the timing of sample and carrier flow, and the timing of the turning of these valves can be the turning of these valves can be important to method development.important to method development.

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Carrier Gas Selection and Flow RatesCarrier Gas Selection and Flow Rates

The carrier gas is the mobile phase in a GC. Typical The carrier gas is the mobile phase in a GC. Typical carrier gases include carrier gases include heliumhelium, , nitrogennitrogen, , argonargon, , hydrogenhydrogen and and airair. Which gas to use is usually determined by the . Which gas to use is usually determined by the detector being used, for example, a detector being used, for example, a DIDDID requires helium requires helium as the carrier gas. When analyzing gas samples, as the carrier gas. When analyzing gas samples, however, the carrier is sometimes selected based on the however, the carrier is sometimes selected based on the sample's matrix, for example, when analyzing a mixture sample's matrix, for example, when analyzing a mixture in argon, an argon carrier is preferred, because the in argon, an argon carrier is preferred, because the argon in the sample does not show up on the argon in the sample does not show up on the chromatogram. Safety and availability can also influence chromatogram. Safety and availability can also influence carrier selection, for example, hydrogen is flammable, carrier selection, for example, hydrogen is flammable, and high-purity helium can be difficult to obtain in some and high-purity helium can be difficult to obtain in some areas of the world. (See: areas of the world. (See: Helium--occurrence and productionHelium--occurrence and production.).)

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The purity of the carrier gas is also frequently The purity of the carrier gas is also frequently determined by the detector, though the level of sensitivity determined by the detector, though the level of sensitivity needed can also play a significant role. Typically, purities needed can also play a significant role. Typically, purities of 99.995% or higher are used. Trade names for typical of 99.995% or higher are used. Trade names for typical purities include "Zero Grade," "Ultra-High Purity (UHP) purities include "Zero Grade," "Ultra-High Purity (UHP) Grade," "4.5 Grade" and "5.0 Grade."Grade," "4.5 Grade" and "5.0 Grade."

The carrier gas flow rate affects the analysis in the same The carrier gas flow rate affects the analysis in the same way that temperature does (see above). The higher the way that temperature does (see above). The higher the flow rate the faster the analysis, but the lower the flow rate the faster the analysis, but the lower the separation between analytes. Selecting the flow rate is separation between analytes. Selecting the flow rate is therefore the same compromise between the level of therefore the same compromise between the level of separation and length of analysis as selecting the separation and length of analysis as selecting the column temperature.column temperature.

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With GCs made before the With GCs made before the 1990s1990s, carrier flow , carrier flow rate was controlled indirectly by controlling the rate was controlled indirectly by controlling the carrier inlet pressure, or "column head carrier inlet pressure, or "column head pressure." The actual flow rate was measured at pressure." The actual flow rate was measured at the outlet of the column or the detector with an the outlet of the column or the detector with an electronic flow meter, or a bubble flow meter, electronic flow meter, or a bubble flow meter, and could be an involved, time consuming, and and could be an involved, time consuming, and frustrating process. The pressure setting was not frustrating process. The pressure setting was not able to be varied during the run, and thus the able to be varied during the run, and thus the flow was essentially constant during the flow was essentially constant during the analysis.analysis.

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Many modern GCs, however, Many modern GCs, however, electronically measure the flow rate, and electronically measure the flow rate, and electronically control the carrier gas electronically control the carrier gas pressure to set the flow rate. pressure to set the flow rate. Consequently, carrier pressures and flow Consequently, carrier pressures and flow rates can be adjusted during the run, rates can be adjusted during the run, creating pressure/flow programs similar to creating pressure/flow programs similar to temperature programs.temperature programs.

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A gas chromatography oven, open to A gas chromatography oven, open to show a capillary columnshow a capillary column

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Column Temperature and Temperature Column Temperature and Temperature ProgramProgram

The column(s) in a GC are contained in an oven, the The column(s) in a GC are contained in an oven, the temperature of which is precisely controlled temperature of which is precisely controlled electronically. (When discussing the "temperature of the electronically. (When discussing the "temperature of the column," an analyst is technically referring to the column," an analyst is technically referring to the temperature of the column oven. The distinction, temperature of the column oven. The distinction, however, is not important and will not subsequently be however, is not important and will not subsequently be made in this article.)made in this article.)

The rate at which a sample passes through the column The rate at which a sample passes through the column is directly proportional to the temperature of the column. is directly proportional to the temperature of the column. The higher the column temperature, the faster the The higher the column temperature, the faster the sample moves through the column. However, the faster sample moves through the column. However, the faster a sample moves through the column, the less it interacts a sample moves through the column, the less it interacts with the stationary phase, and the less the analytes are with the stationary phase, and the less the analytes are separated.separated.

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Column Temperature and Temperature ProgramColumn Temperature and Temperature Program

In general, the column temperature is selected to In general, the column temperature is selected to compromise between the length of the analysis and the compromise between the length of the analysis and the level of separation.level of separation.

A method which holds the column at the same A method which holds the column at the same temperature for the entire analysis is called "isothermal." temperature for the entire analysis is called "isothermal." Most methods, however, increase the column Most methods, however, increase the column temperature during the analysis, the initial temperature, temperature during the analysis, the initial temperature, rate of temperature increase (the temperature "ramp") rate of temperature increase (the temperature "ramp") and final temperature is called the "and final temperature is called the "temperature temperature programprogram."."

A temperature program allows analytes that elute early A temperature program allows analytes that elute early in the analysis to separate adequately, while shortening in the analysis to separate adequately, while shortening the time it takes for late-eluting analytes to pass through the time it takes for late-eluting analytes to pass through the column.the column.

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ApplicationApplication

In general, substances that vaporize below ca. 300 °C In general, substances that vaporize below ca. 300 °C (and therefore are stable up to that temperature) can be (and therefore are stable up to that temperature) can be measured quantitatively. The samples are also required measured quantitatively. The samples are also required to be to be saltsalt-free; they should not contain -free; they should not contain ionsions. Very minute . Very minute amounts of a substance can be measured, but it is often amounts of a substance can be measured, but it is often required that the sample must be measured in required that the sample must be measured in comparison to a sample containing the pure, suspected comparison to a sample containing the pure, suspected substance.substance.

Various Various temperature programstemperature programs can be used to make the can be used to make the readings more meaningful; for example to differentiate readings more meaningful; for example to differentiate between substances that behave similarly during the GC between substances that behave similarly during the GC processprocess

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Professionals working with GC analyze the content of a Professionals working with GC analyze the content of a chemical product, for example in assuring the quality of chemical product, for example in assuring the quality of products in the chemical industry; or measuring toxic products in the chemical industry; or measuring toxic substances in soil, air or water. GC is very accurate if substances in soil, air or water. GC is very accurate if used properly and can measure used properly and can measure picomolespicomoles of a of a substance in a 1 ml liquid sample, or substance in a 1 ml liquid sample, or parts-per-billionparts-per-billion concentrations in gaseous samples.concentrations in gaseous samples.

In practical courses at colleges, students sometimes get In practical courses at colleges, students sometimes get acquainted to the GC by studying the contents of acquainted to the GC by studying the contents of LavenderLavender oil or measuring the oil or measuring the ethyleneethylene that is secreted that is secreted by by Nicotiana benthamianaNicotiana benthamiana plants after artificially injuring plants after artificially injuring their leaves. These GC analyses are done rather quickly their leaves. These GC analyses are done rather quickly (1 to 15 minutes per sample) and therefore suited for (1 to 15 minutes per sample) and therefore suited for such courses.such courses.

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One example of the use of gas chromatography One example of the use of gas chromatography is in the study of the selectivity of Fischer-is in the study of the selectivity of Fischer-Tropsch synthesis catalysts. The outlet from this Tropsch synthesis catalysts. The outlet from this process contains a number of light gases process contains a number of light gases including H2, CO, CO2 and CH4, as well as including H2, CO, CO2 and CH4, as well as heavier parafinic and olefinic hydrocarbons (C2-heavier parafinic and olefinic hydrocarbons (C2-C40+). In a typical experiment, a packed column C40+). In a typical experiment, a packed column is used to separate the light gases, which are is used to separate the light gases, which are then detected with a TCD. The hydrocarbons are then detected with a TCD. The hydrocarbons are separated using a capillary column and detected separated using a capillary column and detected with an FID. with an FID.

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A complication with light gas analyses that include H2 is A complication with light gas analyses that include H2 is that He, which is the most common and most sensitive that He, which is the most common and most sensitive inert carrier (sensitivity is proportional to molecular inert carrier (sensitivity is proportional to molecular mass) has an almost identical thermal conductivity to mass) has an almost identical thermal conductivity to hydrogen (it is the difference in thermal conductivity hydrogen (it is the difference in thermal conductivity between two separate filaments in a Wheatstone Bridge between two separate filaments in a Wheatstone Bridge type arrangement that shows when a component has type arrangement that shows when a component has been eluted). For this reason, dual TCD instruments are been eluted). For this reason, dual TCD instruments are used with a separate channel for hydrogen that uses used with a separate channel for hydrogen that uses nitrogen as a carrier are common. Argon in often used nitrogen as a carrier are common. Argon in often used when analysing gas phase chemistry reactions such as when analysing gas phase chemistry reactions such as F-T synthesis so that a single carrier gas can be used F-T synthesis so that a single carrier gas can be used rather than 2 separate ones. The sensitivity is less but rather than 2 separate ones. The sensitivity is less but this is a tradeoff for simplicity in the gas supply.this is a tradeoff for simplicity in the gas supply.

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GCs in Popular CultureGCs in Popular Culture

Movies, books and TV shows tend to Movies, books and TV shows tend to misrepresent the capabilities of gas misrepresent the capabilities of gas chromatography and the work with these chromatography and the work with these machines.machines.

In the U.S. TV show CSI, for example, GCs are In the U.S. TV show CSI, for example, GCs are used to rapidly identify unknown samples. "This used to rapidly identify unknown samples. "This is gasoline bought at a Chevron station in the is gasoline bought at a Chevron station in the past two weeks," the analyst will say fifteen past two weeks," the analyst will say fifteen minutes after receiving the sample.minutes after receiving the sample.

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In fact, a GC analysis takes much more time; In fact, a GC analysis takes much more time; sometimes a single sample must be run more sometimes a single sample must be run more than an hour according to the chosen program; than an hour according to the chosen program; and even more time is needed to "heat out" the and even more time is needed to "heat out" the column so it is free from the first sample and can column so it is free from the first sample and can be used for the next. Equally, several runs are be used for the next. Equally, several runs are needed to confirm the results of a study - a GC needed to confirm the results of a study - a GC analysis of a single sample may simply yield a analysis of a single sample may simply yield a result per chance (see statistical significance).result per chance (see statistical significance).

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Also, GC does not positively identify most samples; and Also, GC does not positively identify most samples; and not all substances in a sample will necessarily be not all substances in a sample will necessarily be detected. All a GC truly tells you is at which relative time detected. All a GC truly tells you is at which relative time a component eluted from the column and that the a component eluted from the column and that the detector was sensitive to it. To make results meaningful, detector was sensitive to it. To make results meaningful, analysts need to know which components at which analysts need to know which components at which concentrations are to be expected; and even then a concentrations are to be expected; and even then a small amount of a substance can hide itself behind a small amount of a substance can hide itself behind a substance having both a higher concentration and the substance having both a higher concentration and the same relative elution time. Last but not least it is often same relative elution time. Last but not least it is often needed to check the results of the sample against a GC needed to check the results of the sample against a GC analysis of a reference sample containing only the analysis of a reference sample containing only the suspected substance.suspected substance.

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A GC-MS can remove much of this ambiguity, A GC-MS can remove much of this ambiguity, since the mass spectrometer will identify the since the mass spectrometer will identify the component's molecular weight. But this still component's molecular weight. But this still takes time and skill to do properly.takes time and skill to do properly.

Similarly, most GC analyses are not push-button Similarly, most GC analyses are not push-button operations. You cannot simply drop a sample operations. You cannot simply drop a sample vial into an auto-sampler's tray, push a button vial into an auto-sampler's tray, push a button and have a computer tell you everything you and have a computer tell you everything you need to know about the sample. According to need to know about the sample. According to the substances one expects to find the operating the substances one expects to find the operating program must be carefully chosen.program must be carefully chosen.

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A push-button operation can exist for A push-button operation can exist for running similar samples repeatedly, such running similar samples repeatedly, such as in a chemical production environment as in a chemical production environment or for comparing 20 samples from the or for comparing 20 samples from the same experiment to calculate the mean same experiment to calculate the mean content of the same substance. However, content of the same substance. However, for the kind of investigative work portrayed for the kind of investigative work portrayed in books, movies and TV shows this is in books, movies and TV shows this is clearly not the caseclearly not the case