Misplaced Pages

#222777

80-399: Gas chromatography is also sometimes known as vapor-phase chromatography ( VPC ), or gas–liquid partition chromatography ( GLPC ). These alternative names, as well as their respective abbreviations, are frequently used in scientific literature. Gas chromatography is the process of separating compounds in a mixture by injecting a gaseous or liquid sample into a mobile phase, typically called

160-473: A charcoal column that also used mercury. Gerhard Hesse, while a professor at the University of Marburg /Lahn decided to test the prevailing opinion among German chemists that molecules could not be separated in a moving gas stream. He set up a simple glass column filled with starch and successfully separated bromine and iodine using nitrogen as the carrier gas. He then built a system that flowed an inert gas through

240-441: A combustion chamber and scrubber unit supplied by fans and a refrigeration unit which cools the gas. A drier in series with the system removes moisture from the gas before it is supplied to the deck. Cargo tanks on gas carriers are not inerted, but the whole space around them is. Inert gas is produced on board commercial and military aircraft in order to passivate fuel tanks. On hot days, fuel vapour in fuel tanks may otherwise form

320-413: A constant sensitivity over long period of time. In addition, when alkali ions are not added to the flame, AFD operates like a standard FID. A catalytic combustion detector (CCD) measures combustible hydrocarbons and hydrogen. Discharge ionization detector (DID) uses a high-voltage electric discharge to produce ions. Flame photometric detector (FPD) uses a photomultiplier tube to detect spectral lines of

400-519: A current between the electrodes. The increase in current is translated and appears as a peak in a chromatogram. FIDs have low detection limits (a few picograms per second) but they are unable to generate ions from carbonyl containing carbons. FID compatible carrier gasses include helium, hydrogen, nitrogen, and argon. In FID, sometimes the stream is modified before entering the detector. A methanizer converts carbon monoxide and carbon dioxide into methane so that it can be detected. A different technology

480-436: A detector response. Nitrogen–phosphorus detector (NPD), a form of thermionic detector where nitrogen and phosphorus alter the work function on a specially coated bead and a resulting current is measured. Dry electrolytic conductivity detector (DELCD) uses an air phase and high temperature (v. Coulsen) to measure chlorinated compounds. Mass spectrometer (MS), also called GC-MS ; highly effective and sensitive, even in

560-421: A fire and explosion prevention measure. At the bench scale, chemists perform experiments on air-sensitive compounds using air-free techniques developed to handle them under inert gas. Helium, neon, argon, krypton, xenon, and radon are inert gases. Inert gas is produced on board crude oil carriers (above 8,000 tonnes from Jan 1, 2016) by burning kerosene in a dedicated inert gas generator . The inert gas system

640-427: A flammable or explosive mixture which if oxidized, could have catastrophic consequences. Conventionally, Air Separation Modules (ASMs) have been used to generate inert gas. ASMs contain selectively permeable membranes. They are fed compressed air that is extracted from a compressor stage of a gas turbine engine. The pressure drives the separation of oxygen from the air due to the increased permeability of oxygen through

720-427: A gas switching valve system; adsorbed samples (e.g., on adsorbent tubes) are introduced using either an external (on-line or off-line) desorption apparatus such as a purge-and-trap system, or are desorbed in the injector (SPME applications). The real chromatographic analysis starts with the introduction of the sample onto the column. The development of capillary gas chromatography resulted in many practical problems with

800-543: A glass condenser packed with silica gel and collected the eluted fractions. Courtenay S.G Phillips of Oxford University investigated separation in a charcoal column using a thermal conductivity detector. He consulted with Claesson and decided to use displacement as his separating principle. After learning about the results of James and Martin, he switched to partition chromatography. Early gas chromatography used packed columns, made of block 1–5 m long, 1–5 mm diameter, and filled with particles. The resolution of packed columns

880-400: A length of 5–60 metres (16–197 ft). The GC column is located inside an oven where the temperature of the gas can be controlled and the effluent coming off the column is monitored by a suitable detector. A gas chromatograph is made of a narrow tube, known as the column , through which the vaporized sample passes, carried along by a continuous flow of inert or nonreactive gas. Components of

SECTION 10

#1732856132223

960-545: A number of advantages. Flame ionization detectors cannot detect inorganic substances and some highly oxygenated or functionalized species like infrared and laser technology can. In some systems, CO and CO 2 can be detected in the FID using a methanizer , which is a bed of Ni catalyst that reduces CO and CO 2 to methane, which can be in turn detected by the FID. The methanizer is limited by its inability to reduce compounds other than CO and CO 2 and its tendency to be poisoned by

1040-445: A number of chemicals commonly found in gas chromatography effluents. Another important disadvantage is that the FID flame oxidizes all oxidizable compounds that pass through it; all hydrocarbons and oxygenates are oxidized to carbon dioxide and water and other heteroatoms are oxidized according to thermodynamics. For this reason, FIDs tend to be the last in a detector train and also cannot be used for preparatory work. An improvement to

1120-404: A number of problems inherent in the use of syringes for injection. Even the best syringes claim an accuracy of only 3%, and in unskilled hands, errors are much larger. The needle may cut small pieces of rubber from the septum as it injects sample through it. These can block the needle and prevent the syringe filling the next time it is used. It may not be obvious that this has happened. A fraction of

1200-403: A potential difference. The positive electrode acts as the nozzle head where the flame is produced. The other, negative electrode is positioned above the flame. When first designed, the negative electrode was either tear-drop shaped or angular piece of platinum. Today, the design has been modified into a tubular electrode, commonly referred to as a collector plate. The ions thus are attracted to

1280-436: A radioactive beta particle (electron) source to measure the degree of electron capture. ECD are used for the detection of molecules containing electronegative / withdrawing elements and functional groups like halogens, carbonyl, nitriles, nitro groups, and organometalics. In this type of detector either nitrogen or 5% methane in argon is used as the mobile phase carrier gas. The carrier gas passes between two electrodes placed at

1360-506: A small number of samples), to robotic technologies (XYZ robot vs. rotating robot – the most common), or to analysis: The column inlet (or injector) provides the means to introduce a sample into a continuous flow of carrier gas. The inlet is a piece of hardware attached to the column head. Common inlet types are: The choice of carrier gas (mobile phase) is important. Hydrogen has a range of flow rates that are comparable to helium in efficiency. However, helium may be more efficient and provide

1440-421: A small quantity of sample. This detector can be used to identify the analytes in chromatograms by their mass spectrum. Some GC-MS are connected to an NMR spectrometer which acts as a backup detector. This combination is known as GC-MS-NMR . Some GC-MS-NMR are connected to an infrared spectrophotometer which acts as a backup detector. This combination is known as GC-MS-NMR-IR. It must, however, be stressed this

1520-445: A substance can be measured, but it is often required that the sample must be measured in comparison to a sample containing the pure, suspected substance known as a reference standard . Various temperature programs can be used to make the readings more meaningful; for example to differentiate between substances that behave similarly during the GC process. Professionals working with GC analyze

1600-432: A substitute for an inert gas. This is useful when an appropriate pseudo-inert gas can be found which is inexpensive and common. For example, carbon dioxide is sometimes used in gas mixtures for GMAW because it is not reactive to the weld pool created by arc welding. But it is reactive to the arc. The more carbon dioxide that is added to the inert gas, such as argon, will increase your penetration. The amount of carbon dioxide

1680-415: A variety of applications, they are generally used to prevent unwanted chemical reactions with the oxygen ( oxidation ) and moisture ( hydrolysis ) in the air from degrading a sample. Generally, all noble gases except oganesson ( helium , neon , argon , krypton , xenon , and radon ), nitrogen , and carbon dioxide are considered inert gases. The term inert gas is context-dependent because several of

SECTION 20

#1732856132223

1760-820: Is helium . Flame ionization detector A flame ionization detector ( FID ) is a scientific instrument that measures analytes in a gas stream. It is frequently used as a detector in gas chromatography . The measurement of ions per unit time makes this a mass sensitive instrument. Standalone FIDs can also be used in applications such as landfill gas monitoring , fugitive emissions monitoring and internal combustion engine emissions measurement in stationary or portable instruments. The first flame ionization detectors were developed simultaneously and independently in 1957 by McWilliam and Dewar at Imperial Chemical Industries of Australia and New Zealand (ICIANZ, see Orica history ) Central Research Laboratory, Ascot Vale, Melbourne , Australia . and by Harley and Pretorius at

1840-464: Is also frequently determined by the detector, though the level of sensitivity needed can also play a significant role. Typically, purities of 99.995% or higher are used. The most common purity grades required by modern instruments for the majority of sensitivities are 5.0 grades, or 99.999% pure meaning that there is a total of 10 ppm of impurities in the carrier gas that could affect the results. The highest purity grades in common use are 6.0 grades, but

1920-444: Is at 394 nm. With an atomic emission detector (AED), a sample eluting from a column enters a chamber which is energized by microwaves that induce a plasma. The plasma causes the analyte sample to decompose and certain elements generate an atomic emission spectra. The atomic emission spectra is diffracted by a diffraction grating and detected by a series of photomultiplier tubes or photo diodes. Electron capture detector (ECD) uses

2000-595: Is calculated by finding the response of a known amount of analyte and a constant amount of internal standard (a chemical added to the sample at a constant concentration, with a distinct retention time to the analyte). In most modern GC-MS systems, computer software is used to draw and integrate peaks, and match MS spectra to library spectra. In general, substances that vaporize below 300 °C (and therefore are stable up to that temperature) can be measured quantitatively. The samples are also required to be salt -free; they should not contain ions . Very minute amounts of

2080-718: Is due to the valence , the outermost electron shell , being complete in all the inert gases. This is a tendency, not a rule, as all noble gases and other "inert" gases can react to form compounds under some conditions. The inert gases are obtained by fractional distillation of air , with the exception of helium which is separated from a few natural gas sources rich in this element, through cryogenic distillation or membrane separation. For specialized applications, purified inert gas shall be produced by specialized generators on-site. They are often used by chemical tankers and product carriers (smaller vessels). Benchtop specialized generators are also available for laboratories. Because of

2160-643: Is often determined by what kind of transfer you will be using in GMAW. The most common is spray arc transfer, and the most commonly used gas mixture for spray arc transfer is 90% argon and 10% carbon dioxide. In underwater diving an inert gas is a component of the breathing mixture which is not metabolically active and serves to dilute the gas mixture. The inert gas may have effects on the diver, but these are thought to be mostly physical effects, such as tissue damage caused by bubbles in decompression sickness . The most common inert gas used in breathing gas for commercial diving

2240-528: Is the collection of conditions in which the GC operates for a given analysis. Method development is the process of determining what conditions are adequate and/or ideal for the analysis required. Conditions which can be varied to accommodate a required analysis include inlet temperature, detector temperature, column temperature and temperature program, carrier gas and carrier gas flow rates, the column's stationary phase, diameter and length, inlet type and flow rates, sample size and injection technique. Depending on

2320-446: Is the polyarc, by Activated Research Inc, that converts all compounds to methane. Alkali flame detector (AFD) or alkali flame ionization detector (AFID) has high sensitivity to nitrogen and phosphorus, similar to NPD. However, the alkaline metal ions are supplied with the hydrogen gas, rather than a bead above the flame. For this reason AFD does not suffer the "fatigue" of the NPD, but provides

2400-410: Is two to three times more sensitive to analyte detection than TCD. The TCD relies on the thermal conductivity of matter passing around a thin wire of tungsten-rhenium with a current traveling through it. In this set up helium or nitrogen serve as the carrier gas because of their relatively high thermal conductivity which keep the filament cool and maintain uniform resistivity and electrical efficiency of

2480-407: Is used to prevent the atmosphere in cargo tanks or bunkers from coming into the explosive range. Inert gases keep the oxygen content of the tank atmosphere below 5% (on crude carriers, less for product carriers and gas tankers), thus making any air/hydrocarbon gas mixture in the tank too rich (too high a fuel to oxygen ratio) to ignite. Inert gases are most important during discharging and during

Gas chromatography - Misplaced Pages Continue

2560-503: Is very rare as most analyses needed can be concluded via purely GC-MS. Vacuum ultraviolet (VUV) represents the most recent development in gas chromatography detectors. Most chemical species absorb and have unique gas phase absorption cross sections in the approximately 120–240 nm VUV wavelength range monitored. Where absorption cross sections are known for analytes, the VUV detector is capable of absolute determination (without calibration) of

2640-561: The University of Pretoria in Pretoria , South Africa . In 1959, Perkin Elmer Corp. included a flame ionization detector in its Vapor Fractometer. The operation of the FID is based on the detection of ions formed during combustion of organic compounds in a hydrogen flame . The generation of these ions is proportional to the concentration of organic species in the sample gas stream. To detect these ions, two electrodes are used to provide

2720-541: The methanizer is the Polyarc reactor , which is a sequential reactor that oxidizes compounds before reducing them to methane. This method can be used to improve the response of the FID and allow for the detection of many more carbon-containing compounds. The complete conversion of compounds to methane and the now equivalent response in the detector also eliminates the need for calibrations and standards because response factors are all equivalent to those of methane. This allows for

2800-400: The 1990s, carrier flow rate was controlled indirectly by controlling the carrier inlet pressure, or "column head pressure". The actual flow rate was measured at the outlet of the column or the detector with an electronic flow meter, or a bubble flow meter, and could be an involved, time consuming, and frustrating process. It was not possible to vary the pressure setting during the run, and thus

2880-459: The ASMs in comparison to nitrogen. For fuel tank passivation, it is not necessary to remove all oxygen, but rather enough to stay below the lean flammability limit and the lean explosion limit. In contrast to the oxygen concentration of 21% in air, 10% to 12% in the ullage of a passivated fuel tank is common over the course of a flight. In gas tungsten arc welding (GTAW), inert gases are used to shield

2960-465: The area of the peak using the mathematical function of integration , the concentration of an analyte in the original sample can be determined. Concentration can be calculated using a calibration curve created by finding the response for a series of concentrations of analyte, or by determining the relative response factor of an analyte. The relative response factor is the expected ratio of an analyte to an internal standard (or external standard ) and

3040-562: The ballast voyage when more hydrocarbon vapor is likely to be present in the tank atmosphere. Inert gas can also be used to purge the tank of the volatile atmosphere in preparation for gas freeing - replacing the atmosphere with breathable air - or vice versa. The flue gas system uses the boiler exhaust as its source, so it is important that the fuel/air ratio in the boiler burners is properly regulated to ensure that high-quality inert gases are produced. Too much air would result in an oxygen content exceeding 5%, and too much fuel oil would result in

3120-415: The best separation if flow rates are optimized. Helium is non-flammable and works with a greater number of detectors and older instruments. Therefore, helium is the most common carrier gas used. However, the price of helium has gone up considerably over recent years, causing an increasing number of chromatographers to switch to hydrogen gas. Historical use, rather than rational consideration, may contribute to

3200-492: The carrier gas, and passing the gas through a stationary phase. The mobile phase is usually an inert gas or an unreactive gas such as helium , argon , nitrogen or hydrogen . The stationary phase can be solid or liquid, although most GC systems today use a polymeric liquid stationary phase. The stationary phase is contained inside of a separation column. Today, most GC columns are fused silica capillaries with an inner diameter of 100–320 micrometres (0.0039–0.0126 in) and

3280-405: The carrier gas. In a flame ionization detector (FID), electrodes are placed adjacent to a flame fueled by hydrogen / air near the exit of the column, and when carbon containing compounds exit the column they are pyrolyzed by the flame. This detector works only for organic / hydrocarbon containing compounds due to the ability of the carbons to form cations and electrons upon pyrolysis which generates

Gas chromatography - Misplaced Pages Continue

3360-457: The carryover of dangerous hydrocarbon gas. The flue gas is cleaned and cooled by the scrubber tower. Various safety devices prevent overpressure, the return of hydrocarbon gas to the engine room, or having a supply of IG with too high oxygen content. Gas tankers and product carriers cannot rely on flue gas systems (because they require IG with O 2 content of 1% or less) and so use inert gas generators instead. The inert gas generator consists of

3440-424: The collector plate and upon hitting the plate, induce a current. This current is measured with a high-impedance picoammeter and fed into an integrator . The manner in which the final data is displayed is based on the computer and software. In general, a graph is displayed that has time on the x-axis and total ion on the y-axis. The current measured corresponds roughly to the proportion of reduced carbon atoms in

3520-406: The column temperature, the faster the sample moves through the column. However, the faster a sample moves through the column, the less it interacts with the stationary phase, and the less the analytes are separated. In general, the column temperature is selected to compromise between the length of the analysis and the level of separation. A method which holds the column at the same temperature for

3600-399: The column. Generally, chromatographic data is presented as a graph of detector response (y-axis) against retention time (x-axis), which is called a chromatogram. This provides a spectrum of peaks for a sample representing the analytes present in a sample eluting from the column at different times. Retention time can be used to identify analytes if the method conditions are constant. Also,

3680-424: The compounds as they are burned in a flame. Compounds eluting off the column are carried into a hydrogen fueled flame which excites specific elements in the molecules, and the excited elements (P,S, Halogens, Some Metals) emit light of specific characteristic wavelengths. The emitted light is filtered and detected by a photomultiplier tube. In particular, phosphorus emission is around 510–536 nm and sulfur emission

3760-553: The content of a chemical product, for example in assuring the quality of products in the chemical industry; or measuring chemicals in soil, air or water, such as soil gases . GC is very accurate if used properly and can measure picomoles of a substance in a 1 ml liquid sample, or parts-per-billion concentrations in gaseous samples. Inert gas An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds . Though inert gases have

3840-485: The continued preferential use of helium. Commonly used detectors are the flame ionization detector (FID) and the thermal conductivity detector (TCD). While TCDs are beneficial in that they are non-destructive, its low detection limit for most analytes inhibits widespread use. FIDs are sensitive primarily to hydrocarbons, and are more sensitive to them than TCD. FIDs cannot detect water or carbon dioxide which make them ideal for environmental organic analyte analysis. FID

3920-410: The detector being used, for example, a DID requires helium as the carrier gas. When analyzing gas samples the carrier is also selected based on the sample's matrix, for example, when analyzing a mixture in argon, an argon carrier is preferred because the argon in the sample does not show up on the chromatogram. Safety and availability can also influence carrier selection. The purity of the carrier gas

4000-419: The detector(s) (see below) installed on the GC, there may be a number of detector conditions that can also be varied. Some GCs also include valves which can change the route of sample and carrier flow. The timing of the opening and closing of these valves can be important to method development. Typical carrier gases include helium , nitrogen , argon , and hydrogen . Which gas to use is usually determined by

4080-512: The development of the flame ionization detector. Martin and another one of their colleagues, Richard Synge , with whom he shared the 1952 Nobel Prize in Chemistry , had noted in an earlier paper that chromatography might also be used to separate gases. Synge pursued other work while Martin continued his work with James. German physical chemist Erika Cremer in 1947 together with Austrian graduate student Fritz Prior developed what could be considered

SECTION 50

#1732856132223

4160-421: The end of the column, and adjacent to the cathode (negative electrode) resides a radioactive foil such as 63Ni. The radioactive foil emits a beta particle (electron) which collides with and ionizes the carrier gas to generate more ions resulting in a current. When analyte molecules with electronegative / withdrawing elements or functional groups electrons are captured which results in a decrease in current generating

4240-432: The entire analysis is called "isothermal". Most methods, however, increase the column temperature during the analysis, the initial temperature, rate of temperature increase (the temperature "ramp"), and final temperature are called the temperature program. A temperature program allows analytes that elute early in the analysis to separate adequately, while shortening the time it takes for late-eluting analytes to pass through

4320-439: The filament. When analyte molecules elute from the column, mixed with carrier gas, the thermal conductivity decreases while there is an increase in filament temperature and resistivity resulting in fluctuations in voltage ultimately causing a detector response. Detector sensitivity is proportional to filament current while it is inversely proportional to the immediate environmental temperature of that detector as well as flow rate of

4400-531: The first gas chromatograph that consisted of a carrier gas, a column packed with silica gel, and a thermal conductivity detector. They exhibited the chromatograph at ACHEMA in Frankfurt, but nobody was interested in it. N.C. Turner with the Burrell Corporation introduced in 1943 a massive instrument that used a charcoal column and mercury vapors. Stig Claesson of Uppsala University published in 1946 his work on

4480-444: The flame (E) pyrolyzing the eluent. The ions (F) are repelled up toward the collector plates (G) which are connected to a very sensitive ammeter, which detects the ions hitting the plates, then feeds that signal to an amplifier, integrator, and display system(H). The products of the flame are finally vented out of the detector through the exhaust port (J). Flame ionization detectors are used very widely in gas chromatography because of

4560-402: The flame ionization detector varies from manufacturer to manufacturer, but the principles are the same. Most commonly, the FID is attached to a gas chromatography system. The eluent exits the gas chromatography column (A) and enters the FID detector’s oven (B). The oven is needed to make sure that as soon as the eluent exits the column, it does not come out of the gaseous phase and deposit on

4640-461: The flame. Specifically how the ions are produced is not necessarily understood, but the response of the detector is determined by the number of carbon atoms (ions) hitting the detector per unit time. This makes the detector sensitive to the mass rather than the concentration, which is useful because the response of the detector is not greatly affected by changes in the carrier gas flow rate. FID measurements are usually reported "as methane," meaning as

4720-477: The flow was essentially constant during the analysis. The relation between flow rate and inlet pressure is calculated with Poiseuille's equation for compressible fluids . Many modern GCs, however, electronically measure the flow rate, and electronically control the carrier gas pressure to set the flow rate. Consequently, carrier pressures and flow rates can be adjusted during the run, creating pressure/flow programs similar to temperature programs. The polarity of

4800-399: The inert gases, including nitrogen and carbon dioxide, can be made to react under certain conditions. Purified argon gas is the most commonly used inert gas due to its high natural abundance (78.3% N 2 , 1% Ar in air) and low relative cost. Unlike noble gases , an inert gas is not necessarily elemental and is often a compound gas. Like the noble gases, the tendency for non-reactivity

4880-424: The injection technique. The technique of on-column injection, often used with packed columns, is usually not possible with capillary columns. In the injection system in the capillary gas chromatograph the amount injected should not overload the column and the width of the injected plug should be small compared to the spreading due to the chromatographic process. Failure to comply with this latter requirement will reduce

SECTION 60

#1732856132223

4960-409: The interface between the column and FID. This deposition would result in loss of eluent and errors in detection. As the eluent travels up the FID, it is first mixed with the hydrogen fuel (C) and then with the oxidant (D). The eluent/fuel/oxidant mixture continues to travel up to the nozzle head where a positive bias voltage exists. This positive bias helps to repel the oxidized carbon ions created by

5040-399: The linear velocity the faster the analysis, but the lower the separation between analytes. Selecting the linear velocity is therefore the same compromise between the level of separation and length of analysis as selecting the column temperature. The linear velocity will be implemented by means of the carrier gas flow rate, with regards to the inner diameter of the column. With GCs made before

5120-417: The mixture, but functional groups can play a large part in column selection. The polarity of the sample must closely match the polarity of the column stationary phase to increase resolution and separation while reducing run time. The separation and run time also depends on the film thickness (of the stationary phase), the column diameter and the column length. The column(s) in a GC are contained in an oven,

5200-425: The need for detection at very low levels in some forensic and environmental applications has driven the need for carrier gases at 7.0 grade purity and these are now commercially available. Trade names for typical purities include "Zero Grade", "Ultra-High Purity (UHP) Grade", "4.5 Grade" and "5.0 Grade". The carrier gas linear velocity affects the analysis in the same way that temperature does (see above). The higher

5280-623: The non-reactive properties of inert gases, they are often useful to prevent undesirable chemical reactions from taking place. Food is packed in an inert gas to remove oxygen gas. This prevents bacteria from growing. It also prevents chemical oxidation by oxygen in normal air. An example is the rancidification (caused by oxidation) of edible oils. In food packaging , inert gases are used as a passive preservative, in contrast to active preservatives like sodium benzoate (an antimicrobial ) or BHT (an antioxidant ). Historical documents may also be stored under inert gas to avoid degradation. For example,

5360-598: The number of carbon atoms in their molecule (more carbon atoms produce greater current), while oxygenates and other species that contain heteroatoms tend to have a lower response factor. Carbon monoxide and carbon dioxide are not detectable by FID. FID measurements are often labelled "total hydrocarbons" or "total hydrocarbon content" (THC), although a more accurate name would be "total volatile hydrocarbon content" (TVHC), as hydrocarbons which have condensed out are not detected, even though they are important, for example safety when handling compressed oxygen. The design of

5440-688: The number of molecules present in the flow cell in the absence of chemical interferences. Olfactometric detector , also called GC-O, uses a human assessor to analyse the odour activity of compounds. With an odour port or a sniffing port, the quality of the odour, the intensity of the odour and the duration of the odour activity of a compound can be assessed. Other detectors include the Hall electrolytic conductivity detector (ElCD), helium ionization detector (HID), infrared detector (IRD), photo-ionization detector (PID), pulsed discharge ionization detector (PDD), and thermionic ionization detector (TID). The method

5520-479: The original documents of the U.S. Constitution are stored under humidified argon. Helium was previously used, but it was less suitable because it diffuses out of the case more quickly than argon. Inert gases are often used in the chemical industry. In a chemical manufacturing plant, reactions can be conducted under inert gas to minimize fire hazards or unwanted reactions. In such plants and in oil refineries, transfer lines and vessels can be purged with inert gas as

5600-406: The pattern of peaks will be constant for a sample under constant conditions and can identify complex mixtures of analytes. However, in most modern applications, the GC is connected to a mass spectrometer or similar detector that is capable of identifying the analytes represented by the peaks. The area under a peak is proportional to the amount of analyte present in the chromatogram. By calculating

5680-410: The quantity of methane which would produce the same response. The same quantity of different chemicals produces different amounts of current, depending on the elemental composition of the chemicals. The response factor of the detector for different chemicals can be used to convert current measurements into actual amounts of each chemical. Hydrocarbons generally have response factors that are equal to

5760-425: The sample is in liquid, gas, adsorbed, or solid form, and on whether a solvent matrix is present that has to be vaporized. Dissolved samples can be introduced directly onto the column via a COC injector, if the conditions are well known; if a solvent matrix has to be vaporized and partially removed, a S/SL injector is used (most common injection technique); gaseous samples (e.g., air cylinders) are usually injected using

5840-409: The sample may get trapped in the rubber, to be released during subsequent injections. This can give rise to ghost peaks in the chromatogram. There may be selective loss of the more volatile components of the sample by evaporation from the tip of the needle. The choice of column depends on the sample and the active measured. The main chemical attribute regarded when choosing a column is the polarity of

5920-404: The sample pass through the column at different rates, depending on their chemical and physical properties and the resulting interactions with the column lining or filling, called the stationary phase . The column is typically enclosed within a temperature controlled oven. As the chemicals exit the end of the column, they are detected and identified electronically. Chromatography dates to 1903 in

6000-823: The separation capability of the column. As a general rule, the volume injected, V inj , and the volume of the detector cell, V det , should be about 1/10 of the volume occupied by the portion of sample containing the molecules of interest (analytes) when they exit the column. Some general requirements which a good injection technique should fulfill are that it should be possible to obtain the column's optimum separation efficiency, it should allow accurate and reproducible injections of small amounts of representative samples, it should induce no change in sample composition, it should not exhibit discrimination based on differences in boiling point, polarity, concentration or thermal/catalytic stability, and it should be applicable for trace analysis as well as for undiluted samples. However, there are

6080-447: The solute is crucial for the choice of stationary compound, which in an optimal case would have a similar polarity as the solute. Common stationary phases in open tubular columns are cyanopropylphenyl dimethyl polysiloxane, carbowax polyethyleneglycol, biscyanopropyl cyanopropylphenyl polysiloxane and diphenyl dimethyl polysiloxane. For packed columns more options are available. The choice of inlet type and injection technique depends on if

6160-412: The temperature of which is precisely controlled electronically. (When discussing the "temperature of the column," an analyst is technically referring to the temperature of the column oven. The distinction, however, is not important and will not subsequently be made in this article.) The rate at which a sample passes through the column is directly proportional to the temperature of the column. The higher

6240-436: The tungsten from contamination. It also shields the fluid metal (created from the arc) from the reactive gases in air which can cause porosity in the solidified weld puddle. Inert gases are also used in gas metal arc welding (GMAW) for welding non-ferrous metals. Some gases which are not usually considered inert but which behave like inert gases in all the circumstances likely to be encountered in some use can often be used as

6320-468: The work of the Russian scientist, Mikhail Semenovich Tswett , who separated plant pigments via liquid column chromatography. The invention of gas chromatography is generally attributed to Anthony T. James and Archer J.P. Martin . Their gas chromatograph used partition chromatography as the separating principle, rather than adsorption chromatography . The popularity of gas chromatography quickly rose after

6400-528: Was improved by the invention of capillary column, in which the stationary phase is coated on the inner wall of the capillary. The autosampler provides the means to introduce a sample automatically into the inlets. Manual insertion of the sample is possible but is no longer common. Automatic insertion provides better reproducibility and time-optimization. Different kinds of autosamplers exist. Autosamplers can be classified in relation to sample capacity (auto-injectors vs. autosamplers, where auto-injectors can work

#222777