Cromatografia gasosa: diferenças entre revisões
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* '''[[Microextração de fase sólida]] (SPME)''' |
* '''[[Microextração de fase sólida]] (SPME)''' |
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Tradicionalmente, os fabricantes de injetores não são os mesmos que fabricam os cromatógrafos, e atualmente nenhum fabricante de CGs oferece uma linha completa de injetores automáticos. Historicamente, os países mais ativos em tecnologia de amostragem automática são os [[Estados Unidos da América]], a [[Itália]] e a [[Suíça]]. |
Tradicionalmente, os fabricantes de injetores não são os mesmos que fabricam os cromatógrafos, e atualmente nenhum fabricante de CGs oferece uma linha completa de injetores automáticos. Historicamente, os países mais ativos em tecnologia de amostragem automática são os [[Estados Unidos da América]], a [[Itália]] e a [[Suíça]]. |
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=== Inlets === |
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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''. |
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<u>Common inlet types are:</u> |
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* '''S/SL (Split/Splitless) injector'''; a sample is introduced into a heated small chamber via a syringe through a ''septum'' - the heat facilitates [[volatilization]] of the sample and sample matrix. The carrier gas then either sweeps the entirety (''splitless'' mode) or a portion (''split'' mode) of the sample into the column. In split mode, a part of the sample/carrier gas mixture in the injection chamber is exhausted through the ''split vent''. Split injection is preferred when working with samples with high analyte concentrations (>0.1%) whereas splitless injection is best suited for trace analysis with low amounts of analytes. (<0.01%) |
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* '''On-column inlet'''; the sample is here introduced in its entirety without heat. |
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* '''PTV injector'''; Temperature-programmed sample introduction was first described by Vogt in 1979. Originally Vogt developed the technique as a method for the introduction of large sample volumes (up to 250 µL) in capillary GC. Vogt introduced the sample into the liner at a controlled injection rate. The temperature of the liner was chosen slightly below the boiling point of the solvent. The low-boiling solvent was continuously evaporated and vented through the split line. Based on this technique, Poy developed the Programmed Temperature Vaporising injector; PTV. By introducing the sample at a low initial liner temperature many of the disadvantages of the classic hot injection techniques could be circumvented. |
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* '''Gas source inlet''' or '''gas switching valve'''; gaseous samples in collection bottles are connected to what is most commonly a six-port ''switching valve''. The carrier gas flow is not interrupted while a sample can be expanded into a previously evacuated ''sample loop''. Upon switching, the contents of the sample loop are inserted into the carrier gas stream. |
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* '''P/T (Purge-and-Trap) system'''; An inert gas is bubbled through an aqueous sample causing insoluble volatile chemicals to be purged from the matrix. The volatiles are 'trapped' on an absorbent column (known as a trap or concentrator) at ambient temperature. The trap is then heated and the volatiles are directed into the carrier gas stream. Samples requiring preconcentration or purification can be introduced via such a system, usually hooked up to the S/SL port. |
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* '''[[Solid phase microextraction|SPME]]''' (solid phase microextraction) offers a convenient, low-cost alternative to P/T systems with the versatility of a syringe and simple use of the S/SL port. |
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=== Columns === |
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Two types of columns are used in GC: |
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*'''Packed columns''' are 1.5 - 10 m in length and have an internal diameter of 2 - 4 mm. The tubing is usually made of stainless steel or glass and contains a ''packing'' of finely divided, inert, solid support material (eg. [[diatomaceous earth]]) that is coated with a liquid or solid stationary phase. The nature of the coating material determines what type of materials will be most strongly adsorbed. Thus numerous columns are available that are designed to separate specific types of compounds. |
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*'''Capillary columns''' have a very small internal diameter, on the order of a few tenths of millimeters, and lengths between 25-60 meters are common. The inner column walls are coated with the active materials (WCOT columns), some columns are quasi solid filled with many parallel micropores (PLOT columns). Most capillary columns are made of fused-[[silica]] with a [[polyimide]] outer coating. These columns are flexible, so a very long column can be wound into a small coil. |
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*'''New developments''' are sought where stationary phase incompatibilities lead to geometric solutions of parallel columns within one column. Among these new developments are: |
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** Internally heated ''microFAST'' columns, where two columns, an internal heating wire and a temperature sensor are combined within a common column sheath ([http://etd.lsu.edu/docs/available/etd-04012004-193144 microFAST]); |
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** ''[http://www.restek.com/restek/images/external/620-04-001.pdf Micropacked columns]'' (1/16" OD) are column-in-column packed columns where the outer column space has a packing different from the inner column space, thus providing the separation behaviour of two columns in one. They can easily fit to inlets and detectors of a capillary column instrument. |
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The '''temperature-dependence''' of molecular adsorption and of the rate of progression along the column necessitates a careful control of the column [[temperature]] 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. This is referred to as a '''temperature program'''. Electronic pressure control can also be used to modify flow rate during the analysis, aiding in faster run times while keeping acceptable levels of separation. |
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The '''choice of carrier gas''' (''mobile phase'') is important, with hydrogen being the most efficient and providing the best separation. However, helium has a larger range of flowrates that are comparable to hydrogen in efficiency, with the added advantage that helium is non-flammable, and works with a greater number of detectors. Therefore, helium is the most common carrier gas used. |
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=== Detectors === |
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A number of detectors are used in gas chromatography. The most common are the [[flame ionization detector]] (FID) and the [[thermal conductivity detector]] (TCD). Both are sensitive to a wide range of components, and both work over a wide range of concentrations. While TCDs are essentially universal and can be used to detect any component other than the carrier gas (as long as their thermal conductivities are different from that of the carrier gas, at detector temperature), FIDs are sensitive primarily to hydrocarbons, and are more sensitive to them than TCD. However, an FID cannot detect water. Both detectors are also quite robust. Since TCD is non-destructive, it can be operated in-series before an FID (destructive), thus providing complementary detection of the same analytes. |
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Other detectors are sensitive only to specific types of substances, or work well only in narrower ranges of concentrations. They include: |
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*[[discharge ionization detector]] (DID), which uses a high-voltage electric discharge to produce ions. |
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*[[electron capture detector]] (ECD), which uses a radioactive [[Beta particle]] (electron) source to measure the degree of electron capture. |
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*[[flame photometric detector]] (FPD) |
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*[[flame ionization detector]] (FID) |
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*[[Hall electrolytic conductivity detector]] (ElCD) |
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*[[helium ionization detector]] (HID) |
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*[[Nitrogen Phosphorus Detector]] (NPD) |
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*[[mass selective detector]] (MSD) |
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*[[photo-ionization detector]] (PID) |
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*[[pulsed discharge ionization detector]] (PDD) |
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*[[thermal energy(conductivity) analyzer/detector]] (TEA/TCD) |
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Some gas chromatographs are connected to a [[mass spectrometer]] which acts as the detector. The combination is known as [[GC-MS]]. Some [[GC-MS]] are connected to an [[nuclear magnetic resonance spectroscopy|NMR spectrometer]] which acts as a back up detector. This combination is known as [[GC-MS-NMR]]. Some [[GC-MS-NMR]] are connected to an [[infrared spectroscopy|infrared spectrophotometer]] which acts as a back up detector. This combination is known as [[GC-MS-NMR-IR]]. It must, however, be stressed this is very rare as most analyses needed can be concluded via purely GC-MS. |
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== Methods == |
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The '''method''' 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. |
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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 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. |
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[[Image:GeoStrataEclipse.jpg]] |
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This image above shows the interior of a GeoStrata Technologies Eclipse Gas Chromatograph that runs continuously in three minute cycles. Two valves are used to switch the test gas into the sample loop. After filling the sample loop with test gas, the valves are switched again applying carrier gas pressure to the sample loop and forcing the sample through the Column for separation. |
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=== Carrier gas selection and flow rates === |
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Typical carrier gases include [[helium]], [[nitrogen]], [[argon]], [[hydrogen]] and [[air]]. Which gas to use is usually determined by the detector being used, for example, a [[Discharge ionization detector|DID]] requires helium as the carrier gas. When analyzing gas samples, however, the carrier is sometimes 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, for example, hydrogen is flammable, and high-purity helium can be difficult to obtain in some areas of the world. (See: [[Helium#Occurrence and production|Helium--occurrence and production]].) |
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The purity of the carrier gas 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. Trade names for typical purities include "Zero Grade," "Ultra-High Purity (UHP) Grade," "4.5 Grade" and "5.0 Grade." |
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The carrier gas flow rate affects the analysis in the same way that temperature does (see above). The higher the flow rate the faster the analysis, but the lower the separation between analytes. Selecting the flow rate is therefore the same compromise between the level of separation and length of analysis as selecting the column temperature. |
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With GCs made before 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. The pressure setting was not able to be varied during the run, and thus the flow was essentially constant during the analysis. The relation between flow rate and inlet pressure is calculated with [[Hagen-Poiseuille equation#Poiseuille's equation for compressible fluids|Poiseuille's equation for compressible fluids]]. |
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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. |
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=== Inlet types and flow rates === |
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The choice of inlet type and injection technique depends on if 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 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 S/SL injector (SPME applications). |
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=== Sample size and injection technique === |
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==== Sample injection ==== |
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[[Image:GCruleof10.jpg|thumb|350px|right|The rule of ten in gas chromatography]] |
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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 the injection technique. The technique of on-column injection, often used with packed columns, is usually not possible with capillary columns. The injection system, in the capillary gas chromatograph, should fulfil the following two requirements: |
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#The amount injected should not overload the column. |
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#The width of the injected plug should be small compared to the spreading due to the chromatographic process. Failure to comply with this requirement will reduce the separation capability of the column. As a general rule, the volume injected, V<sub>inj</sub>, and the volume of the detector cell, V<sub>det</sub>, 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. |
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Some general requirements, which a good injection technique should fulfill, are: |
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*It should be possible to obtain the column’s optimum separation efficiency. |
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*It should allow accurate and reproducible injections of small amounts of representative samples. |
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*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. |
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*It should be applicable for trace analysis as well as for undiluted samples. |
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{{Expand|date=February 2007}} |
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=== Column selection === |
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{{Expand|date=February 2007}} |
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=== Column temperature and temperature program === |
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[[Image:GC Oven inside.jpg|thumb|300px|right|A gas chromatography oven, open to show a capillary column]] |
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The column(s) in a GC are contained in an oven, 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.) |
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The rate at which a sample passes through the column is directly proportional to the temperature of the column. The higher 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. |
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In general, the column temperature is selected to compromise between the length of the analysis and the level of separation. |
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A method which holds the column at the same temperature for 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 is called the "'''temperature program'''." |
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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 the column. |
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== Data reduction and analysis == |
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'''Qualitative analysis:''' |
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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 [[analyte]]s 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, the pattern of peaks will be constant for a sample under constant conditions and can identify complex mixtures of analytes. In most modern applications however the GC is connected to a [[Mass spectrometry|mass spectrometer]] or similar detector that is capable of identifying the analytes represented by the peaks. |
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'''Quantitive analysis:''' |
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The area under a peak is proportional to the amount of analyte present in the chromatogram. By calculating 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 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). |
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In most modern [[GC-MS]] systems, computer software is used to draw and integrate peaks, and match [[mass spectrometry|MS]] spectra to library spectra. |
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== Application == |
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In general, substances that vaporize below ca. 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 [[ion]]s. Very minute amounts of 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. |
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Various [[GC temperature program|temperature programs]] can be used to make the readings more meaningful; for example to differentiate between substances that behave similarly during the GC process. |
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Professionals working with GC analyze the content of a chemical product, for example in assuring the quality of products in the chemical industry; or measuring toxic substances in soil, air or water. GC is very accurate if used properly and can measure [[picomole]]s of a substance in a 1 ml liquid sample, or [[Parts per notation|parts-per-billion]] concentrations in gaseous samples. |
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In practical courses at colleges, students sometimes get acquainted to the GC by studying the contents of [[Lavender]] oil or measuring the [[ethylene]] that is secreted by ''[[Nicotiana benthamiana]]'' plants after artificially injuring their leaves. These GC analyses hydrocarbons (C2-C40+). In a typical experiment, a packed column is used to separate the light gases, which are then detected with a [[Thermal conductivity detector|TCD]]. The [[hydrocarbon]]s are separated using a capillary column and detected with an [[Flame ionization detector|FID]]. A complication with light gas analyses that include H<sub>2</sub> is that He, which is the most common and most sensitive inert carrier (sensitivity is proportional to molecular mass) has an almost identical thermal conductivity to hydrogen (it is the difference in thermal conductivity between two separate filaments in a Wheatstone Bridge type arrangement that shows when a component has been eluted). For this reason, dual TCD instruments are used with a separate channel for hydrogen that uses nitrogen as a carrier are common. Argon is often used when analysing gas phase chemistry reactions such as F-T synthesis so that a single carrier gas can be used rather than 2 separate ones. The sensitivity is less but this is a tradeoff for simplicity in the gas supply. |
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== GCs in popular culture == |
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Movies, books and TV shows tend to misrepresent the capabilities of gas chromatography and the work done with these instruments. |
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In the U.S. TV show [[CSI: Crime Scene Investigation|CSI]], for example, GCs are used to rapidly identify unknown samples. "This is [[gasoline]] bought at a [[Chevron Corporation|Chevron]] station in the past two weeks," the analyst will say fifteen minutes after receiving the sample. |
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In fact, a typical GC analysis takes much more time; sometimes a single sample must be run more than an hour according to the chosen program; and even more time is needed to "heat out" the column so it is free from the first sample and can be used for the next. Equally, several runs are needed to confirm the results of a study - a GC analysis of a single sample may simply yield a result per chance (see [[statistical significance]]). |
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Also, GC does not positively identify most samples; and not all substances in a sample will necessarily be detected. All a GC truly tells you is at which relative time a component eluted from the column and that the detector was sensitive to it. To make results meaningful, analysts need to know which components at which concentrations are to be expected; and even then a small amount of a substance can hide itself behind a substance having both a higher concentration and the same relative elution time. Last but not least it is often needed to check the results of the sample against a GC analysis of a reference sample containing only the suspected substance. |
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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. |
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Similarly, 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. According to the substances one expects to find the operating program must be carefully chosen. |
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A push-button operation can exist for running similar samples repeatedly, such as in a chemical production environment or for comparing 20 samples from the same experiment to calculate the mean content of the same substance. However, for the kind of investigative work portrayed in books, movies and TV shows this is clearly not the case. |
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== See also == |
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*[[Thin layer chromatography]] |
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*[[Analytical chemistry]] |
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*[[Chromatography]] |
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*[[Gas chromatography-mass spectrometry]] |
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*[[Katharometer]] |
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*[[Standard addition]] |
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*[[Unresolved Complex Mixture]] -->e |
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Revisão das 16h55min de 30 de setembro de 2016
 | Este artigo ou se(c)ção está a ser traduzido. |
Cromatografia gasosa (CG) ou cromatografia gás-líquido (CGL), é um tipo comum de cromatografia usada em química orgânica para separação de compostos que podem ser vaporizados sem decomposição.
História
Usos típicos da Cromatografia Gasosa incluem teste de pureza de uma substância em particular, ou separação de diversos componentes de uma mistura (as quantidades relativas de um determinado componente também podem ser determinadas). Em algumas situações, a Cromatografia Gasosa pode ajudar a identificar um composto. Em química de microescala, Cromatografia Gasosa pode ser usada para preparar compostos puros de uma mistura.[1]
Em cromatografia gasosa, a fase em movimento (ou "fase móvel") é um gás transportador, normalmente um gás inerte tal como o hélio ou um gás não reativo tal como o nitrogênio. A fase estacionária é uma camada microscópica de líquido ou polímero sobre um sólido inerte, dentro de uma peça tubular de vidro ou metal chamada coluna. O instrumento usado para realizar a cromatografia gasosa é chamado cromatógrafo a gás (mais raramente "aerógrafo" ou "separador a gás").
Os compostos gasosos sendo analisados interagem com as paredes da coluna, a qual é revestida com diferentes fases estacionárias. Isto causa que cada composto "elui" a um tempo diferente, conhecido como tempo de retenção do composto. A comparação de tempos de retenção é que dá a Cromatografia Gasosa sua eficiência analítica.
Cromatografia gasosa é em princípio similar à cromatografia em coluna (assim como outras formas de cromatografia, tal como a HPLC, TLC), mas tem diversas diferenças notáveis. Primeiramente, o processo de separação dos compostos em uma mistura é carregada entre uma fase líquida estacionária e uma fase de gás em movimento, enquanto na cromatografia em coluna a fase estacionária é um sólido e a fase móvel é um líquido. (Por este motivo o nome completo adequado do procedimento é "cromatografia gás-líquido", referindo-se às fases móveis e estacionárias, respectivamente.) Secundariamente, a coluna através da qual a fase gasosa passa é localizada em um forno onde a temperatura do gás pode ser controlada, enquanto a cromatografia em coluna (tipicamente) não possui qualquer controle de temperatura. Em terceiro lugar, a concentração de um composto na fase gás é unicamente uma função da pressão de vapor do gás.[1]
Cromatografia gasosa é também similar a destilação fracionada, devido a ambos os processos separarem os componentes de uma mistura primariamente baseando-se em diferentes pontos de ebulição (ou pressões de vapor). Entretanto, a destilação fracionada é tipicamente usada para separar componentes de uma mistura em grande escala, enquanto CG pode ser usada numa escala muito menor (i.e. microescala).[1]
Cromatografia gasosa é também algumas vezes conhecida como cromatografia em fase vapor (CFC), ou cromatografia de partição gás-líquido (CPGL). Estes nomes alternativos, assim como suas respectivas abreviações, são frequentemente encontradas em literatura científica. Estritamente falando, CFGL é a mais correta terminologia, e é então preferível por muitos autores.[1]
Cromatografia data de 1903 no trabalho do cientista russo Mikhail Semenovich Tswett. O estudante graduado alemão Fritz Prior desenvolveu a cromatografia gasosa de estado sólido em 1947. Archer John Porter Martin, quem foi vencedor do Prêmio Nobel por seu trabalho no desenvolvimento das cromatografias líquido-líquido (1941) e em papel (1944), estabeleceu os fundamentos para o desenvolvimento da cromatografia gasosa e posteriormente produziu a cromatografia gás-líquido (1950).
Análise através de Cromatografia Gasosa
Uma cromatografia gasosa é um processo de análise química instrumental por separação de compostos químicos e uma amostra complexa. Uma cromatografia gasosa usa um tubo estreito através do qual se dá o fluxo conhecido como coluna, através do qual diferentes constituintes de uma amostra passam em uma corrente de gás (gás condutor, ou transportador, a fase móvel) em diferentes taxas dependendo de várias propriedades físicas e químicas e suas interações com um específico recheio da coluna, chamada fase estacionária. Como os compostos químicos saem no final da coluna, são detectados e identificados eletronicamente. A função da fase estacionária na coluna é separar componentes diferentes, causando a cada um saída da coluna em um tempo diferente (tempo de retenção). Outros parâmetros que podem ser usados para alterar a ordem ou tempo de retenção são a taxa de fluxo do gás condutor e a temperatura.
Em uma análise CG, um volume conhecido de analito gasoso ou líquido é injetado na entrada da coluna, geralmente com o uso de uma microsseringa (ou com fibras de microextração de fase sólida, ou ). Conforme o gás carregador leva as moléculas do analito através da coluna, essa movimentação é inibida pela adsorção das moléculas do analito nas paredes da coluna ou no material do empacotamento da mesma. A taxa com que as moléculas progridem ao longo da coluna depende da força da adsorção que, por sua vez, depende do tipo de molécula e do material da fase estacionária. Uma vez que cada tipo de molécula tem uma taxa de progressão diferente, os vários componentes da mistura de analito são separados conforme progridem ao longo da coluna, chegado ao fim dela em momentos diferentes (tempos de retenção). Um detector é empregado para monitorar o fluxo de saída da coluna. Assim, o momento em que cada componente sai da coluna, e a quantidade deles, pode ser determinada. Geralmente, as substâncias são identificadas (qualitativamente) pela ordem na qual emergem (eluem) da coluna e pelo tempo de retenção do analito na coluna.
Componentes físicos
Injetores automáticos
O injetor automático permite a introdução automatizada de amostra nos inlets. A injeção manual ainda é possível, porém não é mais comum. A injeção automática fornece melhor reprodutibilidade e otimização de tempo.
Existem diferentes tipos de injetores automáticos. Eles podem ser classificados de acordo com a capacidade (número de amostras com a qual é possível trabalhar), de acordo com a tecnologia robótica (XYZ robot vs. rotating/SCARA-robot – o mais comum), ou conforme a análise:
- Líquido
- Microextração de fase sólida (SPME)
Tradicionalmente, os fabricantes de injetores não são os mesmos que fabricam os cromatógrafos, e atualmente nenhum fabricante de CGs oferece uma linha completa de injetores automáticos. Historicamente, os países mais ativos em tecnologia de amostragem automática são os Estados Unidos da América, a Itália e a Suíça.
Referências
- ↑ a b c d Pavia, Donald L., Gary M. Lampman, George S. Kritz, Randall G. Engel (2006). Introduction to Organic Laboratory Techniques (4th Ed.). [S.l.]: Thomson Brooks/Cole. pp. 797–817. ISBN 978-0-495-28069-9
Ligação externas
- Predefinição:Dmoz (em inglês)
- Gas Chromatography Help Site (em inglês)