gas chromatography
GC gas chromatography
GC gas chromatography is a laboratory equipment used for chemical analysis to separate and characterize chemicals in air. GC gas chromatography uses a long, narrow tube known as a separating column, into which the sample is injected In it, and then separate the chemicals that make up a particular Vue and move through it at different speeds, and that is Depending on the different chemicals and the physical properties of each material, and during the exit of the materials.
The chemical function of the column is from the side of the column, it is detected and distinguished electronically. that Separation and concentration of the different sample components in order to amplify and distinguish the detected signal.
In a GC gas chromatography loading, the general volume of the gaseous or liquid hydrated material is It is injected into the inlet of the column using a microscopic syringe. And although the carrier gas It transports the hydrolyzed particles through the column, but this movement is hindered by adsorption Particles of the material loaded either on the walls of the column or on the materials packed into the column.
And the transmission rate depends Molecules in the column on the extent of the strength of adsorption, which in turn depends on the type of molecular and on the type of Packed items for baptism. Since each substance has its own transmission rate, the components can be separated The sample is all according to the speed of its movement through the column, and thus the variation in the speed of its arrival to the tip column.
A detector is used to detect the components that come out of the column as soon as they exit Detection of each separately according to the time of their exit from the column. The history of GC gas chromatography dates back to
3091, where he invented the chromatographic method of the Russian scientist Mikhail Semenovich Tsoi.
And But his work in this field was not known at the time, because he was writing only in Russian. But in a year 3091 Solid phase GC gas chromatography was developed by the German student Fritz Beriuer. and In 3099 Arthur John Porter Martin, who received the Nobel Prize in Chemistry as a result of His development of -liquid chromatography method in 3093 and paper chromatography in 3099- He developed liquid-gas chromatography.
Gas chromatography is sometimes known as vapor phase chromatography (VPC), or gas-liquid separation chromatography (GLPC). These alternative names, as well as their respective abbreviations, are frequently used in the scientific literature.
Gas chromatography is the process of separating compounds in a mixture by injecting a gaseous or liquid sample into a mobile phase, usually called a carrier gas, and passing the gas through a stationary phase. The mobile phase is usually an inert gas or an inert gas such as helium , argon , nitrogen or hydrogen . The stationary phase is a microscopic layer of viscous liquid on a surface of solid particles on an inert solid support inside a piece of glass or metal tube called a column. The surface of the solid particles may also act as a stationary phase in some columns. The glass or metal column through which the gas phase passes is located in a furnace where the temperature of the gas can be controlled and the rinsing out of the column is monitored by a computerized detector.
date gas chromatography
The history of chromatography dates back to 1903 in the work of the Russian scientist, Mikhail Semenovich Tsoi, who separated plant pigments by means of liquid column chromatography. German physical chemist Erika Kremer in 1947 together with Austrian graduate student Fritz Bree developed the theoretical foundations of GC and made the first gas-liquid chromatograph, but her work was considered irrelevant and long ignored. Archer John Porter Martin, who received the Nobel Prize in 1952 for his work developing liquid and liquid chromatography (1941) and paper (1944), is credited with establishing gas chromatography. The popularity of gas chromatography rose rapidly after the development of the flame ionization detector
gas chromatography analysis
In GC analysis, a known volume of gaseous or liquid analyte is injected through a rubber disc and into a heated, temperature-controlled port attached to the column. As the carrier gas transports the analyte particles through the column, there is adsorption of the analyte particles either onto the column walls or onto the packing materials (stationary phase) in the column to give separation.
Since each type of molecule has a different progression rate, the different components of the analyte mixture are separated as they progress along the column and reach the end of the column at different times (retention time). The detector is used to monitor the time each component reaches the outlet and ultimately the amount of that component can be determined. [1] In general, substances (type) are identified in the order in which they are extracted from the column and the time the analysis is kept in the column.
physical components
entries
Segmented/undivided entrance.
The column inlet (or injector) provides a means to introduce a sample into the continuous flow of the carrier gas. The entrance is a piece of hardware attached to the shaft head.
Common entrance types are:
S/SL injector (split/unsplit); A sample is introduced into a small heated chamber via a trans-septal syringe – the heat facilitates volatilization of the sample and sample matrix. Then the carrier gas scans either the whole (non-split mode) or part (split mode) of the sample into the column. In split mode, part of the carrier gas sample/mixture is exhausted into the injection chamber through the vent hole.
Fragmented injection is preferred when working with samples with high analyte concentrations (>0.1%) while unfractionated injection is best suited for tracer analysis with low analyte volumes (<0.01%). In non-split mode, the split valve opens after a preset period of time to purge heavier items that would contaminate the system.
This predetermined (unsplit) time should be optimized, shorter time (eg 0.2 min) ensures reduction of tail but loss in response, longer time (2 min) increases tail but also signal.
Column Inlet Here the sample is introduced directly into the entire column without heat, or at a temperature below the boiling point of the solvent. The lower temperature condenses the sample into a narrow area. The column and inlet can then be heated, releasing the sample into the gas phase. This ensures the lowest possible chromatographic temperature and prevents samples from decomposing above the boiling point.
PTV injector Temperature programmed sample introduction was first described by Vogt in 1979. Vogt originally developed this technology as a method for introducing large sample volumes (up to 250 μl) into capillary GC.
Vogt introduced the sample into the endothelium at a controlled injection rate. The lining temperature was chosen just below the boiling point of the solvent. The low-boiling solvent was evaporated and aerated through the split line. Based on this technology, Poy developed the programmed temperature evaporation injector; PTV.
By introducing the sample at a lower initial lining temperature, many drawbacks of traditional hot injection techniques can be circumvented.
gas source inlet or gas switch valve; Gas samples in collection bottles are related to what is more commonly a six-port switch valve. The flow of the carrier gas is not interrupted while the sample can be expanded into a previously evacuated sample ring. When switching, the contents of the sample loop are introduced into the carrier gas stream.
P / T system (disinfection and trap); An inert gas is pumped through an aqueous sample which removes insoluble volatile chemicals from the matrix. Volatile matter is “trapped” on an absorbent column (known as a trap or condenser) at ambient temperature. The trap is then heated and the volatiles are directed into the carrier gas stream. Samples requiring pre-concentration or purification can be fed via such a system, usually connected to an S/SL port.
The choice of the 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 better separation if flow rates are improved.
Helium is non-flammable and works with many older detectors and devices. Therefore, helium is the most common carrier gas. However, the price of helium has skyrocketed in recent years, causing more chromatographs to convert to hydrogen gas. Historical use, rather than rational consideration, may contribute to the continued preferential use of helium.
detectors
Commonly used detectors are the flame ionization detector (FID) and the thermal conductivity detector (TCD). While TCDs are useful in that they are non-destructive, the low detection limit of most analyzes prevents their widespread use. FIDs are primarily sensitive to hydrocarbons, and are more sensitive to them than TCDs. FIDs cannot detect water or carbon dioxide which makes them ideal for environmental organic analysis. FID is two to three times more sensitive to analyte detection than TCD.
Thermal conductivity detector (TCD) is based on the thermal conductivity of a material passing around a thin wire of tungsten and rhenium with a current passing through it. In this setup, helium or nitrogen acts as a carrier gas due to the relatively high thermal conductivity that keeps the filament cool and maintains the uniform resistance and electrical efficiency of the filament.
When the analyte molecules are mixed from the column, mixed with the carrier gas, the thermal conductivity decreases while there is an increase in the filament temperature and resistance which leads to voltage fluctuations which eventually leads to the detector response. The sensitivity of the detector is proportional to the current of the filament while inversely proportional to the immediate environmental temperature of that detector as well as the flow rate of the carrier gas.
In a Flame Ionization Detector (FID), the electrodes are placed next to the hydrogen/air fed flame near the outlet of the column, and when carbon-containing compounds exit the column they are thermally hydrolyzed by the flame.
This detector works only with organic/hydrocarbon-containing compounds due to the ability of carbon to form cations and electrons upon pyrolysis that generates a current between the electrodes. The increase in current is localized and shown as a peak on the chromatogram. FIDs have low detection limits (a few picograms per second) but are unable to generate carbonyl-containing ions. FID-compliant carrier gases include helium, hydrogen, nitrogen, and argon.
The Alkaline Flame Ionization Detector (AFD) or Alkaline Flame Ionization Detector (AFID) has a high sensitivity to nitrogen and phosphorous, similar to NPD. However, the alkali metal ions are supplied with hydrogen gas, rather than a bead over the flame.
For this reason, the Arc Fault Detector does not suffer from NPD ‘strain’, but provides constant sensitivity over a long period of time. In addition, when no alkaline ions are added to the flame, the AFD acts like a standard FID. The catalytic combustion detector (CCD) measures flammable hydrocarbons and hydrogen. A Discharge Ionization Detector (DID) uses a high-voltage electrical discharge to produce ions.
A polyarc reactor is an addition to new or existing GC-FID instruments that converts all organic compounds into methane molecules prior to detection by AAA. This technique can be used to improve the response of FID and allow more carbon-containing compounds to be detected.
The complete conversion of compounds to methane and the equivalent response now in the detector also eliminates the need for calibration and calibration because the response factors are all equivalent to those of methane. This allows for rapid analysis of complex mixtures containing particles where standards are not available.
A flame photometric detector (FPD) uses a photomultiplier tube to detect the spectral lines of compounds as they burn in a flame. The compounds eliminated from the column are transferred to a hydrogen-fueled flame which excites specific elements in molecules, and the excited elements (P, S, halogens, some metals) emit light at specific characteristic wavelengths. The emitted light is filtered and detected by a photomultiplier tube.
In particular, the emission of phosphorous is around 510-536 nm and that of sulfur at 394 nm. Using an atomic emission detector (AED), an elliptic sample enters from a column into a chamber that is activated by microwaves that stimulate the plasma. Plasma causes the analysis sample to decay and some elements generate atomic emission spectra. Atomic emission spectra are deflected by diffraction gratings and detected by a series of photomultipliers or photodiodes.
An Electron Capture Detector (ECD) uses a radioactive beta particle (electron) source to measure the degree of electron capture. The ECD is used to detect molecules that contain electronegative/pull elements and functional groups such as halogens, carbonyls, nitriles, nitro groups, and organometallics.
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 located at the end of the column, and next to the cathode (negative electrode) there is a radioactive wafer such as Ni 63. The radioactive foil emits a beta particle (electron) that collides with the carrier gas and ionizes it to generate more ions resulting in a current. When analytic molecules containing electrical elements/draw elements or functional groups are captured, causing a drop in current that generates a detector response.
Nitrogen-phosphorous (NPD) detector, a form of thermal detector where nitrogen and phosphorous alter the action function on a specially coated bead and the resulting current is measured.
The dry electrical conductivity detector (DELCD) uses an air phase and a high temperature (anti-Coulson) to measure chlorinated compounds.
mass spectrometry (MS), also called GC-MS; Very sensitive and sensitive, even in a small amount of the sample. This reagent can be used to identify analytes in chromatograms by their mass spectrometry. Some GC-MS are attached to the 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 spectrophotometer that acts as a back-up detector. This combination is known as GC-MS-NMR-IR. However, it must be emphasized that this is very rare as most of the required analyzes can be completed via purely GC-MS.
Vacuum ultraviolet (VUV) radiation represents the latest development in gas chromatography detectors. They absorb most of the chemical species and have unique gas-phase absorption cross-sections in the ~120-240 nm VUV wavelength range that has been observed.
When absorption cross-sections are known for the analyses, the VUV detector is capable of absolute determination (without titration) of the number of molecules present in the flow cell in the absence of chemical interferences.
The olfactory detector, also known as GC-O, uses a human estimator to analyze the odor activity of compounds. Using the odor port or inhalation port, the odor quality, odor intensity, and duration of odor activity of the compound can be assessed.
Other reagents include the Hall Electrical Conduction Detector (ElCD), Helium Ionization Detector (HID), Infrared Detector (IRD), Photonic Ionization Detector (PID), Pulsed Discharge Ionization Detector (PDD), and Thermal Ionization Detector (TID).
Techniques
A method is the set of conditions under which the GC operates for a given analysis. Method development is the process of determining appropriate and/or ideal conditions for the required analysis.
Conditions that can be changed to accommodate the required analysis include inlet temperature, detector temperature, column temperature, program temperature, carrier gas flow rates, carrier gas, column stationary phase, diameter, length, inlet type, 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 vary. Some GCs also include valves that can alter the sample flow path and holder. The timing of the opening and closing of these valves could be important to the development of the method.
Selection of carrier gas and flow rates
Typical carrier gases include helium, nitrogen, argon, and hydrogen. Which gas to use is usually determined by the detector used, for example, DID requires helium as the carrier gas. When analyzing gas samples, the carrier is also selected based on the sample matrix, for example, when analyzing a mixture in argon, an argon carrier is preferred because argon in the sample does not appear on the chromatogram. Safety and availability can also affect carrier choice.
The purity of the carrier gas is also frequently determined by the detector, although the required sensitivity level can also play an important role. Typically, a purity of 99.995% or higher is used. The most common purities required by modern devices for most sensitivities are 5.0 degrees, or 99.999% pure which means there are a total of 10ppm of impurities in the carrier gas that can affect the results.
The highest purities in common use are 6.0, but the need for detection at very low levels in some forensic and environmental applications has driven the need for carrier gases with a purity of 7.0 now available commercially. Typical purity brand names include ‘Grade Zero’, ‘Ultra Purity (UHP)’, ‘Grade 4.5’ and ‘Grade 5.0’.
The linear velocity of the carrier gas affects the analysis in the same way as the temperature (see above). The higher the linear velocity the faster the analysis, but the lower the separation between the analytes. Thus, the choice of linear velocity is the same compromise between the separation plane and the length of the analysis as the choice of the column temperature. The linear velocity will be carried out by the flow rate of the carrier gas, with respect to the inner diameter of the shaft.
With the manufacture of GCs prior to the 1990s, conveyor flow rate was indirectly controlled by controlling the conveyor inlet pressure, or “shaft head pressure”. The actual flow rate at the outlet of the column or detector was measured with an electronic flowmeter, or bubble flowmeter, and can be an involved, time-consuming, and frustrating process.
It was not possible to change the pressure settings during operation, and thus the flow was essentially constant during the analysis. The relationship between flow rate and inlet pressure is calculated using the Boisei equation for compressible fluids.
However, many modern GCs electronically measure the flow rate, and electronically control the pressure of the carrier gas to adjust the flow rate. Thus, carrier pressure and flow rates can be adjusted during operation, creating pressure/flow programs similar to temperature programs.
Choosing a fixed compound
The polarity of the solute is essential for the selection of a stable compound, which in the optimal case will have a polarity similar to that of the solute. The common stationary phases in open tube columns are cyanopropyl phenyl dimethyl polysiloxane, carboax polyethylene glycol, cyanopropyl cyanopropyl phenyl polysiloxane and diphenyl dimethyl polysiloxane. More options are available for filled columns.
Inlet types and flow rates
The choice of inlet type and injection technique depends on whether the sample is in liquid, gaseous, atomized or solid form, and on whether a solvent matrix is present that must be evaporated.
Dissolved samples can be introduced directly into the column via a COC injector, if conditions are known; If a solvent mold has to be vaporized and partially removed, an S / SL injector (the most common injection technique) is used; Gas samples (for example, air cylinders) are usually injected using a gas exchange valve system; Adsorbed samples are introduced (eg, on adsorbent tubes) using an external absorber (connected or disconnected) such as the purge and trap system, or sucked into the injector (SPME applications).
Sample size and injection technique
sample injection
The rule of ten in gas chromatography
True chromatographic analysis begins with the introduction of the sample into the column. The development of capillary gas chromatography has led to many practical problems in the injection technique. The column injection technique, often used with packed columns, is usually not possible with capillary columns.
In the injection system of a capillary gas chromatograph, the injected amount should not increase the loading of the column and the width of the injected stopper should be small compared to the diffusion caused by the chromatographic process.
Failure to comply with this last requirement will reduce the separation capacity of the shaft. As a general rule, the injected volume, hits V, and the volume of the reagent cell, V det , should be about 1/10 of the volume occupied by the sample portion containing the particles of interest (analytes) as they exit the column.
Some general requirements that a good injection technique must meet are that it should be possible to obtain optimum column separation efficiency, it should allow accurate and reproducible injection of small amounts of representative samples, it should not make any change in the sample composition, nor should It shows discrimination based on differences in boiling point, polarity, concentration or thermal/catalytic stability, and should be applicable for tracer analysis as well as for undiluted samples.
However, there are a number of problems inherent with the use of syringes for injection. Even the best syringes claim only 3% accuracy, and errors in unskilled hands are much greater. The needle may cut small pieces of rubber from the septum as it injects a sample through it.
These can clog the needle and prevent the syringe from being filled the next time it is used. It may not be obvious that this has happened. Part of the sample may become trapped in the rubber to be released during subsequent injections. This can cause ghost peaks to appear in the chromatogram. There may be a selective loss of the more volatile components of the sample by evaporation from the needle tip. [15th]
column selection
The choice of column depends on the sample and the active measurement. The main chemical characteristic considered when choosing a column is the polarity of the mixture, but functional groups can play a large role in the selection of the column.
The polarity of the sample must be closely matched with the polarity of the stationary phase of the shaft to increase accuracy and separation while decreasing runtime. Separation and runtime also depend on film thickness (for stationary phase), shaft diameter and shaft length.
Column temperature and temperature program
Gas chromatography furnace, open to show capillary column
The column(s) of the GC are contained in a furnace, the temperature of which is precisely controlled electronically. (When discussing “column temperature,” the analyst is technically referring to the shaft furnace temperature. However, the distinction is not important and will not be defined later in this article.)
The rate at which the 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 the sample moves through the column, the less it interacts with the stationary phase, and the less the analytes separate.
In general, the column temperature is selected to reconcile the length of the analysis with the level of separation.
The method of holding the column at the same temperature for the entire analysis is called “isothermal”. However, most methods increase the temperature of the column during analysis, the initial temperature, the rate of temperature increase (the “ramp” temperature), and the final temperature called the temperature program.
The temperature program allows analytes extracted early in the analysis to separate appropriately, while shortening the time it takes for analytes that were recently filtered to pass through the column.
Data reduction and analysis
Applications
see also
Basic components of a GC gas chromatography:
Carrier gas (mobile phase) gas carrier with flow regulator.
Chapter column (column packed or capillary column
capillary column
Detector Estimator and Recording Tool.
Carrier GC gas chromatography
The first important part of gas chromatography is the carrier gas. And the carrier gas may Limium and nitrogen, hydrogen, are a mixture of argon and methane. Carrier gas function Carrying the sample through the system. The first choice of carrier gases depends onPrivacy of the application and the type of detector used.
And aluminum is one of the most used gases common. The added gases may include hydrogen and air which are associated with a reagent Specific to be used in gas chromatography, for example, a bulb displacement detector is required Lib, hydrogen and air help in combustion. Gases are generally supplied by a compressed gas cylinder.
The freedom to choose the gas source. The purity of the gas must be taken into account when acquiring a cylinder The sensitivity and selectivity of the reagent must be taken into account when determining the corresponding purity levels (higher selectivity, higher purity).
Classification in regulators, metal tubes, fittings are used As surfaces for gases in gas chromatography. It is useful to remember that moisture traps can Used to reduce the presence of pollutants from gas sources.
Compressed gas cylinders model included Pressure between 299 and 2999 Pa. But the pressure familiar to work with gas chromatography Within a range of 29 to 399 Pa.
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