Gas Chromatography

Gas Chromatography

There are various ways to analyze the components of a certain compound. One of the techniques being commonly used to analyze the components of a sample compound is gas chromatography. This is being used to separate compounds based on its volatilities. Once the compounds are separated, its components can be easily analyzed already. This applies to compounds that can be vaporized without a need for decomposition. It is also applicable in testing the purity of a substance. This provides qualitative analysis for individual compounds in a particular sample. GC also measures giving out quantitative information.


History of Gas Chromatography

The history of gas chromatography can be traced as early as the year 1903. It originated from the work of Mikhail Semenovich Tswett, a scientist from Russia who had used chromatography on his works. A liquid to liquid chromatography was also developed in the year of 1941 and later in 1944 was the paper chromatography. These were all developed by Archer John Porter Martin. Martin got a Nobel Prize for his liquid to liquid chromatography. . In the year 1947, Fritz Prior, a graduate student from Germany, created the combination of solid and gas chromatography. These works have started the GC or best preferred to as the gas liquid chromatography. In the year 1950, Erika Cremer had take charge of the groundwork for this development.


Process of Gas Chromatography

There are two phases being used in the process. In separating the components of a compound, a column is being used. This column is made of a narrow metal tube. It has a filling which is a microscopic layer of liquid. This is being referred to as the stationary phase. The sample will then be transported inside the column using the carrier gas which is usually helium, hydrogen or nitrogen. This is then referred to as the mobile or moving phase.

The above process will use a gas separator or a gas chromatograph. This is an instrument used in GC. The components will be separated through heat. This will enable the component to vaporized and separate from other components and pass through the stationary phase. The time and speed will also be monitored during the process.


Different Components of Gas Chromatograph

To better understand the process and methods of gas chromatography, it’s essential to know the different parts of the instrument and other peripherals.

The Automatic Samplers – The automatic samplers are being used to insert a sample to the inlets of column or the narrow metal tubing. While do insertion of sample can be done manually, this is no longer commonly used nowadays. Currently, automatic sampler is being used as this provides more accurate moves and saves on time. In some cases an automatic injector is also being used, but this can only work on smaller number of samples. The automatic samplers depend on the type of analysis you want to perform. Some automatic samplers available are static head space and dynamic head space. The first one is using a syringe technology while the latter uses transfer line technology. Liquid automatic sampler is also available. Another automatic sampler is the solid phase micro extraction. Automatic sampler is also known as autosampler.

The Column Inlets – The column or the narrow metal tubing has an inlet attached to the head of the column. There are various types of column inlets that are commonly used today. The Split or Split less Injector is one of the inlet types. In using this, a sample is being inserted into a chamber using a syringe through a dividing wall or smaller cavity. During a split mode, only a portion of the sample is swept by the carrier gas. On split less mode, on the other hand, the carrier gas sweeps the entire sample. Split mode is applicable when the required substance that is being analyzed is highly concentrated. When the required substance being analyzed only requires low amount, split less injection is applicable. Another type of column inlet is the On-column inlet. In this type, the sample is directly inserted in the column without heat requirement. Programmed Temperature Vaporizing injector is another type of column inlet. This is also known as the PTV injector. In some instances, gaseous sample is being introduced in the column. With this the inlet type applicable is the gas source inlet. This is also known as the gas switching valve. The sample in gas form is collected in bottles and connected to a switching valve. Once the carrier gas is introduced in the column, the sample will be transferred to a sample loop, and then switching of the contents will be inserted in the carrier gas stream. Purge and Trap system also known as P/T system is another type of column insert which is applicable to liquid or aqueous sample. With this, the volatiles are stuck on a concentrator column at ambient temperature. When the trap is heated already, the volatiles will be directed to the carrier gas stream. This system may be of higher cost, but another alternative to this is the solid phase micro extraction.

The Columns - A gas chromatograph uses two types of columns. Packed columns measure one and a half to ten meters in length. They also have an internal diameter measuring two to four millimeters. The narrow tubing is normally made of glass and some others are made of stainless steel material. The columns are packed with divider, inert and solid support material. The internals are coated with either liquid stationary phase or solid. The coating materials determine the type of materials or samples will be absorbed strongly. Several columns are built up to separate different types of sample compounds. Capillary columns type is the other type of columns that are made of smaller diameter and with lengths measure at about twenty five to sixty meters. The inner walls are coated with active materials and use polyimide to coat the externals. These are all flexible which enable a column to be converted into a coil.

The Detectors – The FID or the flame ionization detector and the TCD or the thermal conductivity detector are the most common detectors. These two detectors are working over on concentrations of wider ranges. These are also sensitive to various choices of components. The thermal conductivity detector works more on any component aside from the carrier gas. However, it will only work if the thermal conductivities of the component differ from the carrier gas. The flame ionization detector on the other hand is working on hydrocarbons. One thing that we need to emphasize is that the flame ionization detector is not capable of detecting water. Both detectors can complement each other as the thermal conductive detector is non-destructive while the flame ionization detector is destructive. Some detectors are only capable of detecting specific types of components. These detectors include the Catalytic Combustion Detector, also known as CCD. This measures hydrocarbon and hydrogen combustion. The DID or the Discharge Ionization Detector requires a high voltage discharge of electric in order to produce ions. The Dry Electrolytic Conductivity Detector or the DELCD is cable to measure chlorinated compounds and uses a high temperature and an air phase. The Electron Capture Detector or the ECD uses radioactive electron particularly the Beta particle in order to determine the degree of the electron capture. Other detectors include The Flame Photometric Detector or the FPD, the Hall Electrolytic Conductivity Detector also known as EICD, the Nitrogen Phosphorus Detector or the NPD, the Mass Selective Detector or the MSD, the Photo-ionization Detector also known as the PID, the PDD or the Pulsed Discharge Ionization Detector, and the Thermionic Ionization Detector which is also known as the TID. A mass spectrometer is sometimes being used as detector in some gas chromatographs.


How to use a gas chromatograph or gas separator?

Here are the methods on how to use a gas chromatograph or gas separator. The initial stage is setting up the preparatory conditions. This is where the conditions are being determined. Set the parameters for the test that you want to expedite. These are the conditions that will be used as basis and include temperature, identified carrier gas, and the gas flow rate. Identify also the stationary phase as to what specific factors to use. The exact measurement of the narrow tube should also be considered and prepared. The conditions should accommodate the analysis being required. In certain circumstances, there are different detectors being used depending on what analysis you want to get into.

Sample Requirements – To start the test, prepare the volume of the sample. This may be a gaseous sample or a liquid sample. This will then be injected into the column or the narrow metal tubing. Depending on the state of the sample, specific methods will be applied. For a gaseous sample, a gas source switching system is being required. For liquid sample, a micro syringe will be applicable. In case a solid sample will be used, micro extraction fibers are needed.

Selection of Gas Carrier – It is essential to determine the type of gas that will be used to conduct the gas chromatography. Helium, nitrogen, hydrogen, argon and air, are the gasses used. The detector is usually the determinant as to what type of carrier gas will be used in this process. The carrier gas is determined based on the matrix of the sample if the sample being used is gaseous sample. It is also significant to note that safety should be taken into consideration. Some gas like helium is scarce and not always available in the market. That is why the availability of type of gas is also a factor in determining which type of gas will be used as carrier. In several cases, hydrogen always replaces helium in some applications. Another factor to consider is the purity of the gas. This can be determined by the detector. It is also important to note that the linear velocity of carrier gas has an effect in the analysis. Just like the temperature, linear velocity of gas affects the process. The analysis will be faster if the linear velocity is high. This can be determined through the gas flow rate. Before the introduction of gas chromatograph the gas carrier flow rate can be controlled through the inlet pressure. The flow rate is measured at the outlet of the narrow metal tubing or the column. A detector with electronic flow meter can also be used to determine the flow rate of the carrier gas. In some cases, bubble flow meter is also being used. However, these things were found to be time consuming. With the invention of gas chromatograph, the flow rate of carrier gas is being determined electronically. During this process, the flow rate and the carrier gas pressures are adjustable during the run.

Stationary Phase - The stationary compound is also important. The selection process should be done tediously. The stationary compound should match the sample compound that will be analyzed. There are various options that can be used especially if the columns or the narrow metal tubing is a packed type.

Choosing the Inlet Types and Flow Rates – In choosing the inlet types and the injection techniques, the state of sample is main determinant. These depend if the sample is in solid form, gas, and liquid. It will also depend on the presence of the solvent matrix. The solvent matrix has to be vaporized to separate the components, thus, this is one of the main considerations in choosing the inlet. Normally, if the sample is in liquid form, a split or split less type of injection is being adapted. If the sample is in gas form a gas switching valve system is applicable.

Choosing the column or columns – Again, the sample is the main consideration in choosing the column. The polarity of the mixture should also be taken into consideration as well as the functional groups. The column or columns are packed in an oven. Thus, the temperature is controllable through electronics. The temperature of the oven is obviously the temperature of the column or the columns. So if there is a need to adjust the column’s temperature, this will be adjusted by adjusting the temperature of the oven. The higher the temperature of the column is, the faster the separation process could be finished. Programming the temperature whether it will be increased or decreased is very important in most methods.

Analysis - Once the sample is injected through the column, the mobile phase comes in. Insert the carrier gas through the column at a required time. It is important to check on the retention time and the temperature. The sample’s outlet stream can now be monitored and analyzed through a detection device. A gas chromatograph is connected to a detector. Normally a mass spectrometer is used as a detection device, but it actually depends on the sample as well as on the application. This will then allow proper analysis of the sample. A gas chromatograph produces the results through graphical presentation. The graph reveals the response of the detection device against the retention time. The output is being recorded in a data recorder that plots the signal coming from the detection device. There are two types of analyses that can be determined through gas chromatography. The qualitative analysis and the quantitative analysis are the two types of analysis that can be produced in the process. The qualitative analysis is being produced through graphical presentation. Obviously the quantitative analysis comprises of calculations of the area of the peak of the concentration and this can be calculated using a calibration curve. During this modern time, a software program is already available to convert the data and come out with the analysis required.


Where do we apply the gas chromatography?

One may wonder where we apply the gas chromatography. It can also be related to the importance of GC in the real world. Why is it developed? What are the uses of gas chromatography?

As it is the technique for separating different components in a compounded substance, the process is being used to determine the complexities of mixtures. It is significant in ensuring the quality control especially in the drug industry. It’s also essential in the quality control in the chemical industry. Gas chromatography is also important in analyzing the pollution in the environment. It is very important to measure the pollutants present in the air. It is also critical in the clinical analysis. Sample of use of gas chromatograph is its application in the natural gas pipeline. It is very important to have an instrument that will measure the different components relative thereto.

The process of GC is part of the laboratory activities during college. With this, we can say that every one has somehow experienced the process at the most simple and basic way. The experiment of separating the contents of lavender oil is usually used during college laboratory or practical application. Some uses the experiment of measuring the ethylene content of plants. This is being done by injuring the leaves and secreting the liquid components out of the leaves.




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