9 Important things about Gas Chromatography
What is gas chromatography’s definition?
Gas chromatography is an analytical technique used to separate components of a mixture by their volatility. Gas Chromatography (GC) is a method for separating compounds based on their volatility. It involves heating samples in a closed container until all the volatile components evaporate leaving behind the nonvolatile compounds. The remaining sample is then injected into a column where it is separated according to its affinity for different materials.
Who invented gas chromatography?
Anthony T. James and Archer J.P. Martin are widely credited with inventing gas chromatography. Instead of adsorption chromatography, their gas chromatograph used partition chromatography as the separating principle. After the development of the flame ionization detector, the popularity of gas chromatography skyrocketed. Martin and another of his colleagues, Richard Synge, with whom he shared the 1952 Nobel Prize in Chemistry, had previously noted in a paper that chromatography could be used to separate gases. While Synge pursued other projects, Martin continued his work with James.
What is the basic principle of gas chromatography?
The basic principle of gas chromatography is that the greater the affinity of the compound for the stationary phase, the more the compound will be retained by the column and the longer it will be before it is eluted and detected. Thus the heart of the gas chromatograph is the column in which separation of the component takes place, and to this must be added the source and control of the carrier gas flow through the column, a mean of sample introduction, and a means of detection of the components as they elute from the end of the column.
The basis of the separation is a retardation of the individual components as they are moved through a long column by a carrier gas, usually helium or nitrogen. The column consists of a steel or glass tube filled with inert packing material such as glass or ceramic beads.
In gas-liquid chromatography (GLC), these are coated with a volatile liquid, so that the surface area of the liquid in contact with the gas is large. For some applications, the packing may be solid without any liquid coating; it is then called gas-solid chromatography (GSC), but this is less widely used than GLC. The sample is injected into the carrier gas stream. As it moves through the column with the carrier gas, the molecules of each substance present in the sample will distribute between the gas and the liquid. Individual molecules will constantly move between the gas and the liquid in a dynamic equilibrium. While a molecule is in the gas phase it will pass along the column, while it remains dissolved in the liquid it will be stationary.
The more volatile a substance, the greater proportion of time its molecules will be moving in the carrier gas, and so the sooner it will emerge from the column. In this way, each substance will become separated within the column and emerge separated by time in the end.
What is the process of gas chromatography?
The process involves injecting a sample into a stream of gas which passes through a column packed with a solid material called a stationary phase. GC has been widely used in chemical analysis since the 1940s. In recent years, it has become increasingly popular as a forensic tool because it provides a fast and accurate means of identifying chemicals. A gas chromatograph consists of three main components: a column, injector, and detector. The column is where the separation takes place. The injector is where the sample is introduced into the system. The detector measures the concentration of each component in the sample.
How does gas chromatography work?
As the sample travels through the column, it interacts with the stationary phase and separates out based on its affinity for the stationary phase. The separation process occurs when the gas molecules interact with the stationary phase. This interaction depends on the type of stationary phase used. A typical GC system uses a glass tube filled with a solid material called a “stationary phase.”
The stationary phase is coated onto the inside wall of the glass tube. The sample enters at one end of the tube and moves down the tube until it reaches the stationary phase. At this point, the different compounds within the sample will interact differently with the stationary phase. The compounds that interact more strongly with the stationary phase will move faster than those that do not. Once the sample passes through the stationary phase, it exits the other end of the tube into a detector where the separated compounds are identified. The separated components exit the column at different times and are detected using detectors. The speed at which the compounds travel through the column is measured by a device called a detector.
Detectors for gas chromatography
There are two main types of detectors: flame ionization (FID) and mass spectrometry (MS). FID detects the compounds based on how much energy they emit as they pass through the flame. MS detects the compounds based on their mass. A computer then analyzes the data from the detector and creates a graph showing how much of each component was present in the original sample. Gas chromatography is often used to analyze organic compounds such as hydrocarbons, alcohols, and other volatile substances. It is also commonly used to detect contaminants in food and water.
Most detectors require one or more gases to function properly. Flow rates for each type of detector vary between GC manufacturers. It is important to follow the recommended flow rates to obtain the optimal sensitivity, selectivity, and linear range. The magnitude of the detector signal is plotted versus time (from the time of injection).
FLAME IONIZATION DETECTOR (FID): A hydrogen-air flame is used to burn compounds. Carbon-containing compounds emit ions that attract the collector. A signal is generated after counting the number of ions that hit the collector.
NITROGEN PHOSPHORUS DETECTOR (NPD): Compounds are burned in a plasma that surrounds a rubidium bead that has been supplied with hydrogen and air. Compounds containing nitrogen and phosphorus emit ions that attract the collector. The number of ions striking the collector is counted, and a signal is produced.
ELECTRON CAPTURE DETECTOR (ECD): The electrons are supplied by a 63Ni foil that lines the detector cell. In the cell, a current is generated. Electronegative compounds absorb electrons, causing the current to decrease. The amount of current loss is measured indirectly, and a signal is generated.
THERMAL CONDUCTIVITY DETECTOR (TCD): A detector cell contains a heated filament that is powered by an applied current. The filament current changes as a carrier gas containing solutes pass through the cell. The current variation is compared to the current in a reference cell. A signal is generated after measuring the difference.
FLAME PHOTOMETRIC DETECTOR (FPD): A hydrogen-air flame is used to burn compounds. Compounds containing sulphur and phosphorus produce light emitting species (sulfur at 394 nm and phosphorous at 526 nm). A monochromatic filter allows only one wavelength to pass through. A photomultiplier tube is used to measure the amount of light and generates a signal. Each detection mode necessitates a different filter.
PHOTOIONIZATION DETECTOR (PID): Compounds eluting into a cell are bombarded with photons of high energy emitted by a lamp. Ionization occurs on compounds with ionization potentials less than the photon energy. The ions produced are attracted to an electrode, measured, and a signal is produced.
ELECTROLYTIC CONDUCTIVITY DETECTOR (ELCD): Compounds are mixed with a reaction gas before being passed through a high-temperature reaction tube. Specific reaction products are formed and mixed with a solvent before passing through an electrolytic conductivity cell. The change in the solvent’s electrolytic conductivity is measured, and a signal is generated. The temperature of the reaction tube and the solvent used determine which compounds are detected.
MASS SPECTROMETER (MS): The detector is maintained under a vacuum. Compounds are bombarded with electrons (EI) or gas molecules (CI) The resulting ions are focused and accelerated into a mass filter. The mass filter selectively allows all ions of a specific mass to pass through. A mass spectrum is obtained for each scan which plots the various ion masses versus their abundance or number.
Stationary & mobile phase in gas chromatography
Gas chromatography is the separation of compounds in a mixture by injecting a gaseous or liquid sample into a mobile phase, commonly referred to as the carrier gas, and passing the gas through a stationary phase. In most cases, the mobile phase is an inert or unreactive gas such as helium, argon, nitrogen, or hydrogen. The stationary phase is a microscopic layer of viscous liquid on a solid particle surface on an inert solid support inside a column of glass or metal tubing. In some columns, the surface of the solid particles may also act as the stationary phase.
Gas chromatography and Mass spectroscopy
Gas chromatography-mass spectrometry (GC-MS) is an analytical technique that combines the characteristics of gas chromatography and mass spectrometry to identify different substances in a test sample. GC-MS has applications in drug detection, fire investigation, environmental analysis, explosives investigation, and sample identification.
Application of gas chromatography
- Used for separating volatile mixtures.
- Food analysis, both qualitative and quantitative
- Contaminants such as environmental pollutants, pesticides, and naturally occurring toxins are detected and analyzed.
- Used to control the quality of chemicals in medicines, automobiles, and other products. Also used to analyze natural products and detect blood alcohol levels.
- GLC separates hundreds of hydrocarbons from petroleum.
- Also used to investigate reaction mechanisms.
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