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Unformatted text preview: Gas Chromatography
By Ashay Mehta What is Gas Chromatography?
A machine that is used to help chemists identify and analyze compounds after separation of the compound Gas Chromatography dates back to 1903 when it was cited in the works of Mikhail Semenovich Tswett Other types of chromatography have been used but this is the most common Uses of Gas Chromatography
Used to separate and analyze materials without decomposition Used to test the purity of a substance as well GC helps in identifying the compound in some cases Columns Components of a GC Packed columns 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 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. Capillary columns have a very small internal diameter, on the order of a few tenths of millimeters, and lengths between 2560 meters are common. The inner column walls are coated with the active materials. Some columns are quasi solid filled with many parallel micropores. Most capillary columns are made of fusedsilica with a polyimide outer coating. These columns are flexible, so a very long column can be wound into a small coil. Components of a GC (cont'd)
Column inlet 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 There are Split/Splitless, Oncolumn, Programmed Temperature Vaporizing (PTV), Gas source, Purge and Trap and Solid phase microextraction inlets as well Components of a GC (cont'd)
There are many types of detectors but the most commonly used are flame ionization detector (FID) and the thermal conductivity detector (TCD) Flame Ionization Detector are sensitive primarily to hydrocarbons, and are more sensitive than TCD. An FID destroys most if not all of the components it is detecting making it destructive. For this reason, in multipledetector situations, the FID is almost always the last detector.An FID essentially can only detect components which can be burned. Thermal Conductivity Detector TCDs are essentially universal and can be used to detect any component other than the carrier gas. In a TCD, the components can continue on to another detector after passing through the TCD. Thus, it is considered a nondestructive detector. The TCD is a good general purpose detector for initial investigations with an unknown sample and can also be used to to analyze permanent gases because it responds to all pure substances. How Does It Work?
The mobile phase is the carrier gas, usually an inert or unreactive gas, and the stationary phase is microscopic layer of liquid on an inert solid support, inside of a column. The gaseous compounds being analyzed interact with the walls of the column, which is coated with different stationary phases. This causes each compound to elute at a different time, known as the retention time of the compound. More Detailed
A syringe needle is placed into a hot injector port of the and the sample is injected. The injector is set to a temperature higher than the components' boiling points. So, components of the mixture evaporate into the gas phase inside the injector. A carrier gas flows through the injector and pushes the gaseous components of the sample onto the GC column. Molecules split between the mobile and stationary phases. After components of the mixture move through the GC column, they reach a detector. Ideally, components of the mixture will reach the detector at varying times due to differences in the partitioning between mobile and stationary phases. The detector sends a signal to the chart recorder which results in a peak on the chart paper. The area of the peak is proportional to the number of molecules generating the signal. The End ...
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This note was uploaded on 06/02/2010 for the course MGMT 121 taught by Professor Morozova during the Spring '10 term at NJIT.
- Spring '10