Gas Chromatography

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Gas Chromatography. - PowerPoint PPT Presentation

Transcript of Gas Chromatography

Slide 1

Gas ChromatographyGas chromatography is a type of chromatography used in chemistry for analyzing and separating compounds that can be convert to gas without decomposition. Uses of this technique include testing the purity of a Gaseous substance and separating the different components of a mixture. Sometimes, it may help in identifying the compound. It can also be used to prepare pure compounds from a mixture. Gas chromatography is similar to fractional distillation, as both processes separate the mixture components primarily based on their vapor pressure differences.

3Principle of GC

Gas chromatography is based on the principle of partition(differential distribution) of an volatile compound(gas) in two phases - a liquid phase covering the adsorbent surface and a gaseous phase of the eluting gas. With a fixed set of parameters (length and diameter of column, temp., nature and flow rate of the eluting gas etc.)

Compounds A and B interact with the stationary phase through intermolecular forces.A interacts more strongly with the stationary liquid phase and is retained relative to B, which interacts weakly with the stationary phase. Thus B spends more time in the gas phase and advances more rapidly through the column and has a shorter retention time than A.Typically, components with similar polarity elute in order of volatility. Thus alkanes elute in order of increasing boiling points; lower boiling alkanes will have shorter retention times than higher boiling alkanes.

Sample is injected into the injection port. Sample vaporizes and is forced into the column by the carrier gas ( = mobile phase which in GC is usually helium) Components of the sample mixture interact with the stationary phase so that different substances take different amounts of time to elute from the column.The separated components pass through a detector. Electronic signals, collected over time, are sent to the GC software, and a chromatogram is generated.

Components of a Gas Chromatograph

Gas Supply: E.g. N2 or HeSample Injector: syringe or septumColumn: Tubing packed with small uniform size, inert support coated with thin film of nonvolatile liquidDetectors: - Thermal conductivity (TC) - Flame ionization detector (FID) - Electron Capture (ECD) - Photo Ionization (PID)

Gas-Supply

Carrier gases, which should be chemically inert and non reactive, include Helium, Nitrogen, and Hydrogen. Along with the gas supply there are pressure regulators, gauges, and flow meters. In addition, the gas carrier system often contains a molecular sieve to remove water and other impurities.

Sample Injection SystemColumn efficiency depends upon that Sample should be of suitable size and introduced as a plug of vapor. Slow injection of oversized samples causes poor resolution and band spreading . The most common method of sample injection involves the use of micro syringe to inject a liquid or gaseous sample through a self-sealing, silicone-rubber diaphragm or septum into a flash vaporizer port located at the head of the column.

Schematics of sample injection

Column Configurations

Two types of columns are commonly used in gas chromatography Packed and open tubular Capillary typeChromatographic columns vary in length from