Gas chromatography

General Principles of Gas Chromatography

There are two types of gas chromatography: gas-liquid chromatography, where the sorbent is a nonvolatile liquid called the stationary liquid phase, coated as a thin layer on an inert, granular solid support; and gas-solid chromatography, where the sorbent is a granular solid of large surface area (1:666). The moving gas phase, called the carrier gas, is an inert gas such as nitrogen or helium. The gas flows through the chromatographic column packed with the sorbent. The solution being chromatographed divides itself between the moving gas phase and the sorbent, and moves through the column at a rate dependent upon its partition coefficient, or solubility (in the liquid phase in gas-liquid chromatography), or upon its adsorption coefficient on the packing (in gas-solid chromatography), and the carrier gas flow rate (1:666).

Gas-solid is the older of the two forms of gas chromatography, and was originally limited to the separation of permanent gases or relatively non

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main part of the gas chromatography equipment is the columns, which lie within an oven. The columns can be glass or metal tubes, from three to six millimeters in diameter, and packed with an inert diatomaceous earth support coated with a nonvolatile liquid to from three to 20 percent by weight. When capillary tubes are used, the support for the film of the liquid phase is the capillary walls themselves. Open tubular columns can be used which are support-coated, and the sample capacity of the columns can be increased by the presence of very loosely packed support material, or by roughening the capillary walls. The choice of the liquid phase is the most crucial factor in the separation of the specimen being analyzed (1:667). Common liquid phases include high molecular weight hydrocarbons such as Apiezon L and the natural product, squalene, silicon gum rubbers, and methyl phenyl silicones, polyethylene glycols, and some esters of low volatility, such as triethylene glycol succinate. Polar liquid phases make good solvents for polar solutes, while nonpolar liquid phases are often the best for separation of nonpolar solutes (1:668). When choosing the liquid phase, solute-solvent interactions such as hydrogen bonding must be tak
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Approximate Pages = 6 (250 words per page)

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