Chromatography
Chromatography is the process of separating mixtures of chemicals into individual components as a means of identification or purification. It derives from the Greek words chroma, meaning color, and graphy, meaning writing. The word was coined in 1906 by the Russian chemist Mikhail Tsvett who used a column to separate plant pigments. Currently chromatography is applied to many types of separations far beyond those of just color separations. Common chromatographic applications include gas-liquid chromatography (GC), liquid-solid chromatography (LC), thin layer chromatography (TLC), ion exchange chromatography, and gel permeation chromatography (GPC). All of these methods are invaluable in analytical environmental chemistry, particularly GC, LC, and GPC.
The basic principle of chromatography is that different compounds have different retentions when passed through a given medium. In a chromatographic system, one has a mobile phase and a stationary phase. The mixture to be separated is introduced in the mobile phase and passed through the stationary phase. The compounds are selectively retained by the stationary phase and move at different rates which allows the compounds to be separated.
In gas chromatography, the mobile phase is a gas and the stationary phase is a liquid fixed to a solid support. Liquid samples are first vaporized in the injection port and carried to the chromatographic column by an inert gas which serves as the mobile phase. The column contains the liquid stationary phase, and the compounds are separated based on their different vapor pressures and their different affinities for the stationary phase. Thus different types of separations can be optimized by choosing different stationary phases, and by altering the temperature of the column. As the compounds elute from the end of the column, they are detected by one of a number of methods that have specificity for different chemical classes.
Liquid chromatography consists of a liquid mobile phase and a solid stationary phase. There are two general types of liquid chromatography: column chromatography and high pressure liquid chromatography (HPLC). In column chromatography, the mixture is eluted through the column containing stationary packing material by passing successive volumes of solvents or solvent mixtures through the column. Separations result as a function of both chemical-solvent interactions as well as chemical-stationary phase interactions. Often this technique is used in a preparative manner to remove interferences from environmental sample extracts. HPLC refers to specific instruments designed to perform liquid chromatography under very high pressures to obtain a much greater degree of resolution. The column outflow is passed through a detector and can be collected for further processing if desired. Detection is typically by ultraviolet light or fluorescence.
A variation of column chromatography is gel permeation chromatography (GPC), which separates chemicals based on size exclusion. The column is packed with porous spheres, which allow certain size chemicals to penetrate the spheres and excludes larger sizes. As the sample mixture traverses the column, larger molecules move more quickly and elute first while smaller molecules require longer elution times. An example of this application in environmental analyses is the removal of lipids (large molecules) from fish tissue extracts being analyzed for pesticides (small molecules).
Resources
Books
McNair, H. M., and E. J. Bonelli. Basic Gas Chromatography. Palo Alto, CA: Varian Instruments, 1969.
Peters, D. G., J. M. Hayes, and G. M. Hieftje. Chemical Separations and Measurements. Philadelphia: Saunders, 1974.
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