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In addition to reading this lab, be sure to view the ChemPages modules on:  Chromatography, paper (plus Self-Check Exercises)

Chromatography (from the Greek chroma, for color, and graphein, to write) is a technique often used by chemists to separate components of a mixture. Paper chromatography is only one example of many different chromatographic methods. The first type of chromatography to be discovered was . In 1906, the Russian botanist Mikhail Tsvett separated color pigments present in leaves by allowing a solution of these pigments to flow down a column packed with an insoluble material such as starch, alumina (Al2O3), or silica (SiO2). Because different color bands appeared along the column, he called the procedure chromatography. Color is not a requisite property to achieve separation of compounds by this procedure. Colorless compounds can be made visible by being allowed to react with other reagents, or they can be detected by physical means. Consequently, because of its simplicity and efficiency, this technique has wide applicability for separating and identifying compounds such as drugs and natural products.

Mixtures of volatile liquids are commonly separated by a method called . In this method, a mixture of liquids is vaporized into a gas stream flowing through a long tube of solid absorbent material coated with an appropriate liquid. The carrier gas is usually helium. As with paper chromatography, the components of the mixture will have different in the liquid coating. Separation of the components of the mixture thus occurs as the mixture progresses through the tube. The individual components of the mixture exit the tube one by one and are usually detected by electronic means.

General Introduction to Paper Chromatography

Paper chromatography is a simple technique. A single drop or spot of the mixture to be analyzed is applied about 1 cm from the end of a strip of filter paper. The end of the filter paper strip is then placed in a beaker or jar in a shallow layer of . Since filter paper is very permeable to liquids, the solvent begins rising up the filter paper strip by capillary action. The solvent is called the mobile phase; the paper is the stationary phase.

As the solvent rises to the level at which the spot of mixture was applied to the filter paper, various effects can occur, depending on the constituents of the spot. Those components of the spot that are completely soluble in the solvent will be swept along with the solvent front as it continues to wick up the paper. Those components that are not at all soluble in the solvent will be left behind at the original location of the spot. Most components of the spot mixture will take an intermediate approach as the solvent front passes, however. Components that are somewhat soluble will be swept along by the solvent front, but to different extents, reflecting their specific solubilities. By this means, the original spot of mixture is spread out into a series of spots, with each spot representing one single component of the original mixture.

The separation of a mixture by chromatography is not solely a function of the of the components in the solvent used, however. The filter paper used in chromatography is not inert, but consists of molecules which may interact with the components of the mixture being separated. Each component of the mixture is likely to have a different extent of interaction with the filter paper. This differing extent of interaction between the components of a mixture and the filter paper forms an equally important basis for the separation. Filter paper adsorbs molecules on its surface to differing extents, depending on the structure and properties of the molecules. This process is different from absorption!

To place a paper chromatographic separation on a quantitative basis, a mathematical function called the retention factor, Rf, is defined: distance traveled by spot (cm) R = f distance traveled by solvent (cm)

Paper Chromatography 2-1