CHROMATOGRAPHY


Meaning of CHROMATOGRAPHY in English

method for separating chemical substances that makes use of the relative rates at which they are adsorbed from a moving stream of gas or liquid on a stationary substance, which is usually a finely divided solid, a sheet of filter material, or a thin film of a liquid on the surface of a solid. It is a versatile method that can separate very complex mixtures even in the absence of detailed previous knowledge of the number, nature, or relative amounts of the individual substances present. Chromatography depends for its separating ability on differential retardation or retention caused by unequal adsorption of the different components carried along by a stream of inert liquid or gas. The method is widely used for the separation of chemical compounds of biological origin (for example, amino acid fragments of proteins) and of complex mixtures of petroleums and volatile aromatic mixtures, such as perfumes and flavours (which can contain hundreds of different components). Chromatography is carried out by passing a gas or liquid (the mobile phase) containing the unknown mixture over or through an adsorbing material (the stationary phase) of high surface area. The components of the mixture will be separated by the stationary phase and can be identified individually. Further analysis can be carried out by subjecting the separated components to mass spectrometry. Chromatography was originally used for the separation of coloured compounds (hence the name), notably by the Russian botanist Mikhail Tsvet (or Tswett), who investigated mixtures of plant pigments by adding solutions of these mixtures to the top of a column of powdered alumina and washing the column with an organic solvent. The pigments then separated into series of discrete coloured bands in the column. Each component thus isolated can be recovered either by extruding the solid column and physically separating the different bands or by using carefully chosen solvents to wash them off the column separately, a technique known as elution. Tsvet's method was neglected for many years, but since the 1930s it has been extensively used for identifying many biologically important materials, especially after it was adapted to use filter paper as the support. With suitable apparatus, mixtures applied to the paper can be separated in two dimensions; paper chromatography has been widely used to study colourless amino acids, steroids, carbohydrates, and other complex materials of natural origin. The components on the eluted paper appear as coloured spots when sprayed with suitable developing fluids. Gas chromatography, which employs a gaseous mobile phase, is carried out in either a packed or capillary column configuration. In a packed column a solid support, contained in a metal or glass tube, serves as the stationary phase or is coated with a liquid stationary phase. Capillary columns are small-diameter tubes whose walls are coated with the stationary phase. Mixtures injected into the inlet of the column are driven by compressed gas (usually hydrogen or helium), and the components appear in well-separated zones. Several methods of detection of the eluted components are employed, notably devices that compare the thermal conductivity of the separated gas with that of a reference gas stream. Alternatively, in the flame ionization detector, the separated gas stream is mixed with hydrogen and burned; the resulting ions are detected by their ionization potential. This method is widely used for detection of microgram quantities of hydrocarbons. Liquid chromatography is performed either in a column or on a plane. The most widely used columnar liquid technique is high-performance liquid chromatography, in which a pump forces the liquid mobile phase through a high-efficiency, tightly packed column at high pressure. Detection is accomplished by measuring some property of the sample components, such as absorbance or fluorescence. supercritical-fluid chromatography, the mobile phase is a supercritical fluid, a vapour phase that has reached a specific temperature beyond which it can no longer condense to a liquid regardless of how high the pressure is raised. This method is particularly useful in separating compounds that are thermally unstable, nonpolar, or nonvolatile. technique for separating the components, or solutes, of a mixture on the basis of the relative amounts of each solute distributed between a moving fluid stream, called the mobile phase, and a contiguous stationary phase. The mobile phase may be either a liquid or a gas, while the stationary phase is either a solid or a liquid. Kinetic molecular motion continuously exchanges solute molecules between the two phases. If, for a particular solute, the distribution favours the moving fluid, the molecules will spend most of their time migrating with the stream and will be transported away from other species whose molecules are retained longer by the stationary phase. For a given species, the ratio of the times spent in the moving and stationary regions is equal to the ratio of its concentrations in these regions, known as the partition coefficient. (The term adsorption isotherm is often used when a solid phase is involved.) A mixture of solutes is introduced into the system in a confined region or narrow zone (the origin), whereupon the different species are transported at different rates in the direction of fluid flow. The driving force for solute migration is the moving fluid, and the resistive force is the solute affinity for the stationary phase; the combination of these forces, as manipulated by the analyst, produces the separation. Chromatography is one of several separation techniques defined as differential migration from a narrow initial zone. Electrophoresis is another member of this group. In this case, the driving force is an electric field, which exerts different forces on solutes of different ionic charge. The resistive force is the viscosity of the nonflowing solvent. The combination of these forces yields ion mobilities peculiar to each solute. Chromatography has numerous applications in biological and chemical fields. It is widely used in biochemical research for the separation and identification of chemical compounds of biological origin. In the petroleum industry the technique is employed to analyze complex mixtures of hydrocarbons. Roy A. Keller As a separation method, chromatography has a number of advantages over older techniques-crystallization, solvent extraction, and distillation, for example. It is capable of separating all the components of a multicomponent chemical mixture without requiring an extensive foreknowledge of the identity, number, or relative amounts of the substances present. It is versatile in that it can deal with molecular species ranging in size from viruses composed of millions of atoms to the smallest of all molecules-hydrogen-which contains only two; furthermore, it can be used with large or small amounts of material. Some forms of chromatography can detect substances present at the picogram (10-12 gram) level, thus making the method a superb trace analytical technique extensively used in the detection of chlorinated pesticides in biological materials and the environment, in forensic science, and in the detection of both therapeutic and abused drugs. Its resolving power is unequaled among separation methods. Additional reading Chromatography is introduced in Colin F. Poole and Sheila A. Schuette, Contemporary Practice of Chromatography (1984); and E. Heftmann (ed.), Chromatography: Fundamentals and Applications of Chromatographic and Electrophoretic Methods, 2 vol. (1983). The history of chromatography is chronicled in L.S. Ettre, "Evolution of Liquid Chromatography," in Csaba Horvth (ed.), High-performance Liquid Chromatography, vol. 1 (1980), pp. 1-74; and L.S. Ettre and A. Zlatkis (eds.), 75 Years of Chromatography: A Historical Dialogue (1979). Discussions of specific methods of chromatography include Robert L. Grob (ed.), Modern Practice of Gas Chromatography, 2nd ed. (1985); Milton L. Lee, Frank J. Yang, and Keith D. Bartle, Open Tubular Column Gas Chromatography (1984); Roger M. Smith, Gas and Liquid Chromatography in Analytical Chemistry (1988); L.R. Snyder and J.J. Kirkland, Introduction to Modern Liquid Chromatography, 2nd ed. (1979); R.P.W. Scott, Liquid Chromatography Detectors, 2nd rev. ed. (1986); Bernard Fried and Joseph Sherma, Thin-layer Chromatography, 2nd ed. rev. and expanded (1986); Joseph C. Touchstone and Murrel F. Dobbins, Practice of Thin Layer Chromatography, 3rd ed. (1992); Frank J. Yang (ed.), Microbore Column Chromatography: A Unified Approach to Chromatography (1989); Milos V. Novotny and Daido Ishii (eds.), Microcolumn Separations: Columns, Instrumentation, and Ancillary Techniques (1985); W.W. Yau, J.J. Kirkland, and D.D. Bly, Modern Size-exclusion Liquid Chromatography: Practice of Gel Permeation and Gel Filtration Chromatography (1979); Hamish Small, Ion Chromatography (1989); Roger M. Smith (ed.), Supercritical Fluid Chromatography (1989); A.F. Bergold et al., "High Performance Affinity Chromatography," in Csaba Horvth (ed.), High-performance Liquid Chromatography, vol. 5 (1988), pp. 95-209; and Josef Janca, Field-flow Fractionation, trans. from Czech (1988). Advances in Chromatography (1965- ), is a multivolume series on various topics of chromatography. Periodicals include Journal of Chromatography (irregular); Chromatographia (monthly); Preparative Chromatography (quarterly); and LC GC: Magazine of Liquid and Gas Chromatography (monthly). Roy A. Keller

Britannica English vocabulary.      Английский словарь Британика.