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FIGURE 3.1 The principle of a chromatographic separation. (a) A mixture of solutes is introduced to the chromatographic system in the form of a sharp band and is carried through the system by the mobile phase. (b) Molecules of an unretained solute (marked by circles) do not interact with the stationary phase; other molecules partition into the stationary phase according to their partition coefficient. Partitioning is a dynamic process, with equal number of molecules going into and out of the stationary phase at any given time when the system is at equilibrium. (c) Unretained solute molecules travel at the same speed as the mobile phase and elute from the system in the time tm. (d) Molecules of the first retained solute reach the outlet of the system in time tr1. At any given time during the separation, the concentration of the solute marked by squares in the stationary phase is greater than the concentration of the solute marked by triangles. Thus, the solute marked by squares spends on average more time in the stationary phase and elutes from the system last.

FIGURE 3.1 The principle of a chromatographic separation. (a) A mixture of solutes is introduced to the chromatographic system in the form of a sharp band and is carried through the system by the mobile phase. (b) Molecules of an unretained solute (marked by circles) do not interact with the stationary phase; other molecules partition into the stationary phase according to their partition coefficient. Partitioning is a dynamic process, with equal number of molecules going into and out of the stationary phase at any given time when the system is at equilibrium. (c) Unretained solute molecules travel at the same speed as the mobile phase and elute from the system in the time tm. (d) Molecules of the first retained solute reach the outlet of the system in time tr1. At any given time during the separation, the concentration of the solute marked by squares in the stationary phase is greater than the concentration of the solute marked by triangles. Thus, the solute marked by squares spends on average more time in the stationary phase and elutes from the system last.

range of methods classified as liquid chromatography (LC). Gases can be used as the mobile phase whenever the components to be separated have appreciable vapor pressures. Methods based on this principle are classified as gas chromatography (GC). Highly compressed, dense gases (fluids) kept above their critical temperatures are used in supercritical fluid chromatography (SFC). This last type of fluid has very peculiar properties — its density and solvating power are close to those of a liquid, while its viscosity is only somewhat greater than that of a gas. Consequently, SFC bridges the gap between LC and GC. In spite of its potential, SFC did not find any significant applications in environmental analysis and hence it will not be discussed in more detail.

Another classification of chromatographic methods is based on the physical form of the stationary phase used. In the vast majority of chromatographic separations the stationary phase is confined within a tube through which the mobile phase is fed. The tube is called a chromatographic column, and all such methods are classified as column chromatography. The stationary phase in column chromatography can have the form of a compact bed of small, usually porous particles packed inside the column, or can be spread on the walls of the column. Columns of the first type are called packed columns, while columns of the second type are called open tubular columns. Alternatively, the stationary phase can be spread as a thin, homogenous layer on a flat, inert support. Methods utilizing this approach are termed thin-layer chromatography (TLC). In fact, TLC is a representative of a broader group of methods in which the stationary phase has a planar form, so called planar chromatography. Another representative of this group is paper chromatography, in which the support material itself (paper) constitutes the stationary phase.

LC uses mostly packed columns, as the use of open tubular columns in this method is not practical because of the extremely small column diameters required for good separation. In gas chromatography, both packed and open tubular columns can be used, but the latter are far more popular because of their vastly superior properties. The mobile phase is usually forced through the stationary phase at elevated pressure, although other approaches are also possible (e.g., electrically driven flow in electrochromatography (EC), gravity driven flow in classical LC or flow driven by capillary forces in TLC).

Chromatographic methods can be classified according to the type of interaction between the solute and the stationary phase. Of the several possibilities, sorption is by far the most common. The term applies to a class of processes in which one material (in this case the solute) is taken up by another (the stationary phase). Whenever the solute is confined to the surface of the stationary phase, we call the process adsorption, and the method is called adsorption chromatography. Whenever the solute penetrates the stationary phase and enters the bulk of it absorption occurs; however, chromatographic methods based on this principle are usually called partition chromatography. Ion exchange makes it possible to separate ions by liquid chromatography. Methods based on this principle are referred to as ion chromatography. Polymers and other high molecular weight compounds can be separated according to their size by using materials whose pores cover a specific range of sizes. Such methods are called size exclusion chromatography. Specific interactions between stationary phase and one particular type of solute form the basis of affinity chromatography. This last method differs from all the others in that it is typically used to isolate a single solute from a complex mixture rather than to separate the components of the mixture from each other.

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