Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 14:07

Another useful set of techniques for separating mixtures is called chromatographyThe separation of a mixture into its components by taking advantage of each components' different affinities for a stationary phase and a mobile phase.. Perhaps the simplest of these techniques to describe is paper chromatography, an example of which is shown in the video below.

Three substances are applied to a strip of chromatography-grade paper (the stationary phase of this experiment). As the liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container level rises and meets the spots, the sample partially dissolves in the liquid (the mobile phase because it is moving) and travels up the plate within the solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture.. Different substances will travel different distances along the plate. The distance that a substance will travel depends on how strongly it adheres to the stationary phase (a process called adsorptionFormation of a layer of gas, liquid, or solid on the surface of a solid; the adsorbed substance is held by either covalent bonds or by non-covalent (intermolecular) forces. Adsorption differ from absorption in that the adsorbed substance is on the surface only.) versus how much time it spends dissolved in the mobile phase. The more a substance adsorbs, the less it dissolves and the less it moves along the plate. The first purple spot is an example of a substance highly adsorbed to the stationary phase. The less a substance adsorbs, the more it dissolves and the farther it travels, such as the yellow spot in this example. The process is continued until a good separation is created. In this manner, a mixture of substances may be separated such as the middle sample, which was composed of the yellow and purple substances originally. Notice that while it was not initially obvious that the middle spot contained both substances, this fact is clear after performing paper chromatography.

All forms of chromatography work on the same general principle as paper chromatography. There is always a stationary phase which does not move and a mobile phase which does. The various components in the mixture being chromatographed separate from each other because they are more strongly held by one phase or the other. Those which have the greatest affinity for the mobile phase move along the fastest.

The most important form of chromatography is gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. chromatography or vaporThe gaseous state of a substance that typically exists as a liquid or solid; a gas at a temperature near or below the boiling point of the corresponding liquid.-phase chromatography. A long column is packed with a finely divided solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. whose surface has been coated with an inertUnreactive. Used to describe coordination complexes that exchange ligands slowly or an electrode in an electrochemical cell that serves only as a surface where reaction can occur and is neither consumed nor added to during reaction. liquid. This liquid forms the stationary phase. The mobile phase is provided by an inert carrier gas, such as He or N2, which passes continuously through the interstices, among the solid particles. A liquid sample can be injected into the gas stream and vaporized just before it enters the tube. As this sample is carried through the tube by the slow stream of gas, those components which are most solubleAble to dissolve in a solvent to a significant extent. in the inert liquid are held up, while the less-soluble components move on more rapidly. The components thus emerge one by one from the end of the tube. In this way it is possible to separate and analyze mixtures of liquids which it would be impossible to deal with by distillationA technique for separating the components of a liquid mixture or solution by vaporizing a portion of the liquid and condensing its vapor in a separate container. or any other technique.

The development of chromatography is one of the major revolutions in technique in the history of chemistry, comparable to that which followed the development of an accurate balance. Separations which were previously considered impossible are now easily achieved, sometimes with quite simple apparatus. This technique is particular essential to the science of biochemistry, in which complexA central metal and the ligands surrounding it; also called coordination complex. mixtures are almost always encountered. In the field of environmental chemistry, chromatography has helped us separate and detect very low concentrations of contaminants like DDT or PCB (polychlorinated biphenyls). The major drawback to chromatography is that it does not lend itself to large-scale operation. As a result it remains largely a laboratory, rather than an industrial, technique for separating mixtures.