Chapter 26 - Chapter 26 An Introduction to Chromatographic...

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Chapter 26 An Introduction to Chromatographic Separations Chromatography permit the scientist to separate closely related components of complex mixtures. In all chromatographic separations the sample is transported in a mobile phase, which may be a gas, a liquid, or a supercritical fluid. This mobile phase is then forced through an immiscible stationary phase, which is fixed in place in a column or on a solid surface. The two phases are chosen so that the components of the sample distribute themselves between the mobile and stationary phase to varying degrees.
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Classification of Chromatographic Methods Chromatographic methods can be categorized in two ways. The first classification is based upon the physical means by which the stationary and mobile phases are brought into contact. In column chromatography, the stationary phase is held in a narrow tube through which the mobile phase is forced under pressure. In planar chromatography, the stationary phase is supported on a flat plate or in the interstices of a paper; here, the mobile phase moves through the stationary phase by capillary action or under the influence of gravity.
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Classification of Chromatographic Methods A more fundamental classification of chromatographic methods is one based upon the types of mobile and stationary phases and the kinds of equilibria involved in the transfer of solutes between phases. Three general categories of chromatography: (1) liquid chromatography, (2) gas chromatography, and (3) supercritical-fluid chromatography. The mobile phases in the three techniques are liquids, gases, and supercritical fluids respectively.
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Elution Chromatography on Columns Elution involves washing a species through a column by continuous addition of fresh solvent. The sample is introduced at the head of a column, whereupon the components of the sample distribute themselves between the two phases. Introduction of additional mobile phase (the eluent) forces the solvent containing a part of the sample down the column, where further partition between the mobile phase and fresh portions of the stationary phase occurs.
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Chromatograms If a detector that responds to solute concentration is placed at the end of the column and its signal is plotted as function of time (or of volume of the added mobile phase), a series of peaks is obtained. Such a plot, called a chromatogram, is useful for both qualitative and quantitative analysis. The positions of peaks on the time axis may serve to identify the components of the sample; the areas under the peaks provide a quantitative measure of the amount of each component.
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MIGRATION RATES OF SOLUTES The effectiveness of a chromatographic column in separating two solutes depends in part upon the relative rates at which the two species are eluted. These rates are determined by the magnitude of the equilibrium constants for the reactions by which the solutes distribute themselves between the mobile and stationary phases.
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Distribution Constants The distribution equilibria involved in chromatography
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This note was uploaded on 05/06/2010 for the course CHEM 4414 at Arkansas Tech.

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Chapter 26 - Chapter 26 An Introduction to Chromatographic...

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