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CHROMATOGRAPHY i S EPARATION OF A M IXTURE BY C OLUMN C HROMATOGRAPHY =========================================================================== Review From General Chemistry review: polarity, vapor pressure; London dispersion forces; dipole-dipole interactions; hydrogen bonding; intermolecular forces; =========================================================================== Introduction Chromatography is a technique used to analyze, identify, and/or separate mixtures of compounds. There are several types of chromatography techniques, for example, column, thin-layer, paper, gas, and liquid chromatography. All types of chromatography, however, have two common features: a mobile phase which is gaseous or liquid and a stationary phase which can be a solid or liquid adsorbent through which the mobile phase passes. The way chromatographic separation works is well illustrated by column chromatography , a technique that uses a solid adsorbent as the stationary phase packed inside a glass or plastic column, and a liquid solvent mobile phase called the eluent . In column chromatography a column is filled with small particles or beads of the stationary phase (insoluble in the eluent) and the column is equilibrated with a small amount of the eluent. Then a mixture of compounds, for instance, A and B , are dissolved in the solvent and applied to the top of the column as a narrow layer (part i. of the diagram below). If the stopcock of the column is carefully opened, a layer of the eluent solvent (mobile phase) moves down the packed column by gravity. As the eluent moves through the column, the compounds in the mixture partition between the moving (mobile) phase and adsorbent (stationary) phase due to the differences in the compounds’ physical properties (e.g., polarity, molecular weight, vapor pressure, etc .) and therefore the different interactions the compounds have with the stationary phase and mobile phase (e.g., London dispersion forces, dipole-dipole interactions, hydrogen bonding, etc .). A compound that is attracted less strongly to the stationary phase will move through the column faster and elute, or come off, the column more rapidly in the eluent. In contrast, a compound that is more strongly attracted to the stationary phase will move through the column more slowly and elutes later. Let us assume, for the purpose of this illustration, that component B binds more strongly to the stationary phase than component A does (see diagram below). Then, as the mixture travels down the column, the molecules of B will be retarded with respect to the molecules of A and, in time, the two components will separate into two bands and they will be eluted at different times. That is, complete separation of the initial mixture into its components is achieved. The collected volumes of the eluent are called fractions. i.
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