Chemical fractionation methods adsorption chromatography

SRC, a detailed examination of the composition of these coal liquids is of fundamental importance. Numerous procedures have been published previously for investigating the composition of liquids derived from coal. In general, these procedures combine separation techniques with a variety of spectroscopic methods to provide the desired quantity of structural information. The separation techniques used include methods based on solubility fractionation (4,5), methods combining solubility fractionation and adsorption chromatography (6), and liquid chromatographic procedures for chemical fractionation (7,8). Chemical reactions also have been used to separate coal liquid asphaltenes into acidic and basic fractions (9). 

However, fractional separation has been the basis for most asphalt composition analysis (Fig. 15.5). The separation methods that have been used divide asphalt into operationally defined fractions. Three types of asphalt separation procedures are now in use (a) chemical precipitation in which n-pentane separation of asphaltenes is followed by chemical precipitation of other fractions with sulfuric acid of increasing concentration (ASTM D-2006) (b) adsorption chromatography with a clay-gel procedure in which, after removal of the asphaltenes, the remaining constituents are separated by selective adsorption/desorption on an adsorbent (ASTM D-2007 and ASTM D-4124) and (c) size exclusion chromatography in which gel permeation chromatographic (GPC) separation of asphalt constituents occurs based on their associated sizes in dilute solutions (ASTM D-3593). 

The fractionation technique for unsaponifiable components, always based on chromatographic methods, has recently been greatly improved. Because of the complex composition of most unsaponifia-bles studied, several steps are required before compounds are sufiS-ciently pure to enable their identification directly by traditional chemical means, or by spectrographic methods. The method giving the best results, especially preparative, consists of a series of adsorption chromatographies on silica or alumina columns or, less frequently, on Florisil or other inert adsorbents. The use of ionic absorbents (DEAE and TEAE) or of exchange-resins offers no advantage. 

The most important minerals of the lanthanide elements are monazite (phosphates of La, Ce, Pr, Nd and Sm, as well as thorium oxide) plus cerite and gadolinite (silicates of these elements). Separation is difficult because of the chemical similarity of the lanthanides. Fractional crystallization, complex formation, and selective adsorption and elution using an ion exchange resin (chromatography) are the most successful methods. 

From amongst the numerous applications of adsorption column chromatography, an interesting example is provided by the separation of a mixture of two enantiomers into its constituents whose separation cannot be carried out by the other usual physical methods like fractional crystallization or fractional distillation. A well-known illustration of the separation of enantiomers without their having to be converted into diastereoisomers by chemical reaction with optically active acids is the resolution of Troger s base, effected by V. Prelog and P. Wieland in 1944,... 

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