Solvents for Adsorption Chromatography

The accumulation of an organic substance on the surface of an adsorbent is determined by its dipolarity as well as its polarizability and molecular size. This is also true for solvents, which are adsorbed more strongly the more dipolar their molecules. The mole- 

This series applies generally to partition separations by paper-, column-, or thin-layer chromatography. The solvents listed go from most to least hydrophilic behaviour. 

Hecker, Chimia 8, 229 (1954) E. Elecker Verteilungsver-fahren in Laboratorien, Verlag Chemie, Weinheim, 1955, p. 92 and 139. 

Eleftmann (ed.) Chromatography, 2 ed.. Reinhold Publishing Company, New York, 1967. 

Mikes Laboratory Handbook of Chromatographic Methods, Van Nostrand, London, 1970. 

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Table 25-2 Eluotropic series and ultraviolet cutoff wavelengths of solvents for adsorption chromatography on silica...

Selection of solvents for adsorption chromatography. The choice of solvent for transferring the mixture to be chromatographed to the column will naturally depend upon the solubility characteristics of the mixture. If it is already in solution, for example as an extract, this is usually evaporated to dryness under reduced pressure and the residue dissolved in the minimum volume of the most non-polar solvent suitable. As concentrated a solution as possible is desirable to achieve a compact band at the top of the column of adsorbent, so that during subsequent development the separation will hopefully proceed with formation of discrete bands. 

Graded Adsorbents and Solvents. Materials used in columns for adsorption chromatography are grouped in Table 12 in an approximate order of effectiveness. Other adsorbents sometimes used include barium carbonate, calcium sulfate, calcium phosphate, charcoal (usually mixed with Kieselguhr or other form of diatomaceous earth, for example, the filter aid Celite) and cellulose. The alumina can be prepared in several grades of activity (see below). 

The correlation of Snyder s solvent strength e° with molecular dipolarity and polarizability (7t ) and the hydrogen-bond acidity (a) and the hydrogen-bond basicity ((3) solvatochromic parameters for adsorption chromatography can be achieved, although most papers on solvatochromic parameters deal with reversed-phase systems [18]. 

FIG. 1.27. Column for adsorption chromatography. Typical size 14/35 3T joints. 45-mm-diameter upper section, 20-mm-diameter column, and a 500-mm column length. O-jointsand Teflon-in-glass valves provide greater solvent resistance than the greased Fittings illustrated here. 

As the polarities of the solute and solvent molecules increase, the interactions of these molecules become much stronger with the adsorbent, and they adsorb with localization. The net result is that the fundamental equation for adsorption chromatography with relatively nonpolar solutes and solvents has to be modified. Several localization effects have been elucidated, and the modified equations that take these factors into consideration are rather complex [7,8,10]. Nevertheless, the equations provide a very important framework in understanding the complexities of adsorption chromatography and in selecting mobile phases and stationary phases for the separation of solutes. 

Sections 9.4 and 10.3 have already provided the basis for optimization by attempting to work with three different solvent mixtures hexane-ether, hexane-dichloromethane and hexane-ethyl acatate for adsorption chromatography and water-methanol, water-acetonitrile, water-tetrahydrofuran for reversed-phase systems. However, this concept is not restricted to binary mixtures but a third or even a fourth component may be added in an attempt to improve the separation. An arrangement of seven different mixtures (Figure 18.11) provides the best basis for systematic evaluation. An example is outlined below. 

Solvents can be grouped for adsorption chromatography into a so-called elutropic series according to their elution strengths. The choice of solvent system is made in relation to the polarity of the adsorbent and the mixture to be separated, utilizing the elutropic solvent series. This series orders solvents according to their hydrophilic or hydrophobic character. A representative elutropic series beginning with the most hydrophobic solvent is as follows ... 

Competition between the sample ion and the counter ion for the fixed ionic site is very similar to the competition between solute and solvent for adsorption sites in LSC. In fact, sometimes ion exchange is referred to as adsorption chromatography involving electrostatic interactions. However, as the nature of the stationary and mobile phases, as well as the samples handled, are quite unlike those used for LSC, we prefer to classify ion-exchange chromatography separately. 

This polarity scale was first developed for adsorption chromatography, and it can be extended to reversed phase chromatography by assigning 0 to the F value of water while neat methanol, acetonitrile, and tetrahydrofuran have F values of 2.6, 3.2, and 4.S, respectively (33). This amounte to a significant reduction of the original selectivity triangle, which is a consequence of the fact that miscibility with water is a prerequisite for the selection of a solvent in reversed-phase HPLC. 

Silica is used for adsorption chromatography with organic solvents as mobile phases. Typical mobile phases are hexane mixed with a more polar solvent, such as dichloromethane or ethyl acetate, often with 0.1-0.5% of methanol or acetonitrile. Acetone, otherwise with many good properties, cannot be used in HPLC with UV detection due to the high UV cutoff. 

The enantioselective determination of 2,2, 3,3, 4,6 -hexachlorobiphenyl in milk was performed by Glausch et al. (21). These authors used an achiral column for an initial separation, followed by separation of the eluent fraction on a chiral column. Fat was separated from the milk by centrifugation, mixed with sodium sulfate, washed with petroleum ether and filtered. The solvent was evaporated and the sample was purified by gel permeation chromatography (GPC) and silica gel adsorption chromatography. Achiral GC was performed on DB-5 and OV-1701 columns, while the chiral GC was performed on immobilized Chirasil-Dex. 

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