Solvent modified adsorbents

In normal-phase chromatography the native, non-modified adsorbent is employed with organic solvent mixtures as eluents. Normal phase chromatography was the classical chromatography mode performed with native silica or alumina, i.e. the adsorbent s surface is hydrophilic and the interaction with the solutes takes place via the hydroxy] groups on the surface. As an example the surface of silica consists of... 

The broad selectivity potential of different solvent systems in NP-LC results from the adjustable concentration of the polar components that are adsorbed into the mono-layer. This leads to an enormous variety of possible stationary phases (adsorbed solvent plus adsorbent) and a corresponding broad range of potential selectivities. Due to differences in the content of the polar modifier, different stationary phases are created. 

The model assumes that the modifier adsorbs reversibly on modifiable sites (Figure 1). With ethanol as solvent, the measured data is well described. For toluene and AcOH, however, adsorption of a second HCd molecule had to be introduced to explain the effects at high modifier concentration. 

For solvent desorption, adsorbents such as modified silica containing cyanopropyl groups with acetone elution and GC-FID/GC-MS detection (Woitowitz and Knecht,... 

It is proposed that a monolayer of solvent is adsorbed to the surface of the silica and that the solute is adsorbed onto the surface when the polarity is less than that of the solvent. No displacement or site competition is envisaged. When the polar modifier concentration is high, a bilayer is formed and the solute then competes with a molecule of the polar solvent in the second layer (Fig. 6.3). Therefore, the major difference between the two models is the level of impor-... 

The foremnner of the modern methods of asphalt fractionation was first described in 1916 (50) and the procedure was later modified by use of fuller s earth (attapulgite [1337-76-4]) to remove the resinous components (51). Further modifications and preferences led to the development of a variety of fractionation methods (52—58). Thus, because of the nature and varieties of fractions possible and the large number of precipitants or adsorbents, a great number of methods can be devised to determine the composition of asphalts (5,6,44,45). Fractions have also been separated by thermal diffusion (59), by dialysis (60), by electrolytic methods (61), and by repeated solvent fractionations (62,63). 

Trainor (1979) modified the above method (1) In the initial extraction, luciferin was extracted with 50 mM acetate buffer (pH 4.75) at 95°C, instead of boiling 20% methanol, to increase the extraction yield. (2) In the DEAE-cellulose chromatography, the column, on which the luciferin sample had been adsorbed, was washed with the following solvents before the elution of luciferin water, lOmM HC1 in methanol, methanol, and NaCl-saturated methanol. (3) To eliminate salts in purified luciferin, the solution was evaporated to dryness, and the luciferin in the residue was extracted with... 

Replacement of gas by the nonpolar (e.g., hydrocarbon) phase (oil phase) has been sometimes used to modify the interactions among molecules in a spread film of long-chain substances. The nonpolar solvent/water interface possesses an advantage over that between gas and water in that cohesion (i.e., interactions between adsorbed molecules) due to dipole and van der Waals s forces is negligible. Thus, at the oil/water interfaces, the behavior of adsorbates is much more ideal, but quantitative interpretation may be uncertain, in particular for the higher chains, which are predominantly dissolved in the oil phase to an unknown extent. The oil phase is poured on the surface of an aqueous solution. Thus, the hydrocarbon, such as heptane or decane, forms a membrane a few millimeters thick. It is thicker than the adsorbed monolayer. Owing to the small difference in dielectric constant between the air and a hydrocarbon oil, the... 

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