Displacement chromatography adsorbents

Narahari, C.R., Strong, J.C., and Frey, D.D., Displacement chromatography of proteins using a self-sharpening pH front formed by adsorbed buffering species as the displacer, /. Chromatogr. A, 825, 115, 1998. 

Liquid/Solid Chromatography (LSC) is adsorption chromatography. Adsorbents such as alumina and silica gel are packed in a column and the sample components are displaced by a mobile phase. Thin layer chromatography and most open column chromatography are considered liquid/solid chromatography. 

The Langmiur isotherm—used by Guiochon and others in the study of preparative scale chromatography—is based on the concept that adsorption nonlinearity occurs when there are so many molecules that they compete with one another for a limited number of adsorption sites. It is obvious that when two concentrated solutes are present at the same time, they will interfere with one another s adsorption. The one that adsorbs most strongly will almost totally displace the weaker adsorber. This is the basis of displacement chromatography, a nonlinear form developed by Tiselius in 1943 (17) and revived recently by Horvath [18]. 

FIGURE 9 Displacement chromatography on a retained pH gradient. The presaturation and elution buffers are formed by titrating 0.05 M NaOH with MOPS and acetic acid, respectively. Adsorbent TosoHaas TSK Q-5PW (5 /x particles) flow rate 0.1 mL/min proteins /3-lactoglobulins A and B (from Narahari et a/.,69 with permission from Elsevier Science). 

Displacement chromatography A chromatographic process in which, after the sample is injected, the mobile phase is replaced by a solution of a compoxmd more strongly adsorbed than any feed component, the displacer. Eventually, the feed components are separated into a series of successive zones, forming an isotachic train which propagates without further change. If the column efficiency, the sepa-... 

The same equipment is used here as with displacement chromatography, with one exception. The inert phase is coated with a thin film of liquid, which is strongly adsorbed to the surface of the inert phase particles. Because this liquid does not move, it is called a stationary phase. A mobile phase is selected that (1) will not dissolve or mix appreciably with the stationary phase and (2) will not adsorb to the inert phase as did the stationary phase. Refer to Figure 14-6A. 

However, it is possible to modify almost any adsorbent so as to give linear isotherms (Chapter 4). so that this theoretical advantage of displacement over elution is of academic interest only. A quite specialized form of displacement chromatography, with fluorescent indicators added to the sample so as to mark band boundaries on the column 37), is still widely used in the petroleum industry for the routine analysis of gasolines. The extension of this technique to other sample types has been discussed 38) but has so far found no reported applications. 

Dipole interact ions,. tee Electrostatic forces Dispersion forces (energies), 44-47 on alumina, 245 in gas-solid adsorption, 243-245 Displacement chromatography, 34-36 Distribution coefficient A, lOi-ll calculation (examples), 385-396 correlation between different adsorbent batches, 148-149... 

As the sample concentration in an adsorption system is increased, the relative coverage of the adsorbent surface by sample likewise increases, and eventually the entire surface may be covered by an adsorbed layer of sample which is one molecule thick (monolayer adsorption). Further adsorption of sample commonly occurs in gas-solid systems, giving an adsorbed phase lich may average several molecules in thickness (multilayer adsorption). Multilayer adsorption is almost nonexistent in liquid-solid systems, and it seems to be confined to hydrophilic solvents plus heavily hydrated adsorbents [e.g., Ref. (14)). With the exception of frontal analysis and displacement chromatography, which are today of... 

The displacement train is a result of the roll-up effect that takes place when species interfere. Although displacement chromatography produces very pure and concentrated products, it has some severe disadvantages. One drawback is that the bands in the isotachic train are very narrow, which makes the harvest of the pure products a non-trivial task. A second drawback is the need to introduce an additional, strongly adsorbing species, which is undesirable as it is hard to remove from the resin. 

Displacement Desorption. The temperature and pressure are maintained essentially constant, as in purge gas stripping, but instead of an inert purge the adsorbed species are displaced by a stream containing a competitively adsorbed species, as in displacement chromatography. The method is applicable to both gas and liquid systems. 

In displacement chromatography, a pulse of mixture is injected, and this is followed by a step of a single component called a displacer, which is adsorbed more strongly than any of the mixture components. After a certain period of time, during which the profiles of each pulse become reorganized, an isotachic pulse train is formed. In the isotachic train, each component forms its own concentration boxcar, with a height that depends on the component and displacer isotherms as well as on the displacer concentration, and a width proportional to the amount of the corresponding component in the sample [14], [15]. 

The model is used to describe nonlinear protein (macromolecule) separations by ion exchange chromatography and as such it is not necessarily restricted to displacement chromatography. The model takes into account that salt gradients are induced in front of the displacer zone (caused by the displacement of the small counter ions by the adsorbing displacer) and that... 

The ftxed-bed processes studied so far are generally useful for separating one solute or one ionic species from the solvent in some cases, more than one solute or one ionic species are also separated from the solvent Separation of each individual species present in a multicomponent gas mixture or liquid solution can be achieved in a fixed bed of adsorbent particles if the mode of operation, e.g. the method of feed introduction, is changed from that used so far. A number of methods are commonly used to this end. They are elution chromatography, displacement chromatography and firontal chromatography (see Figure 7.1.5(c)). 

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