Adsorbent gradient

Two-dimensional thin-layer chromatography (TLC) with adsorbent gradient has allowed the separation of quinolizidine alkaloids in the herb and in several alkaloids from Genista sp. <2004MI89>. 

Glensk, M., Czekalska, M., and Cisowski, W. (2001). Resolution of saponins from a ginseng preparation by 2D TLC with an adsorbant gradient. /. Planar Chromatogr. 14,454-456. 

In TLC a commercial spreading device is usually used to prepare the adsorbent bed [e.g.. Refs. (2,3)]. Many laboratories have dispensed with this aspect of separation, however, since commercially prepared plates are now available for the commonly used adsorbents. The preparation of plates with an adsorbent gradient (Section 2-5F) requires special equipment [e.g.. Ref. ( )]. 

Layers containing a discontinuous adsorbent gradient usually consist of a narrow zone of adsorbent A along the lower edge of the plate and an adsorbent B on the remaining part of the plate [layers with five zones of different adsorbents were also proposed (53)]. Discontinuous adsorbent gradients are used for three purposes ... 

Many examples of continuous and discontinuous adsorbent gradients applied in practice are given by Niederwieser (31) and by Liteanu and Gocan (3). 

Abscisic acid (ABA), 784-785 Absorption measurement, 208 Acid dyes, 1015,1016 Adsorbent gradient, ISO Adsorbents, 17 Adsorption chromatography aluminas and, 17,107 Kowalska model of, 63-64 O ik model of, 61-62 Scott-Kucera model of, 62-63 silica gels and, 17,104 Synder-Soczewinski model of, 60 Alcoholic products, dyes from, 1005 Alkaloids from plants, 1040 pyrrolizidine, 1036-1040 Aluminas, 17,106-107 adsorption chromatography and, 107 partition chromatography and, 107 physical and chemical properties, 106-107 for separation of carbohydrates, 494 Amikacin, 458-461... 

IHP) (the Helmholtz condenser formula is used in connection with it), located at the surface of the layer of Stem adsorbed ions, and an outer Helmholtz plane (OHP), located on the plane of centers of the next layer of ions marking the beginning of the diffuse layer. These planes, marked IHP and OHP in Fig. V-3 are merely planes of average electrical property the actual local potentials, if they could be measured, must vary wildly between locations where there is an adsorbed ion and places where only water resides on the surface. For liquid surfaces, discussed in Section V-7C, the interface will not be smooth due to thermal waves (Section IV-3). Sweeney and co-workers applied gradient theory (see Chapter III) to model the electric double layer and interfacial tension of a hydrocarbon-aqueous electrolyte interface [27]. 

Hydrophobic Interaction Chromatography. Hydrophobic interactions of solutes with a stationary phase result in thek adsorption on neutral or mildly hydrophobic stationary phases. The solutes are adsorbed at a high salt concentration, and then desorbed in order of increasing surface hydrophobicity, in a decreasing kosmotrope gradient. This characteristic follows the order of the lyotropic series for the anions ... 

Because RPSA is appHed to gain maximum product rate from minimum adsorbent, single beds are the norm. In such cycles where the steps take only a few seconds, flows to and from the bed are discontinuous. Therefore, surge vessels are usuaHy used on feed and product streams to provide unintermpted flow. Some RPSA cycles incorporate delay steps unique to these processes. During these steps, the adsorbent bed is completely isolated and any pressure gradient is aHowed to dissipate (68). The UOP Polybed PSA system uses five to ten beds to maximize the recovery of the less selectively adsorbed component and to extend the process to larger capacities (69). 

Pore dijfusion in fluid-filled pores. These pores are sufficiently large that the adsorbing moleciile escapes the force field of the adsorbent surface. Thus, this process is often referred to as macropore dijfusion. The driving force for such a diffusion process can be approximated by the gradient in mole fraction or, if the molar concentration is constant, by the gradient in concentration of the diffusing species within the pores. 

A separation can sometimes be obtained even in the absence of any foam (or any floated floe or other surrogate). In bubble fractionation this is achieved simply by lengthening the bubbled pool to form a vertical column [Dorman and Lemlich, Nature, 207, 145 (1965)]. The ascending bubbles then deposit their adsorbed or attached material at the top of the pool as they exit. This results in a concentration gradient which can serve as a basis for separation. Bubble fractionation can operate either alone or as a booster section below a foam fractionator, perhaps to raise the concentration up to the foaming threshold. 

Bubble Fractionation Figure 22-45 shows continuous bubble fractionation. This operation can be analyzed in a simplified way in terms of the adsorbed cariy-up, which furthers the concentration gradient, and the dispersion in the liquid, which reduces the gradient [Lemhch, Am. Inst. Chem. Fng. J., 12, 802 (1966) 13, 1017 (1967)]. 

A small amount of collector (surfactant) or other appropriate additive in the liquid may greatly increase adsorption (Shah and Lemlich, op. cit.). Column performance can also be improved by skimming the surface of the liquid pool or, when possible, by removing adsorbed solute in even a tenuous foam overflow. Alternatively, an immiscible liquid can be floated on top. Then the concentration gradient in the tall pool of main hquid, plus the trapping action of the immiscible layer above it, will yield a combination of bubble fractionation and solvent sublation. 

It has been seen that this resin has also some important advantages over the other resins in the literature like high total ion exchange capacity, easy synthesis, lower cost, simple regeneration. Furthermore, very good sepai ations were obtained using a concentration gradient of elution. In these elutions, very low concentrations of sodium trimetaphosphate were used. As a result, the resin synthesized can be used as an adsorbent for the effective removal of Pb, Cd, Co, Cu, Fe, Ni, Zn and Cr from aqueous solutions. 

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