Solutions sorbents

Fig. 9. Adsorption isotherms where the adsorbed mass f/rngm 2, of various sorbent surfaces is plotted against the protein concentration cl g dm 3, in solution. Sorbents Teflon (o), polystyrene (x),... 

Various binders have been used to give mechanical stability to the layer spread on the suppon plate. Basic requirements are that the binder should not interfere with solute-sorbent interactions, with elution, and detection procedures. At the same time, these binders have to provide compact and adherent layers together with the sorbent. There are various binders that have been applied to prepare the stationary phase for planar chromatography (Table 10.7). 

Specific solute-sorbent interactions related to steric effects. Sorption of solutes to NOM has been explained in terms of association or trapping of the solutes in hydrophobic cavities embedded within NOM aggregates [discussed by Schlautman and Morgan (34)]. The NOM is a mixture of chromophores that are capable of quenching excited states or photosensitizing reactions. The net effect may depend on the position of the sorbed solute within hydrophobic cavities of this mixture. The position may, in turn, depend on the size and shape of the solute. 

Organic contaminants in surface and subsurface systems are typically distributed by sorption between the aqueous phase and natural solid phases. The extent to which such contaminants are sorbed significantly affects their transport and distribution, their impacts on the ecosystem, and the selection of strategies for their removal. In cases of hydrophobic contaminants, sorption is governed by a complex combination of interactions associated with solute repulsion from the aqueous phase and solute attraction to particular solid phases and interfaces. The variety of thermodynamically driven and kinetic or mass-transport-controlled solute-sorbent interactions that may occur in natural systems were summarized by Weber et al. (1). 

Describe the assumptions about solute-sorbent interactions for the following isotherms ... 

Alumina. The alumina surface is capable of exhibiting different types of solute-sorbent interaction. This may be attributed to, first, the very strong positive fields surrounding the Al, which allow interaction with easily polarisable molecules, such that alumina would be preferred to silica for the resolution of aromatics from olefins and second, the pressure of basic sites (probably 0 ) which allow interaction with proton donors. 

Finally, it should be said that the above-discussed osmotic pressure-forced retention of one of the components of a mixture subjected to separation according to size exclusion mechanism does not exclude the possibility of true retention-type solute/sorbent interactions. Hypercrosslinked polystyrene may well enter attractive interactions with soft and lipophilic (chaotropic [163]) anions, such as sulfide, thiosulfate, rhodanide, perchlorate, tetrafluoroborate, and hexafluorophosphate. Also, some cations such as silver, copper, and mercury may interact with the aromatic 71-systems of the polystyrene matrix, which will retard the movement of the ions. These interactions may contribute to the separation of the ions from their stiU larger competing ions, but they may also be counterproductive to the size exclusion effect and deteriorate the separation of the retarded ions from smaller species. 

Minimization of solute-sorbent hydrophobic interactions can be attained using organic modifiers such as ethylene glycol, propanol, and acetonitrile in the mobile phase buffers. Even though ethylene glycol is the most viscous, causing the most band spreading of the three solvents, it is the most desirable because it is the least likely to initiate protein denaturation. 

A5 ssoc, . of the system to the overall Gibbs free energy ehange, AG associated with the solute-sorbent interaction, as well as the phase ratio of the system, d), will also be independent of temperature. When such conditions... 

Maity, N. Payne, G.F., and Chipchosky, J.L.. Adsorptive separations based on the differences in solute-sorbent hydrogen-bonding strengths, Ind. Eng. Chem. Res.. 30(11). 2456-2463 (1991). 

For ion-exchange sorbents, deionized water is usually used first, but if the sample is in a nonpolar solvent, the SPE should be conditioned with 2-5 mL of this solvent instead. Aqueous samples require use of 2 mL of methanol followed by 2 mL of water. The sorbent is then adjusted to the organic solvent content and salt concentration of the sample solution. Sorbent must not be allowed to... 

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