Size discrimination

Y-L He, S Murby, G Warhurst, L Gifford, D Walker, J Ayrton, R Eastmond, M Rowland. Species differences in size discrimination in the paracellular pathway reflected by oral bioavailability of poly (ethylene glycol) and D-peptides. J Pharm Sci 87 626-633, 1998. 

Kasper-Zubillaga, J.J. Carranza-Edwards, A., 2005.Grain size discrimination between sands of desert and coastal dunes from northwestern Mexico. Revista Mexicana de Ciencias Geologicas, 22, 383-390. 

Although SPMDs are simple in design, the mechanisms governing their performance as passive samplers of HOCs can be quite complex (see Chapter 3). The underlying principle of molecular-size discrimination in the uptake and loss of chemicals by SPMDs is shown in Eigure 2.1. The sizes of the molecules shown in the illustration are scaled to the postulated 10 A diameter of the transient pores in the membrane. Temperature and the presence of plasticizers/solvent will affect the effective pore sizes. 

Size discriminating CE (Capillary gel Oligo- and polynucleotides, oligo- and Slab-gel... 

The assumption has generally been made in SEC with matrices greater than lOOA pore diameter that there is little, if any, size discrimination of molecules less than 500 daltons, i.e., they would all elute at V-p. 

Kasper-Zubillaga, J.J. Carranza-Edwards, A., 2005. Grain size discrimination between sands of desert and coastal dunes from northwestern Mexico. Revista Mexicana de Ciencias Geologicas, 22, 383-390. Kasper-Zubillaga, J.J., Acevedo-Vargas, B., Morton-Bermea, O., Ortiz-Zamora, G., 2008a. Rare earth elements of the Altar Desert dune and coastal sands. Northwestern Mexico. Chemie Der Erde Geochemistry, 68, 45-69. 

The asymmetric allylic C-H activation of cyclic and acyclic silyl enol ethers furnishes 1,5-dicarbonyl compounds and represents a surrogate of the Michael reaction [136]. When sufficient size discrimination is possible the C-H insertion is highly diastereoselective, as in the case of acyclic silyl enol ether 193 (Eq. 22). Reaction of aryldia-zoacetate 192 with 193 catalyzed by Rh2(S-DOSP)4 gives the C-H insertion product 194 (>90% de) in 84% enantiomeric excess. A second example is the reaction of the silyl enol ether 195 with 192 to form 196, a product that could not be formed from the usual Michael addition because the necessary enone would be in its tautomeric naphthol form (Eq. 23). 

Gifford, L Walker, D., Ayrton, J Eastmond, R. and Rowland, M. (1998) Species differences in size discrimination in the paracellular pathway refrected by oral bioavailabibty of poly(efhylene glycol) and D-peptides. Journal of Pharmaceutical Sciences, 87, 626-633. 

Three reviews describing applications of diffusion denuders have been published. The doctoral dissertation of Ferm (31) reflects considerable experience with single-tube denuders for the measurement of a variety of species. The review by Ali et al. (32) is extensive it provides an excellent historical and theoretical background and summarizes the literature based on the type of analyte gas determined. The focus of the most recent review, by Cheng (19), is diffusion batteries used for size discrimination of aerosols as well as diffusion denuders. Various physical designs are discussed in some detail in that review. 

This form of selectivity is based on the concept of selectively blocking the access of all interfering species to the active region of the transducer. It is a form of filtration. The blockage can be achieved by size discrimination. For instance, a dialysis... 

Cyclo-paraffins react more readily with REHY, presumably as a result of pore size differences between the 10 ring ZSM-5 and the 12 ring faujasite, resulting in size discrimination (reactant shape selectivity). Over both zeolites, C9-C10 cyclo-olefins are completely converted so that size discrimination is not observed. However, where conversion of cyclo-olefins is not complete (i.e. C6-C8) there is clear evidence for discrimination between the two zeolites (Figure 7). Presumably this high reactivity of C9/C10 cyclo-olefins over either zeolite can be explained by initial facile attack at outer surfaces. 

The size resolution ideally depends on the monotonic nature of the response curve. Since the counter response is strongly dependent on the index of refraction of particles of a given size, the size discrimination will be quite variable for an aerosol of heterogeneous composition. 

In the other pressure-driven separations in Table I, the difference in size between the permeating component A and rejected components B, C, etc., is progressively reduced in NF vs RO vs GS. This shift in size discrimination requirements is illustrated in Table II. 

Membranes used for separation are thin selective barriers. They may be selective on the basis of size and shape, chemical properties, or electrical charge of the materials to be separated. As discussed in previous sections, membranes that are microporous control separation predominantly by size discrimination, charge interaction, or a combination of both, while nonporous membranes rely on preferential sorption and molecular diffusion of individual species. This permeation selectivity may, in turn, originate from chemical similarity, specific complexation, and/or ionic interaction between the permeants and the membrane material, or specific recognition mechanisms such as bioaffinity. 

Another means of classifying membranes is according to their ability to retain substances of different sizes. Some membranes are capable of size discrimination at the molecular level—for example, with gases or liquids— while others exhibit selectivity toward particles of microscopic dimensions. As will be shown in the following sections, membrane processes in conjunction with appropriate membranes can achieve separation over abroad size spectrum. 

These responses correspond to a void volume of 9.8% using toluene as the void-filling compound in the calculation and 13.3% when chloroform, a smaller compound, is used in the calculation. The voids apparently have not only maximum capacity but also pores with some size-discrimination ability. These amounts correspond to 86% of the saturation limit of toluene in water and only 65% for that of chloroform. Larger molecules with even greater partitions were found not capable of being adsorbed into Teflon AF . The assumption here is that the pore size in Teflon AF prevents large molecules, such as naphthalene and bis(2-ethylhexyl phthalate), a plasticizer, from getting in. 

The technique has been evaluated by Langhorst et. al. [44,45] and reviewed by Miller and Lines [46]. HDC suffers from poor resolution and particle size discrimination. In principle, more accurate particle size determinations are possible but require special software with correction for the extensive band dispersion [47,48]. It has also been shown that the molecular weight and concentration of non-ionic surfactants adsorbed on latex particles have a significant effect on their separation factor [49]. 

In an intriguing example of size discrimination, a dendritic Lewis acid derived from 214 and Cu(OTf)2 can selectively activate the smaller dienophile 212 (Sch. 48) [91]. Diels-Alder reaction of 185 and an equimolar mixture of 212 and 213 provides the adduct 215 in higher yield. A generation-three dendrimer is required to obtain adequate size discrimination. 

It has been claimed in the literature [6] that a size discriminating effect can be observed with PdSalen complexes encaged in faujasite-type zeolites In the competitive hydrogenation of an equimolar mixture of hexene-(l) and cyclohexene in the liquid phase, hydrogenation of the slim 1-alkene is strongly prefered, whereas in the homogeneous phase both alkenes are hydrogenated with a comparable rate. In... 

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