Size exclusion membranes

PURPOSE AND RATIONALE The ultrafiltration (UF) with semipenneable, size-exclusion membranes produces a separation of the free drug from macromolecules by employing a pressure... 

Herr et al. [137] developed a microfluidic chip for saliva-based analysis of proteins (see Fig. 15) [137], which allowed for automated measurement of total salivary metalloproteinase-8 (MMP-8) concentration. The chip performed three tasks (1) enrichment of targets from saliva via a size-exclusion membrane inside... 

CA films by using the phase inversion process. These CA films were cast from solvent/nonsolvent solutions to yield size exclusion membranes consisting of a thin permselective outer layer and a more porous sublayer. These membranes permitted the rapid permeation of a 1500-dalton poly (ethylene glycol) ester of ferrocene however the reproducibility of results presents a problem with these CA mem-branes. Christie et demonstrated that thin films of plasticized polyvinylchloride (PVC), normally used for potentiometric ion-selective electrode applications, applied to electrodes over a polycarbonate dialysis membrane offered improved selectivity ratios for the amperometric detection of phenolic compounds and H2O2 in the presence of the common biological interferents, ascorbic acid and uric acid, over those observed at the dialysis membrane alone or at a composite dialysis/membrane. 

Biocatalysts are not always immobilized on membranes in bioreactors, though. As enzymes are macromolecules and often differ greatly in size from reactants they can be separated by size exclusion membrane filtration with ultra- or nano-filtration. This is used on an industrial scale in one type of enzyme membrane reactor for the production of enantiopure amino acids by kinetic racemic resolution of chemically derived racemic amino acids. The most prominent example is the production of L-methionine on a scale of 400 t/y (Liese et al, 2006). The advantage of this method over immobilization of the catalyst is that the enzymes are not altered in activity or selectivity as they remain solubilized. This principle can be applied to all macromolecular catalysts which can be separated from the other reactants by means of filtration. So far, only enzymes have been used to a significant extent. 

Yuan CJ, Hsu CL, Wang SC, Chang KS (2005) Eliminating the interference of ascorbic acid and uric acid to the amperometric glucose biosensor by cation exchangers membrane and size exclusion membrane. Electroarralysis 17(24) 2239-2245... 

The silicate network can also act as an efficient size exclusion membrane. The pore size distribution can be controlled, at least to some extent, by incorporation of water soluble polymers such as poly(ethyleneglycol) or by pH changes (Brinker, 1989). Carefully tailored silicate membranes, using low pH synthesis even exhibit Knudsen flow, facilitating the separation of air components (Brinker, 1993). Incorporation of zeolites, surfactants, templated mesoporous silicates, and leaving template materials can be used to generate tailored size excluding films. 

To measure the incorporation of deuterium, the sample must contain high doses of D2O, which usually requires dilution or buffer exchange. Dilution is the easiest and fastest method but only attractive if the sample molecules are available in high concentrations. Thanks to the high detection sensitivity of ESI and MALDI MS, this is not a rare exception. However, in many cases dilution is not option and, instead, a rapid buffer exchange is performed, e.g., using a small gel filtration spin column or a size-exclusion membrane. 

Validation Considerations. Mechanisms other then size exclusion maybe operative ia the removal of vimses from biological fluids. Thus vims removal must be vaUdated within the parameters set forth for the production process and usiag membrane material representative of the product line of the filter. 

Aqueous-detergent solutions of appropriate concentration and temperature can phase separate to form two phases, one rich in detergents, possibly in the form of micelles, and the other depleted of the detergent (Piyde and Phillips, op. cit.). Proteins distribute between the two phases, hydrophobic (e.g., membrane) proteins reporting to the detergent-rich phase and hydrophilic proteins to the detergent-free phase. Indications are that the size-exclusion properties of these systems can also be exploited for viral separations. These systems would be handled in the same way as the aqueous two-phase systems. 

In addition to the insoluble polymers described above, soluble polymers, such as non-cross-linked PS and PEG have proven useful for synthetic applications. However, since synthesis on soluble supports is more difficult to automate, these polymers are not used as extensively as insoluble beads. Soluble polymers offer most of the advantages of both homogeneous-phase chemistry (lack of diffusion phenomena and easy monitoring) and solid-phase techniques (use of excess reagents and ease of isolation and purification of products). Separation of the functionalized matrix is achieved by either precipitation (solvent or heat), membrane filtration, or size-exclusion chromatography [98,99]. 

To ensure a better separation, molecular sieving will act much better This size exclusion effect will require an ultramicroporous (i.e pore size D < 0.7 nm) membrane Such materials should be of course not only defect-free, but also present a very narrow pore size distribution. Indeed if it is not the case, the large (less separative and even non separative, if Poiseuille flow occurs) pores will play a major role in the transmembrane flux (Poiseuille and Knudsen fluxes vary as and D respectively). The presence of large pores will therefore cancel any sieving effect... 

We report here the results of our recent studies of poly(alkyl/arylphosphazenes) with particular emphasis on the following areas (1) the overall scope of, and recent improvements in, the condensation polymerization method (2) the characterization of a representative series of these polymers by dilute solution techniques (viscosity, membrane osmometry, light scattering, and size exclusion chromatography), thermal analysis (TGA and DSC), NMR spectroscopy, and X-ray diffraction (3) the preparation and preliminary thermolysis reactions of new, functionalized phosphoranimine monomers and (4) the mechanism of the polymerization reaction. 

Selectivity is one of the most important characteristics of an electrode, as it often determines whether a reliable measurement in the sample is possible or not. In practice, most pH sensitive membranes will also respond slightly to some interfering ions. As can be seen in Eq. (5), the potential of such a membrane is governed mainly by the activity of the hydrogen ion and also by the concentration of other interfering ions. To improve selectivity, advanced membrane compositions or protection membranes with size-exclusion or ion-exchange properties are often utilized. 

It is reported that the greatest interference that affects selectivity of an oxide-based pH electrode is from redox couples. For practical applications, some oxide-based sensors are covered with a thin layer of a size-exclusive protection membrane such as Nafion or polyphenol [43], However, the improvement in selectivity is at the expense of the response slopes and, sometimes, the response times. 

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