Molecular weight cut off

You may be surprised, but fouling is not always detrimental. The term dynamic membrane describes deposits that benefit the separation process by reducing the membrane s effective MWCO Molecular Weight cut-off) so that a solute of interest is better retained. Concentration polarization refers to the reversible build-up of solutes near the membrane surface. Concentration polarization can lead to irreversible fouling by altering interactions between the solvent, solutes and membrane. 

UF membranes are often rated by molecular weight cut-off (MWCO) solutes above the MWCO are retained and those below the MWCO permeate through the... 

As RO membranes become looser their salt rejection falls (see Section 31.8.1). Eventually a point is reached at which there is no rejection of salts, but the membrane still rejects particulates, colloids and very large molecules. The membrane pore size can be tailored to a nominal molecular weight cut-off. The resulting filtering process is called ultra-filtration. 

Commercially available plate- and frame- type ultrafiltration equipment are used for exopolysaccharide concentration. The membranes are polysulphone or polyvinylidine fluoride with molecular weight cut-off between 20-60,000. There is a relatively low eneigy requirement (1-2 kWh m 3) for pumping the fluid through the filtration unit at the desired pressure. Pressure difference across the membrane is of the order 2-14 atmospheres. 

The nominal molecular weight cut-off is normally defined as the molecular weight of a solute for which R = 0.95. Values of MWCO typically lie in the range 2000-100,000, with values of the order of 10,000 being most common. Figure 16.10 shows an AFM scan of 30,000 MWCO membrane. 

Until this point, the sample preparation techniques under discussion have relied upon differences in polarity to separate the analyte and the sample matrix in contrast, ultraflltration and on-line dialysis rely upon differences in molecular size between the analyte and matrix components to effect a separation. In ultrafiltration, a centrifugal force is applied across a membrane filter which has a molecular weight cut-off intended to isolate the analyte from larger matrix components. Furusawa incorporated an ultrafiltration step into his separation of sulfadimethoxine from chicken tissue extracts. Some cleanup of the sample extract may be necessary prior to ultrafiltration, or the ultrafiltration membranes can become clogged and ineffective. Also, one must ensure that the choice of membrane filter for ultrafiltration is appropriate in terms of both the molecular weight cut-off and compatibility with the extraction solvent used. 

The paracellular pathway, between the epithelial cells, is both size- (MW, volume) and charge-dependent [60, 109, 110]. In general, compounds that are limited to paracellular transport are not efficiently absorbed due to the small available absorptive area and the restriction by tight junctions. The molecular weight cut-off seems to be around 400 g mol-1 and 300 g mol-1 for the small and large intestine respectively, and 300 g mol-1 for the Caco-2 cell monolayers [60], which shows the more colonic nature of the Caco-2 monolayer model. Compounds with a... 

PEG (20 kDa) [43]. This demonstrates the strong hydrodynamic properties of PEGylated molecules. The increase in hydrodynamic radius significantly decreases renal clearance. Although the threshold of the molecular weight cut-off of renal filtration of protein is about 65 kDa, the 30-kDa PEG demonstrates minimal renal permeability [44]. 

The conjugate may be purified to remove unconjugated protein using gel filtration on a column of resin having a molecular weight cut-off able to accommodate the proteins being separated. 

Purify the protein conjugate by dialysis or gel filtration using a molecular weight cut-off appropriate for the sizes of the proteins being separated. 

To find a suitable membrane for a certain application, an important parameter is the molecular weight cut-off (MWCO). The MWCO is defined as the molecular weight at which 90% of the solutes are retained by the membrane. It should be taken into account that the pore size of many ultra- and nano filtration membranes is greatly influenced by the solvent and by the temperature used under experimental conditions. This particularly concerns polymeric membranes as will be discussed in the next paragraph. 

An immobilized-enzyme continuous-flow reactor incorporating a continuous direct electrochemical regeneration of NAD + has been proposed. To retain the low molecular weight cofactor NADH/NAD+ within the reaction system, special hollow fibers (Dow ultrafilter UFb/HFU-1) with a molecular weight cut-off of 200 has been used [32],... 

Ultrafiltration was carried out in an Amicon stirred cell, 65 ml capacity, at 4 C with continuous stirring under C02 pressure until 25 ml filtrate had been collected. MWC0 = nominal molecular weight cut-off of the membrane. 

Micelle solutions of PlPAAm-Ci8H35 was prepared by direct dissolution of the polymer in cold water (4°C) due to its good water solubility [23]. Each solution of PIPAAm-PSt, PlPAAm-PBMA, and PIPAAm-PLA was prepared by dissolving each copolymer in DMF, A-ethylacetamide, and DMAc, respectively. The solutions were put into a dialysis bag (MWCO = 13,000) and dialyzed against distilled water at 10°C, 20°C, and 4°C, respectively, for 24 hours. The micelles were purified with ultrafiltration membrane of 200,000 molecular weight cut off at 4°C. The aqueous solution was lyophilized to leave a white powder of micelles. 

The sources and molecular weights of polymers used in this study are given in Table I. The elimination of simple electrolyte residues from polymers was performed by ultrafiltration using a Diaflo PM 10 (Amicon) membrane of 10 000 nominal molecular weight cut-off. Water was purified by a Milli-Q system (Millipore). All other rea-geants were of analytical grade. 

Fig. 4. Application of dendritic ligand 2 in the continuous allylic alkylation of allyl trifluoroacetate and sodium diethyl 2-methyhnalonate in a membrane reactor (Koch MPF-60 NF membrane, molecular weight cut-off = 400 Da) 18a).

The in vitro diffusion studies for each sample were carried out by using the Franz diffusion cells with a diffusional area of about 1.76cm2. The acceptor compartment of the apparatus was filled with the buffer solution pH 6, USP [21], and maintained at 37 0.5°C via a circulating water system. The diffusion membrane (the cellulose membrane with a molecular weight cut-off point of 1000 or the hairless mouse skin) previously prepared was placed between die donor and the acceptor compartments of the assembly. An accurately weighed 4g of sample was then placed in the donor cell and the diffusion process was started. The solution in the acceptor compartment was continuously stirred with a small magnetic stirrer to maintain the sink conditions. Aliquots from the receptor cells were removed at 0.5,2,4, 8 and 24 h time intervals and replaced with equal... 

Figure 24 shows the rejections of polymer solutes, polyethylene glycols) (PEG) with monodispersed molecular weights. From Fig. 24, it is apparent that the composite membrane can find application for ultrafiltration. The molecular weight cut-off drastically decreased by more than 10 fold from the swollen state at 25 °C to the shrunken state at 45 °C. Thus the switching ability of the gel was demonstrated in the permeation experiments. 

Polymer-Assisted Ultrafiltration of Boric Acid. The Quickstand (AGT, Needham, MA) filtration apparatus is pictured schematically in Figure 3. The hollow fiber membrane module contained approximately 30 fibers with 0.5 mm internal diameter and had a nominal molecular weight cut-off of 10,000 and a surface area of 0.015 m2. A pinch clamp in the retentate recycle line was used to supply back pressure to the system. In a typical run, the transmembrane pressure was maintained at 25 psig and the retentate and permeate flow rates were 25 ml/min and 3 ml/min, respectively. Permeate flux remained constant throughout the experiments. 

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