Nominal molecular weight cut-off

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. 

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. 

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. 

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. 

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. 

Figure 6.3 Ultrafiltration membranes are rated on the basis of nominal molecular weight cut-off, but the shape of the molecule to be retained has a major effect on retentivity. Linear molecules pass through a membrane, whereas globular molecules of the same molecular weight may be retained. The table shows typical results obtained with globular protein molecules and linear polydextran for the same polysulfone membrane [8]...

Figure 6.13 The retention of albumin (MW 65 000) in the presence of varying concentrations of y-globulin (MW 156000) by a membrane with a nominal molecular weight cut-off based on one-component protein solutions of MW 200000. As the concentration of y-globulin in the solution increases, the membrane water flux decreases, and the albumin rejection increases from 25% at 0.01 wt% y-globulin to 80% rejection at 0.1 wt% y-globulin [8]...

Figure 13.7 An illustration of the varying ability of different nominal molecular weight cut-off ultra filters to separate the small oxidised species of plutonium from the much larger reduced species.

Continuous mns were carried out in a stirred cell ultrafiltration module using the fluoropolymer membrane FS61PP with a nominal molecular weight cut-off of 20kDa. The reactor, loaded with an appropriate amount of resting cells, was fed with a buffered substrate solution by a peristaltic pump with the flow rate set at... 

Membrane manufacturers prefer to use a parameter called the nominal molecular weight cut-off (NMWCO) or simply the molecular weight cut-off (MWCO) to specify the sieving properties of their membranes. For a given membrane this is defined as the molecular weight of a solute that has an apparent sieving coefficient of 0.1. Polymers, such as dextran or polyethylene glycol, are usually used to determine MWCO. 

Humic Acid Fractionation. The two different size fractions of humic acid used in this study were obtained by ultrafiltration of previously characterized HA from Lake Bradford, FL. An Amicon ultrafiltration cell (model 8050) was used for fractionation with a filtration membrane of the desired pore size. Molecules exceeding that pore size were retained above the membrane. The nominal molecular weight cut-off (MWCO), or pore sizes, were reported by the manufacturer (Amicon), and are based on globular protein standards. Membranes used in this study were XM300 (MWCO 300,000 daltons, lot number AI), YMIOO (MWCO 100,000 daltons, lot number AT), and XM50 (MWCO 50,000 daltons, lot number AG). A 5 liter fiberglass reservoir (Amicon model RG5) was filled with the solution to be fractionated. The solution was ultrafiltered under pressure (50 psi) with constant stirring. 

Fig. 38.11 Effects of p-CyDs (0.1 M) and EDTA (0.2 mM) on permeation of insulin (0.1 mM) through ultrafiltration membranes in phosphate buffer (pH 6.8, p. = 0.2) at 25°C. (A) Insulin alone, (B) with EDTA, (C) with HP-p-CyD, (D) with G2-p-CyD. The membranes of YM-10, YM-30 and XM-50 were rated at pore sizes 10 000, 30 000, and 50 000 nominal molecular weight cut-off, respectively.

All the fractions were subjected to dialysis to remove salts and low molecular weight hydrolysis products. This procedure was performed for 3 days against running deionised water. The nominal molecular weight cut-off of the tubing was given as 12-14 thousand Daltons by the manufacturer (Spectra/Por 5). All fractions were freeze dried. Yield is defined as the amount of total extracted material based on lOOg of treated wood chips. The amount of extracted material comprised both solubles in the extraction liquids and solids in pellets. 

The selection of a membrane for ultrafiltration will require determining the molar mass of the species to be separated and selecting a membrane with a limiting rejection under anticipated conditions of operation. Small-scale application tests will generally need to be performed. Ultrafiltration membranes are rated in terms of their nominal Molecular Weight Cut-off (MWCO). There are no industry-wide standards for this rating, hence manufacturers use different criteria for assigning ultrafiltration... 

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