Molecular Weight Cut-off Values Research Articles

1H nuclear magnetic resonance (NMR) cryoporometry as a tool to determine the pore size distribution of ultrafiltration membranes

The pore size distribution (PSD) of an ultrafiltration (UF) membrane is of great importance as it governs liquid permeability, filtration ability, and the interaction between molecules and the membrane matrix. Two series of UF membranes were studied: commercial regenerated cellulose (PL) membranes and polyethersulfone (PB) membranes, each with molecular weight cutoff (MWCO) values of 5, 10, and 30 kDa. This study was conducted to determine the PSDs of UF membranes using 1H nuclear magnetic resonance (NMR) cryoporometry. NMR cryoporometry is based on the theory of the melting point depressions (MPDs) of liquid confined within a pore, which are dependent on the pore diameter. The MPDs were determined by analyzing the variation of the NMR signal intensity with temperature. From spin-echo intensity versus temperature curves, it was found that for each series of UF membranes the maximum MPDs of cyclohexane confined within the membrane pores were inversely proportional to the MWCO. For the same MWCO, the MPDs of the PL membrane were larger than that of the PB membrane, implying that the pores in the PL membranes were smaller than those in the PB membranes, consistent with their water and solute flux performances reported by the manufacturers. These findings indicate that NMR cryoporometry is a very effective method for determining the PSDs of UF membranes with asymmetric pore structures.

High infusion pressure in conjunction with vitreous surgery alters the morphology and function of the retina of rabbits

To investigate the effects of high infusion pressure in conjunction with pars plana vitrectomy (PPV) on retinal morphology and function in rabbits. Pars plana vitrectomy was performed under urethane (0.8 mg/kg) anaesthesia in the right eye of albino rabbits following phacoemulsification and aspiration (PEA). The left eyes were not touched. After PEA, the animals were divided into two groups. In six eyes, intraocular pressure (IOP) was increased to 80 mmHg for 30 mins (high-pressure group) and in five eyes IOP was maintained at 40 mmHg for 30 mins (low-pressure group). The IOPs were regulated by the height of the bottle of balanced salt solution (BSS) and monitored with a pressure transducer. After the pressure elevation, vitreous fluid was collected to measure the glutamate concentration. Then, PPV was performed for 15 mins in both groups under an infusion pressure of 40 mmHg. In five additional rabbits, PEA alone was performed in the right eye, and vitreous fluid was collected (PEA group). Functional alterations were assessed by recording visual evoked potentials (VEPs) and electroretinograms (ERGs). Ten days after the IOP changes, the animals were killed with intravenous pentobarbital sodium and the eyes were prepared for histological analysis. Damage to retinal ganglion cells (RGCs) was quantified by counting the number of cells in the ganglion cell layer (GCL). The contralateral eyes in the high-pressure group served as controls (n = 6). The mean implicit time (IT) of the VEPs in the high-pressure group was significantly longer than that before the IOP elevation, by 114-124% (p < 0.05, paired t-test), and also than that of control eyes (p < 0.05, anova followed by t-test). No significant changes in the VEPs were detected in either the low-pressure group or the PEA group. There were significantly fewer cells in the GCL in the high-pressure group (24.7/mm) than in the control animals (41.4/mm; p < 0.05, Dunnett's test). The number of cells in the GCL in the low-pressure and PEA groups did not significantly differ to that in the controls. The amplitudes of the ERG a- and b-waves were not significantly changed (p > 0.05, paired t-test). These results suggest that high infusion pressure in conjunction with PPV leads to morphological and functional changes in the retina. The absence of ERG changes and presence of VEP changes suggest that these changes were due to damage to RGCs, which supports the morphological observations.

A Semi-Quantitative Method for Prediction of the Rejection of Uncharged Organic Micropollutants with Nanofiltration

A method for predicting the rejection of uncharged organic pollutants with nanofiltration, based on solute-membrane interactions affecting this rejection and on readily available knowledge about solute- and membrane parameters, was developed. Rejection values of model organic contaminants were measured with bench-scale NF experiments, using Trisep TS80-TSF membranes. Organic contaminants were subdivided into different categories, based on their hydrophobicity (expressed as log Kow-value). Predicted rejection curves, modeled with a log-normal model based on the molecular weight of the solutes, were in close agreement with the experimental data for the different categories of solutes. However, large differences were found between rejection curves for hydrophilic and hydrophobic molecules: rejection curves for hydrophobic molecules have an inflection point at higher molecular weight values compared to hydrophilic molecules and the incline of the rejection curves is less steep for hydrophobic molecules compared to hydrophilic molecules. This indicates that hydrophobic and hydrophilic solutes demonstrate different molecular weight cut-off values on a single membrane and that hydrophobic-hydrophobic interactions between solute and membrane affect rejection of hydrophobic solutes, while rejection is mainly a sieving phenomenon for hydrophilic solutes.

Microdialysis of proteins: Performance of the CMA/20 probe

Unlike all other currently available sampling techniques, microdialysis allows the continuous recovery of dialysed fluid from the interstitial space of living tissue. Microdialysis has been extensively used to study small molecules such as neurotransmitters, metabolites and drugs in the brain and other tissues. There has been increasing interest in measuring proteins using microdialysis. Optimizing protein recovery requires slow buffer flow rates, large pore membranes and osmotic balancing. An examination of a widely used commercially available large pore (100 kDa MWCO) polymeric microdialysis probe, the CMA/20 (polyethersulfone) over 6 days of continuous microdialysis showed that: (1) published molecular weight cut-off values may not predict the size of proteins that can be recovered. This membrane had an effective molecular weight cut-off values (MWCO) of about 29 kDa, (2) protein recoveries decrease over time, (3) small proteins are much less affected than larger ones and (4) there can be significant differences in the recovery of proteins similar in size perhaps due to reaching of a critical size, protein aggregation, shape, surface charge or hydrophobicity.

Influence of operating parameters on separation of green syrup colored matter from sugar beet by ultra- and nanofiltration

The interest in membrane filtration in the sugar industry has recently increased. In sugar beet processing, the main problem is the separation of non-sucrose compounds, especially of colored matter. Separation of colored matter from green syrup was examined by ultra- and nanofiltration of a solution with 39.2% d.m. content. In the process of ultrafiltration (UF), two polymer membranes were used: membrane I with an MWCO (molecular weight cut-off) value of 15–20 kDa and membrane II with MWCO of 6–8 kDa. Nanofiltration (NF) was carried on a polymer membrane with MWCO of 0.5 kDa (membrane III). Effects of UF at 30 and 60 °C, in the range of transmembrane pressure of 1–4 bar, and NF at 30 and 50 °C, in the range of 5–30 bar were examined. In the course of experiments, interdependence between the permeate flux, transmembrane pressure, and volume concentration ratio were followed. The influence of operating parameters (transmembrane pressure, temperature and flow rate) on the efficiency of colored matter removal was examined. Highest separation effects by UF on the examined membranes were achieved for the transmembrane pressure in the range of 2.5–4.0 bar at flow rates ranging from 220 to 360 l/h. Under these conditions, the permeate color was lower by about 55%nd 58% compared with the feed, when using membrane I and membrane II, respectively. Separation of colored matter on membrane III was most efficient when the pressure was held on the level of about 30 bar and at a flow rate ranging from 300 to 400 l/h. Under these conditions, the permeate color decreased by 76%.

UV-Assisted Graft Polymerization of Synthetic Membranes: Mechanistic Studies

Mechanistic studies were conducted on UV-assisted graft polymerization of N-vinyl-2-pyrrolidinone (NVP) onto poly(ether sulfone) (PES) ultrafiltration (UF) membranes using a dip process. The molecular weight cutoff (MWCO) values of the UF membranes were 50, 70, and 100 kDa and the range of NVP monomer concentrations was 2−10 wt %. Our standard photooxidation protocol with 300-nm lamps was used. By operating below an irradiation energy, E < 4 kJ/m2, both homopolymerization and main-chain scission can be minimized with up to CNVP = 5 wt % NVP concentration for all three UF membranes. A universal linear plot of the net amount grafted versus monomer concentration times irradiation energy (CNVP × E) for E < 4 kJ/m2 was obtained for four NVP concentrations and three membranes. Resulting from graft polymerization, there appears to be a qualitative relationship between the thickness of the dense skin layer and the membrane surface roughness as measured by atomic force microscopy. Post-modification washing with ethanol was able to effectively remove entrapped homopolymer and other fragments. Also, UV-assisted graft polymerization of NVP on PES UF membranes effectively reduced irreversible membrane fouling. A protocol for optimizing graft-assisted photopolymerization of PES UF membranes is provided.

Effect of various additives on pore size of polysulfone membrane by phase‐inversion process

AbstractThe phase‐inversion process was used to prepare integrally skinned asymmetric polysulfone (PSf) membranes with different pore sizes. Membranes were prepared from a casting solution of PSf; N‐methyl‐2‐pyrrolidone (NMP) as solvent; and 1,4‐dioxane, diethylene glycol dimethyl ether (DGDE), acetone, and γ‐butyrolactone (GBL) as additives by immersing them in water as a coagulant. The effect of the additives on membrane performance and structure was investigated. The low miscibility of 1,4‐dioxane, DGDE, and acetone with the coagulant resulted in reduced membrane pore size. However, by using GBL as additive pore size of the membrane was slightly increased because of its higher miscibility with the coagulant than NMP. Changing the amount of additives in the casting solution could control the molecular‐weight cutoff values of asymmetric membranes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2562–2566, 2003

Formation of integrally skinned asymmetric polyetherimide nanofiltration membranes by phase inversion process

AbstractIntegrally skinned asymmetric polyetherimide (PEI) membranes were prepared by the phase inversion process from casting solution containing dimethylformamide (DMF) as a solvent and 1,4‐dioxane as a cosolvent. Deionized water was used as a coagulation medium in preparing asymmetric membranes. The effect of 1,4‐dioxane was investigated by measuring casting solution properties, permeation properties, and membrane structures. Various effects of polymer concentration, evaporation time, and coagulation bath temperature were also studied. Low miscibility of 1,4‐dioxane with coagulant (water) resulted in reducing membrane pore size. The molecular weight cutoff values of asymmetric membranes could be controlled by changing the amount of 1,4‐dioxane in the casting solution. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1300–1307, 2002; DOI 10.1002/app.10452

The effect of sodium dodecyl sulfate solutions as gelation media on the formation of PES membranes

Sodium dodecyl sulfate (SDS) aqueous solutions were used as gelation media in the preparation of polyethersulfone (PES) membranes. The casting solution composition was the same for all the tested membranes. The temperatures of gelation media were 4 and 20°C. The concentration of SDS was changed from 0 to 3.0 g/l at 4°C and 0 to 1.6 g/l at 20°C. The surface tension of the gelation media was measured by drop weight method and the electrical conductivities were also determined. The membranes were characterized by transport parameters obtained from separation experiments and roughness parameters, obtained by the atomic force microscopic (AFM) technique. The molecular weight cut-off (MWCO) values of the studied membranes were found to be between 9 000 and 88 000 Da for membranes gelled at 4°C, and between 28 000 and 85 000 Da for membranes gelled at 20°C. The pore sizes were found to be between 3.04 and 10.73 nm for the membranes gelled at 4°C and between 4.48 and 10.74 nm for membranes gelled at 20°C, respectively. In general, both MWCO and pore size decreased with an increase of SDS concentration in gelation media when the concentration was below critical micelle concentration (CMC) and increased with an increase with SDS concentration when the concentration was above CMC. Images of membrane surfaces, taken by AFM, showed that the size of nodules and depressions decreased with a decrease in pore size. The roughness of membranes increased with an increase in pore size and MWCO.

Use of ultrafiltration and chromatography to assess aluminum speciation in serum after deferoxamine administration

Deferoxamine effectively chelates aluminum by forming aluminoxamine, a low-molecular-weight compound removable by dialysis. However, aluminum-bound species other than aluminoxamine might be present in serum after the administration of deferoxamine. To study aluminum speciation after the administration of deferoxamine, high-performance liquid chromatography (HPLC) and ultrafiltration techniques were used. Samples of serum were obtained from six dialysis patients 44 hours after the administration of a single dose of deferoxamine. HPLC and ultrafiltration studies were performed. In the HPLC studies, samples underwent ultrafiltration, the filtrate was injected into the chromatographic system, and detection was performed by UV light and atomic absorption spectrometry. Unknown species of aluminum other than aluminoxamine were found in the early elution fractions. In the ultrafiltration studies, the same samples of serum from the six patients underwent ultrafiltration using membranes with different molecular-weight cutoff values from 1 to 30 kd. The percentages of aluminum found by ultrafiltration using membranes with cutoff values of 5, 10, and 30 kd were greater (64.4% ± 2.5%, 63.5% ± 3.7%, and 65.6% ± 4.3%, respectively) than the percentages obtained with membranes with a 1-kd cutoff value (38.7%), suggesting that the unknown species of aluminum have a molecular weight between 1 and 5 kd. The unknown species of aluminum cannot be aluminoxamine because they behaved in a different way with HPLC.

Atomic force microscopy characterization of ultrafiltration membranes: correspondence between surface pore dimensions and molecular weight cut-off

Atomic force microscopy (AFM) has been used to quantify the mean surface pore diameters of a family of five ultrafiltration membranes with molecular weight cut-off (MWCO) in the range 1000–10 000 aspect ratio tips were used and the images were treated with fast Fourier transform filtering to remove distortions. The mean pore diameters were found to be in the range 1.93–3.14 nm, increasing systematically with the MWCO. Mean pore diameters have also been evaluated from the MWCO values using three published expressions relating MWCO to solute radius. Depending on the expression used, the ratio of the mean pore diameters obtained from MWCO data and AFM measurements (Rr) was in the range 1.04–2.42. Agreement between the diameters obtained in the two ways was best for the membranes with the lowest MWCO values. The overall agreement was substantially better than has been reported previously. The good correspondence between mean surface pore dimensions and MWCO demonstrates the usefulness of AFM in quantifying such a key membrane property. Copyright © 2000 John Wiley & Sons, Ltd.

PASSAGE OF MS2 BACTERIOPHAGE THROUGH VARIOUS MOLECULAR WEIGHT FILTERS

MS2 bacteriophage has a reported nominal molecular weight of 2M daltons. It would be expected that this phage would not pass through filters of various sizes with low molecular weight cutoff (MWCO) values of less than 1M daltons. It was discovered that MS2 bacteriophage will pass through filters with 750K-, 500K-, and 300K-dalton MWCO values. MS2 was retained on the 100K-dalton filter. A cross-flow hollow fiber apparatus was used for the 750K- and 500K-dalton analysis. Centrifuge filters of 1M and 300K and 100K daltons were used. The rate of passage of MS2 through the cross-flow filters is dependent on the tangential flow rate and pressure. Passage through the centrifuge filters depended on the gravitational force applied.

Polyetherketones based on para-carborane: synthesis, sulfonation, and membrane-forming characteristics

Polycondensation of bis(4-phenoxyphenyl)-1,12-dicarbadodecaborane with aromatic and aliphatic dicarboxylic acids in trifluoromethanesulfonic acid affords a new family of high-MW poly(aryletherketone-carborane) s. The polymers derived from 4,4′-biphenyldicarboxylic acid and 4,4′-oxydibenzoic acid are both essentially amorphous when isolated, but the former undergoes an unusual cold-crystallization process beginning at, rather than above, its glass-transition temperature (267°C), and melts at 386°C. Solutions of the polycarborane derived from 4,4′-oxydibenzoic acid afford high-performance ultrafiltration membranes when gelled in water, with molecular weight cut-off values in the range 9000–16000 daltons. Poly(aryletherketone-carborane)s undergo quantitative, structure-defined sulfonation in 98% sulfuric acid, resulting in the controlled introduction of two sulfonic acid groups per repeat unit. The resulting ionomers are insoluble in water but readily soluble in the lower alcohols, to give solutions from which thin-film composite nanofiltration membranes may be fabricated. Such membranes exhibit water-fluxes and ionselectivities which compare favourably to those of many commercial nanofiltration membranes.

A Micromethod for Concentration and Desalting Utilizing a Hollow Fiber, with Special Reference to Capillary Electrophoresis

Concentration of solutes in small volumes offers special problems. The difficulties are particularly great when the sample concentration is low, because the adsorption onto the surfaces with which the solutes come into contact may be very strong. The adsorption not only decreases the original sample concentration but also often alters the relative concentrations of the solutes, causing serious deviations from the original composition of the sample. A novel concentration method is described using a hollow fiber filled with the dilute sample solution. The technique is based on the transport of water out of the fiber through the pores in its wall and has the advantage that solute adsorption is negligible, at least when the solute concentration is not extremely low. The water transport can be achieved by passive or active evaporation or by the Donnan effect, accomplished by immersing the fiber filled with sample into a polymer solution. The method has the advantage that it permits concentration of all substances independently of their molecular weight. Proteins, for instance, were enriched 3000-fold in one step. By using an array of fibers and collecting all of the concentrated zones for a second concentration, the degree of concentration can theoretically become 30002 = 9 × 106-fold, provided that the limit of solubility is not exceeded. The speed of enrichment of proteins was 14-fold per minute for a fiber with an inner diameter of 0.2 mm, length 280 mm, and wall thickness about 0.01 mm in the dry state (the speed can be increased considerably by blowing air over the fiber, e.g., by means of a fan). Simultaneously with the concentration, some purification of the sample can often be achieved by using a fiber with an appropriate molecular weight cutoff value.

Protein Structure Change on Adherence to Ultrafiltration Membranes: An Examination by Electron Paramagnetic Resonance Spectroscopy

Electron paramagnetic resonance spectroscopy was used to examine changes in the distance between two spin-labels on hen egg lysozyme (HEL, 45 Å × 30 Å × 30 Å, 14,600 Da) when interacting with membranes during ultrafiltration (UF). Membranes with nominal molecular weight cutoff (MWCO) values ranging from 10,000 to 300,000 Da were used. Both cellulosic (hydrophilic) and polysulfone (hydrophobic) membranes were studied. The technique used is based on spin–spin interaction of protein-bound spin-labels that cause dipolar broadening of the EPR spectra. This line broadening is related to spin-label distance and was subsequently used to infer protein conformational changes. The results demonstrate that the distance between the two labels on HEL, while in aqueous solution, was approximately 14.4 Å. Using the known crystal structure of hen egg lysozyme, molecular computer modeling of the doubly labeled protein suggests that the most probable spin–spin distance in solution is 13.9 Å. This distance is in excellent agreement with our experimentally observed result. Protein–membrane interaction reduced this distance approximately by 5 Å, irrespective of the membrane morphology. It is speculated that the lack of variation seen in all the membranes, under our test conditions, may be due to limitations associated with spin-label locations.

Characteristics of Ovalubumin Gel Layer Formed on Ceramic Microfiltration Membranes.

Ovalbumin solution, which is known to form a gel layer on membrane surfaces, was filtrated in a cross-flow mode using ceramic microfiltration membranes. The influences of pH and ionic strength of the ovalbumin solution on the properties of the gel layer and membrane permeability were studied.At a pH near the isoelectric point of the ovalbumin solution, both the permeate flux and ovalbumin rejection showed the lowest value. The cohesion force acting among ovalbumin molecules became strongest because the electrostatic repulsion force among the molecules was weakened, and then the gel layer was tightly adsorbed onto the membrane surface. Therefore, the membrane permeate flux obtained at the pH of the isoelectric point was smaller than that obtained at the other pH. The ovalbumin rejection increased at the other pH, except for the isoelectric point, because of the electrostatic repulsion of the ovalbumin gel layer formed on the membrane surface. The influence of electric charge was also recognized in the case of the filtration of sodium L-glutamate solution using the ovalbumin gel layer.When the ovalbumin solution was filtered with a high ionic strength, the permeate flux and the ovalbumin rejection were almost constant, independent of the solution pH.The molecular weight cut-off values of the ovalbumin gel layer were about 200,000–300,000 as self-rejecting dynamic membranes.The specific resistance of the ovalbumin gel layer obtained in dead-end filtration increased in pH near the isoelectric point and with increasing ionic strength of the feed solution. This tendency of the specific resistances was in good agreement with the behavior of the permeate flux of the ovalbumin solution.

Ceramic Membranes for Novel Separations

Abstract A number of advances have been made in the fabrication and performance of supported ceramic membranes. Improvements in techniques for the synthesis of particulate sols containing particles in the range of 4 to 10 nm in diameter are discussed. Several deposition techniques for the fabrication of supported ceramic membranes via solgel technology are examined, including slipcasting, filtration, coating, and permformation. Supported titania membranes have been fabricated and both liquid phase permselectivity and gas phase permeability experiments have been performed. Results of the liquid phase separations indicate that these membranes are characterized by a molecular weight cutoff value of approximately 200. Gas phase permeabilities reveal that Knudsen diffusion is not the only mechanism of transport through these membranes.

Molecular Mass Distribution Immunological Properties Nutritive Value of Whey Protein Hydrolysates

Whey protein concentrate was hydrolyzed using the technical food-grade enzyme Corolase 7092 in order to abolish the allergenicity of whey proteins. The immunological properties of the hydrolysates were tested in vitro with a human-immunoglobulin E (human-IgE) enzyme-linked immunosorbent assay (ELISA) using sera obtained from children allergic to milk proteins and in vivo with a mouse-rat heterologous passive cutaneous anaphylactic test and an anaphylactic shock test in mice. The protein efficiency ratio, determined in young growing rats, was compared to that of casein. Ultrafiltration of the hydrolysates appeared to be necessary to obtain a hypo-allergenic product. The minimal molecular mass to elicit immunogenicity and allergenicity of whey protein hydrolysates appeared to be between 3,000 and 5,000 Da, so the molecular weight cut-off value of the filters required must be in this range. Although there was no evidence that extensively hydrolyzed whey protein is nutritionally inferior to casein, the slightly bitter taste might reduce food intake.

On-Line Dialysis-Column Liquid Chromatography for the Determination of Polar Pesticides in Water Samples Containing Humic Substances

Abstract The potential of the on-line combination of dialysis, for the removal of interfering humic substances from environmental samples, and trace enrichment on a precolumn packed with C18-bonded silica for the determination of polar pesticides by column liquid chromatography with UV detection is investigated, using six phenylurea herbicides as model compounds. The influence of the membrane material, thickness and molecular weight cut-off value, the configuration of the system-i.e. planar or hollow-fibre membranes-and, especially, the sample volume on analyte recovery and removal of humic Substances is studied. Systems which allow the determination of the test compounds at the 1.0 μg/1 level in river and lake water and at the 0.1 μg/l level in tap water are presented.

Osmotic pressure model of membrane fouling applied to the ultrafiltration of whey

An osmotic pressure model for membrane fouling was investigated for predicting flux behaviour during ultrafiltration of reconstituted whey solutions, using a membrane of molecular weight cut-off value 20 000 daltons. Estimations of flux were conducted for three solutions: whey protein, whey protein + lactose and reconstituted whey, using physical properties of whey protein and lactose measured or obtained from published papers. Estimated fluxes of whey protein and whey protein + lactose solutions compared closely with measured values. Concerned with concentration polarization, this model made it clear that operating pressures were reduced mainly by the osmotic pressure of lactose rejected by the fouled membrane, not of whey protein. In the range of usual operating pressures (0·1 ∼ 0·4 MPa), fouling was an important factor of flux decline for ultrafiltration of whey + lactose solution. Thus, a less fouling membrane is preferable.