Molar mass cut-off

Practically, polymers with molar masses between 2 x 104 and 2 x 106 g/mol can be characterized by membrane osmometry, but measurements of Mn <104 g/mol have also been reported with fast instruments and suitable membranes [16]. The lower limit is set by insufficient retention of short polymer chains. Above M 2 x 106 g/mol, the osmotic pressure, which is proportional to Mr1, is too low for a reasonable signal-to-noise ratio. An advantage of the low molar mass cut-off is that impurities with a very low molar mass can permeate through the membrane and, hence, do not contribute to the measured osmotic pressure. Their equilibration time may, however, be different from that of the solute, leading to complex time-dependent signals. 

If the protein concentration of the eluate is too low for further investigation, concentrate the sample using a centrifuge membrane concentrator (Centricon) with molar mass cut-off significantly lower than the Mr of the analyzed protein or by precipitation with TCA or acetone. 

The fibers can be made of different materials, such as cellulose esters and polysulfone. The total surface area of a hollow-fiber bioreactor varies in the range of 0.5-3.5 m2. The pore size of fibers commonly employed in animal cell culture corresponds to a molar mass cut-off between 10 and 100 kDa. 

In ultrafiltration, water and other low molar mass molecules are forced through a semi-permeable membrane by the application of high pressures (1-7 bar) or of a centrifugal field. This technique involves membranes with pore diameters in the range of 1.0-20 nm, which are most commonly characterized and selected based on their nominal molar mass cut-off... 

Reverse osmosis is a process for the removal of particles or ions from a solution for the purification of water and/or removal of salts and other impurities. Tlie solution is allowed to pass through a semipermeable membrane, while the contaminants are rejected. The typical molar mass cut-off (MMCO) is in the range of 150-250 g/mol. 

Molar mass cut off = 200 = 0.2 kDa. The usual range is 0.01-1 kDa. Can handle fluid with significant osmotic pressure, sugars, dissociated acids and divalent salts although the latter two are better handled by RO. 

Experimental considerations Sample preparation and data evaluation are similar to membrane osmometry. Since there is no lower cut-off as in membrane osmometry, the method is very sensitive to low molar mass impurities like residual solvent and monomers. As a consequence, the method is more suitable for oligomers and short polymers with molar masses up to (M)n 50kg/mol. Today, vapour pressure osmometry faces strong competition from mass spectrometry techniques such as matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS) [20,21]. Nevertheless, vapour pressure osmometry still has advantages in cases where fragmentation issues or molar mass-dependent desorption and ionization probabilities come into play. 

Membrane osmometry measurements were carried out with the capillary osmometer shown in Figure 3. Owing to the short equilibration time of the instrument and the low cut-off molar mass of the membrane, solute permeation through the membrane, which would show up as a drift of the baseline, did not cause problems even for the lowest molar mass fraction. M was obtained from... 

The important errors were produced by the drifts of base line and by the improper peak limits and base line settings. For example, the presence of a low-molar-mass tail in a polystyrene sample inflicted an error of 800% in its value. In some cases, the low molecular tail was overlapped with the system peaks, while in other laboratories the software cut-off the low-molar-mass tail of chromatogram. It was likely considered just the base line perturbation by the computing system. 

Desalting or buffer exchanges are often required between purification steps. At the laboratory scale, the protein solution is placed in a tube of a semipermeable polymer membrane immersed in the desired buffer. The membrane pore size determines the minimum molar mass of the compounds that are retained. Small molecules with a molar mass below the membrane cut-off will flow freely across the membrane until the osmotic pressure equilibrium is reached. Complete buffer exchange requires several changes of the dialysis liquid. The process should be carried out at a temperature around 4°C, to avoid loss of activity. 

MWCO), usually defined as the molar mass at which the membrane rejects 90% of solute molecules. However, as in microfiltration, the molecular shape can affect permeability through the membrane pores. For example, a membrane with a nominal cut-off of 100 kDa, which does not allow globular molecules with a molar mass of 100 kDa to flow through, may allow fibrous molecules with higher molar masses to flow across the pores. As in microfiltration, the membrane pore size is not uniform, with a normal distribution around an average value. 

Solvent Relative molar mass Boiling point CC) Viscosity at 20>C (cP) Refractive index at 25-C UV cut-off (nm) Polarity (AI O "... 

Impedance spectra of NETs can be considered as similar to those of DNA as they consist mainly of dsDNA. Additionally, the attached granula proteins and the irregular distribution of the NETs should be taken into account. Dielectric properties of dsDNA were first studied in [6]. The authors observed a dispersion mainly caused by rotation of several DNA molecules. The cut-off frequency depends on the relative molar mass M of DNA... 

One problem of HT-AF4 is that macromolecules with a molar mass below 100 kg/mol are not fully recovered due to the relatively high cut-off of the ceramic membrane. This leads to an overestimation of the calculated molar masses for samples which contain such small molecules. The examination of the data below the cut-off mass range, however, has shown that the error of the missing low part is very low compared to the effect of missing shear degradation. At present, novel ceramic membranes with a lower cut-off are under development. 

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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