超滤原理（二）

Molecular Weight Cut Off
The selectivity and retentivity of a membrane are characterized by its molecular weight cut off (MWCO). This method is reasonably precise when dealing with a single type of solute in which all members are similar in shape, charge and hydration. Most membranes do not have completely uniform pores, but exhibit a pore size distribution that may also affect the degree of separation attainable. By convention, a molecular weight cut off of 10,000 means that the membrane will retain 90% or more of solutes of molecular weight 10,000 or more. Molecules and ions with low molecular weight (e.g. less than 500) pass freely through the membrane, and species with higher molecular weights up to 10,000 are partially retained according to the pore size distribution of the membrane. Molecular/Por® membranes show a narrow pore size distribution which guarantees high selectivity and sharp MWCO.
Adsorption
Adsorption is the phenomenon in which retained solutes in the process solution bind to the surface of the molecular filtration membrane. The adsorption process tends to block or occlude the pores of the membrane resulting in increased retention and decreased flux for the membrane and thus may substantially reduce the recovery of the solute of interest. Adsorption is dependent upon the characteristics of the membrane polymer. With proteinaceous solutes adsorption loss is directly related to membrane area when Type F membranes are used. Adsorptive loss of 2 - 3 x 104 g serum protein/cm2 have been observed with Type F membranes. Type F membranes should be employed in applications where chemical resistance is more critical than yield of processed materials.
When adsorptive loss must be minimized Type C membranes are strongly recommended. Type C membranes have the lowest adsorption of any Molecular/Por membrane and demonstrate negligible loss of protein and other macromolecules.
The following table lists adsorption of cytochrome c (150 µg/mL) for several brands of molecular filtration membranes while concentrating 50 mL solution to 10 mL with a 43 mm disc membrane.
Adsorption Losses on Different Membranes
Membrane Type        Adsorption Loss (%)        Retention of Cytochrome C (%)
Molecular/Por® C-10K
0.8        97
Amicon® YM-10
2.3        97
Amicon UM-10        4.3        26
Millipore® PTGC (104)
11.3        >99
Amicon PM-10        24.6        53
Amicon® is a registered trademark of W.R. Grace & Company.
Millipore® is a registered trademark of Millipore Corporation.
Chemical Resistance
Type C membranes (pH 2 to 9) made from cellulose ester are not compatible with many organic solvents. Type F membranes are exceptionally unreactive and exhibit excellent organic solvent resistance, as well as outstanding overall chemical resistance (pH 1 to 11).
Temperature Resistance
Type F membranes are temperature resistant and can be autoclaved or steam sterilized. Type C membranes are not visibly changed by use in warm water but temperatures above 37°C will change the MWCO and the user should determine the change before use at higher temperature.