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A microporous membrane filter removes particles according to pore size. By contrast, an ultrafiltration (UF) membrane functions as a molecular sieve. It separates dissolved molecules on the basis of their size–often reported as the “molecular weight“ (both parameters are related, but not always directly)—by passing a solution through an infinitesimally fine filter.

Benefits & Limitations
  • Effectively removes most particles, pyrogens, enzymes, microorganisms and colloids above their rated size, retaining them above the ultrafilter surface.
  • Efficient operation throughout their lifetime, unless they are damaged.
  • Their lifetime can be extended by a regular water flush at high speed.
  • Will not remove dissolved inorganics or organic substances.
  • May clog when challenged by an excessive level of high-molecular-weight contaminants.

UltrafiltrationThe ultrafilter is a tough, thin, selectively permeable membrane that retains most macromolecules above a certain size (Nominal Molecular Weight Limit, or NMWL) including colloids, microorganisms and pyrogens. Smaller molecules, such as solvents and ionized contaminants, are allowed to pass into the filtrate. Thus, UF provides a retained fraction (retentate) that is rich in large molecules and a filtrate that contains few, if any, of these molecules.

Ultrafilters are available in several selective ranges. In all cases, the membranes will retain most, but not necessarily all, molecules above their rated size. In water purification, ultrafilters are routinely used to provide pyrogen-free and nuclease-free water for critical cell culture or molecular biology experimentation. The key point here is the validation process, which ensures that the ultrafilter, when challenged by pyrogens, RNases or DNases at levels far above those likely to occur during regular operation, will be able to reliably deliver water within specification.


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