Protein Sample Ultrafiltration

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Original design for a centrifugal ultrafiltration device, from the patent owned by Amicon Separation Sciences Bowers, William F., and Peter N. Rigopulos. Centrifugal Microconcentrator and Methods for Its Use. W. R. Grace & Co., assignee. Patent US 4632761 A. 30 Dec. 1986. Print.

The Original Amicon® Filter: Concentrating on Your Data Since 1965.

Since our original research on protein concentration using ultrafiltration1, we’ve arrived at perfection.

Today’s Amicon® Ultra filters are made with high-quality Ultracel® regenerated cellulose membrane. On this page, you’ll find technical information on the properties of ultrafiltration membranes and how they have been optimized for protein concentration and the fine separation of other macromolecular solutes.

1Blatt, WF et al. “Protein Solutions: Concentration by a Rapid Method.” Science. October 8, 1965, Vol. 150, No. 3693, p. 224-226.

Ultrafiltration Membranes

Ultrafiltration (UF) is the process of separating extremely small particles and dissolved molecules from fluids. The primary basis for separation is molecular size, although in all filtration applications, the permeability of a filter medium can be affected by the chemical, molecular or electrostatic properties of the sample.

Ultrafiltration can only separate molecules which differ by at least an order of magnitude in size. Molecules of similar size cannot be separated by ultrafiltration. Materials ranging in size from 1K to 1000K molecular weight (MW) are retained by certain ultrafiltration membranes, while salts and water will pass through. Colloidal and particulate matter can also be retained.

Ultrafiltration membranes can be used both to purify material passing through the filter and also to collect material retained by the filter. Materials significantly smaller than the pore size rating pass through the filter and can be depyrogenated, clarified and separated from high molecular weight contaminants. Materials larger than the pore size rating are retained by the filter and can be concentrated or separated from low molecular weight contaminants.

Ultrafiltration is typically used to:

  • Separate proteins from buffer components for buffer exchange, desalting, or concentration
  • Remove or exchange of sugars
  • Remove or exchange non-aqueous solvents
  • Separate free from protein-bound ligands
  • Remove materials of low molecular weight
  • Rapidly change ionic and/or pH environment
Depending on the protein to be retained, the most frequently used membranes have a nominal molecular weight limit (NMWL) of 3 kDa to 100 kDa.

Comparing Ultrafiltration and Microfiltration Membranes

Ultrafiltration membranes, such as Ultracel® regenerated cellulose membrane, generally have two distinct layers:
  1. Thin (0.1-1.5 µm), dense skin with a pore diameter of 10-400 angstroms
  2. More porous substructure below the skin
Any species capable of passing through the pores of the skin can therefore freely pass through the membrane. In regenerated cellulose, pore size is precisely controlled during a hydrolysis step inherent to the manufacturing process. Another advantage of regenerated cellulose is that it is amenable to heat-sealing and welding, which makes adhesives unnecessary. As a result, it is possible to manufacture regenerated cellulose-based devices that leach very few extractable impurities into the sample.

Microporous membranes (such as polyethersulfone) are generally rigid, continuous meshes of polymeric material with defined pore sizes. They are used to retain bacteria, colloids and particulates. Species are either retained on the membrane or are trapped in its substructure.

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Cross-section of ultrafiltration membrane with skin and porous substructure.
Cross-section of traditional microporous membrane with uniform pore structure from top to bottom.

Which Membrane Should You Choose for Ultrafiltration?

Our customers have asked for a quick summary of membranes commonly used in filter devices to help understand the difference between ultrafiltration and microfiltration. We’ve summarized a few popular membranes in the table below.

MembraneAppropriate for Ultrafiltration?Appropriate for Microfiltration?
Ultracel® regenerated cellulose BEST N/A
Polyethersulfone (PES) (e.g. Biomax® membrane or Millipore Express® PLUS PES) 2nd BEST BEST
PVDF No Yes