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Fundamentals of Membrane Dialysis

Dialysis is a simple process in which small solutes diffuse from a high concentration solution to a low concentration solution across a semipermeable membrane until equilibrium is reached. Since the porous membrane selectively allows smaller solutes to pass while retaining larger species, dialysis can effectively be used as a separation process based on size rejection. The conditions of dialysis can be controlled or manipulated to produce desired results for a variety of dialysis applications. The application depicts which Molecular Weight Cut Off (MWCO) yields the preferred molecular separation.

Benefits

  • Very Gentle Conditions
  • Easy Operation
  • Wide Range of Sample Volumes
  • Many Membrane Types & MWCO's
  • Disposable Membranes & Devices

Applications

  • Macromolecular Purification
  • Protein Concentration
  • Solute Fractionation
  • Contaminant Removal
  • pH Change
  • Desalting
  • Buffer Exchange
  • Binding Studies
  • Electro-elution
 

Selection Guide for Spectrum Dialysis Membranes

Successful dialysis requires the correct membrane selection for your working conditions. Consideration should be given to membrane:

  • Grade (Biotech or Standard) and/or treatment based on application
  • Type (CE, RC or PVDF) based on sample/buffer conditions
  • Configuration (tubing, RDD or flat-sheet system) based on application
  • MWCO (16 cut-offs available) based on solute size
  • Device or flat widths (tubing) based on volume

Membrane Types

Spectra/Por® Biotech

Biotech membranes are specifically engineered for more critical separation constraints in which an ultrapure membrane with a precise MWCO is required. Since these synthetic membranes are manufactured using a process without metal salts, no pre-treatment or cleaning is required. Biotech membranes are available in 3 membrane types:

  • Biotech Cellulose Ester (CE) - Wide range of selectivity & purity
  • Biotech Regenerated Cellulose (RC) - Combined selectivity, purity & resistance
  • Biotech Polyvinylidene Difluoride (PVDF) - Strong organics, acids, bases & high temperatures

Spectra/Por® Standard Grade Membranes

Ideal for general-purpose dialysis, this lower cost membrane can be used for a wide variety of dialysis applications in which only general separations are required and low level membrane impurities are of little concern. The standard RC type is composed of natural cellulose regenerated from cotton linters for strength and durability. These membranes are also available pre-treated or can be treated for the removal of trace metals or sulfide contaminants.

Overview

Membrane Grade:   Biotech   Standard
Membrane Type: CE RC PVDF RC
MWCO (Daltons): 100, 500, 1k, ,2k, 3.5k, 5k, 8k, 10k, 15k, 25k, 50k, 100k, 300k, 1,000k 3.5k, 8k, 10k, 15k, 25k 250k, 500k 1k, 2k, 3.5k, 5k, 6-8k, 10k,
12-14k, 15k, 25k, 50k
Physical Properties: Opaque, Rigid, Hydrophilic Opaque, Flexible, Hydrophilic Opaque, Rigid, Hydrophobic Clear, Flexible, Hydophilic
Organic Resistance: Fair Good Excellent Good
pH Range: 2-9 2-12 1-14 2-12
Temperature Limit: 37oC 60oC 130oC
(Autoclavable)
121oC
(Autoclavable)
Flat Sheet: Asymmetric Porosity Not Available Not Available Symmetic Porosity
Available Irradiated: Yes Yes No No
Recommended Closures: Universal Only Universal Only Universal Only Both
Ready-to-Use Dialysis Devices: DD, FAL DD, FAL Not Available Dialysis Sacks

DD = DispoDialyzer®, FAL = Float-A-Lyzer®

MWCO & Pore Size consideration

Dialysis membranes consists of a spongy matrix of crosslinked polymers, the pore rating referred to as Molecular Weight Cut Off (MWCO), is an indirect measure of the retention performance.

Membrane MWCO is determined as the solute size that is retained by at least 90%.

However, since a solute's permeability is also dependent upon molecular shape, degree of hydration, ionic charge and polarity, it is recommended to select a MWCO that is half the size of the MW of the species to be retained and/or twice the size of the MW of the species intended to pass through.

A general guide to molecular Pore size is available to download in the resources section below.