The Molecular Weight Cut-Off (MWCO) value often is used by membrane manufacturers to characterize their porous ultrafiltration membranes. It is a typical permeation-related membrane characteristic. The value is defined as the molecular weight (MW) which is rejected for 90%. The rejection is defined as
R = (cF – cP)/cF = 1 – (cP/cF)
where R is the rejection [-], cF the feed concentration of a solute and cP the permeate concentration of this solute. The rejection depends on many solute and process parameters like the type of solute, concentration, hydrodynamics, pressure, temperature and pH. MWCO measurements usually are carried out in separate experiments using different solutes, each with a certain MW. This results in a time consuming procedure for MWCO determination, in addition also neglecting differences in shape and flexibility of the solute and phenomena like concentration polarization and adsorption of the solute on the membrane.
At the EMI Twente we therefore do not use single solutes but a mixture of dextranes or polyethylene glycols (PEG) covering a broad molecular weight range around the expected MWCO. Tests are carried out in a standardized cell, where we carefully control pressure and cross flow velocity in order to create standard conditions for the measurements. After the permeation experiment is carried out, the MW distributions of feed and permeate are determined using Gel Permeation Chromatography (GPC). By calculating the rejection for each MW using the equation mentioned before, the rejection as a function of MW can be determined in a single permeation experiment. The molecular weight where the rejection is 90% now easily can be verified.