Nominal molecular weight cutoff ultrafiltration membranes

Figure 7. Association of predissociated kraft lignin sample (92% retained by nominally 500 molecular weight cutoff ultrafiltration membrane) during incubation at 160 gL 1 in 1.0 M ionic strength aqueous 0.40 M NaOH for (l)0h, (2) 50 h and (3) 480 h.

UF membranes can retain macromolecules such as polysaccharides, proteins, biomolecules, polymers, and colloidal particles. Generally, ultrafiltration membranes are classified by the type of material and their nominal molecular weight cutoff (MWCO), which is usually defined as the smallest molecular weight species for which the membrane has more than 90% rejection. 

The retention characteristics of ultrafiltration membranes are measured in Nominal Molecular Weight Cutoff (NMWCO). The NMWCO can be controlled during membrane formation and is typically available in 1,000 10,000 100,000 and 1,000,000 daltons. For cell harvesting, ultrafiltration membranes of 100,000 NMWCO and microporous membranes are generally employed. While the separation is based on size, the efficiency of the operation can be influenced by other factors. 

Ultrafiltration membrane manufacturers frequently characterize their membranes using the cutoff concept rather than pore size. The nominal molecular weight cutoff (MWCO) is a performance-related parameter, defined as the lower limit of a solute molecular weight (e.g., dextran) for which the rejection is 95-98% (Boerlage, 2001). As the MWCO decreases, the mean pore diameter for most UF membranes has been foimd to decrease (Kim et al., 1990). However, the MWCO may be sharp or diffuse, that is, thoe is a range of MWCO, and in reality MWCO is only a rough indication of the membrane s ability to remove a given compound as molecular shape, polarity, and interaction with the membrane affect rejection (Mulder, 1996). Moreover, membrane surface characteristics (e.g., surface porosity and pore size distribution) may influence the apparent size of particles retained. 

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