MWCO

MWCO stands for molecular weight cut-off. It is a parameter used to describe semipermeable membranes in filtration and dialysis. The MWCO is typically defined as the molecular weight (in daltons) of a solute that is 90% rejected by the membrane under specified conditions, meaning that solutes with higher molecular weights are largely retained while smaller molecules pass through. Some manufacturers define the cut-off at different rejection levels, so MWCO is a guideline rather than a precise pore size.

MWCO informs membrane selection for separating macromolecules from solvents. Membranes are marketed with values such as 1 kDa, 10 kDa, 100 kDa, etc., representing ranges of retained solutes. In practice, a membrane with a given MWCO will retain a substantial fraction of species near that weight, with higher rejection for larger solutes. The actual retention depends on solute shape, charge, concentration, and process conditions.

MWCO is influenced by membrane material and pore structure, as well as test conditions such as temperature, pH, ionic strength, and flow rate. It is not a fixed pore size and can vary between manufacturers and even between batches.

In applications, MWCO is used in designing protein concentration and diafiltration steps, wastewater treatment, and sample preparation. It is also a concept in size-exclusion chromatography, where resin pore sizes approximate a cut-off for solutes by size.