The molecular weight cut-off rates retention of reference molecules. One manufacturer's definition uses the mass at which approximately remains after seventeen-hour dialysis, tested with globular proteins. A rating uses dalton units: a mass, not a length or an absolute passage threshold.[ref]
Dialysis: an overview, Pierce Biotechnology technical note. Tech Tip 20, p. 1, Factors Affecting Dialysis Rate: MWCO definition and globular reference molecules
A semipermeable membrane has a distribution of passage sizes, and a molecule's shape affects passage as well as its mass. Near the nominal MWCO, slow passage can make substantial retention compatible with eventual leakage. The protein-retention experiment below makes retention an observable quantity rather than a universal yes-or-no property inferred from the label.
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Molecular weight cut-off (MWCO) refers to the maximum molecular weight of a substance that can pass through a semipermeable membrane or filter. It is a critical parameter used in various applications, including protein purification, dialysis, ultrafiltration, and other processes in biochemistry and molecular biology. MWCO is typically expressed in daltons (Da) or kilodaltons (kDa) and indicates the size or molecular weight at which molecules are retained by the membrane.