= Molecular weight cut-off
= MWCO
{c}
{synonym}
The <molecular weight cut-off> rates retention of reference <molecules>. One manufacturer's definition uses the <mass> at which approximately $90\%$ remains after seventeen-hour <dialysis>, tested with globular <proteins>. A $10\,\mathrm{kDa}$ rating uses <dalton (unit)> units: a <mass>, not a length or an absolute passage threshold.<Dialysis: an overview, Pierce Biotechnology technical note>[Tech Tip 20, p. 1, Factors Affecting Dialysis Rate: MWCO definition and globular reference molecules]{ref}
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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