 The exclusion limit or "cut-off" of the membrane is usually specified in the form of "NMWC" (nominal molecular weight cut-off, or "MWCO", molecular weight cut off, with units in Dalton).It is defined as the minimum molecular weight of a globular molecule that is retained to 90% by the membrane.The cut-off, depending on the method, can by converted to so-called "D90", which is then expressed in a metric unit.