The molecular weight cut-off (MWCO) is a term used in membrane filtration to denote the pore size distribution and retention capability of a membrane. In general, for optimal recovery of most macromolecules, the recommendation is to use an ultrafiltration membrane with an MWCO that’s 1/3 to 1/2 of the target molecule’s molecular weight. 

MWCO Selection for RNA

To select the right Molecular Weight Cut-Off (MWCO) for RNA:

• Find the molecular weight of the target mRNA, based on its ribonucleotide composition.
• Choose an ultrafiltration membrane with an MWCO roughly one-third of the molecular mass of the target mRNA.

For instance, the average molecular weight of ribonucleotide in RNA is 321.5 Da.

Hence, mRNA that is 1000 nucleotides long would have a molecular weight of 321.5 x1000 = 321,500 Da, which is = 321.5 kDa. 

In this case, you would need a membrane with an MWCO of 107 kDa (1/3 of 321).