Dear Foroogh,

The following text might shed light on the answer to your question.

Molecular Weight Cut-Off

Many membranes, especially those used in ultrafiltration, are also characterized in terms of their molecular mass cut-off. Here, the rejection of a given membrane for various molecular mass components is determined in filtration tests. Rejection is plotted versus molecular mass and the resulting curve describes the membrane's solute retention properties. Components with a diameter smaller than the smallest pores pass through the membrane unhindered; all components with a diameter smaller than the largest pores but larger than the smallest pores are rejected in proportion to the pore size distribution of the membrane.

The retention R is a dimensionless number expressed simply as:

For the equation to calculate R (rejection), please see the following link:

https://encrypted.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwiI1J6V_JbNAhVB2xoKHQR8BbIQIAgmMAE&url=http%3A%2F%2Fwebcache.googleusercontent.com%2Fsearch%3Fq%3Dcache%3AHrOyex3pigoJ%3Awww.emdmillipore.com%2FWeb-US-Site%2Fen_CA%2F-%2FUSD%2FShowDocument-Pronet%253Fid%253D201306.5278%2B%26cd%3D2%26hl%3Den%26ct%3Dclnk%26gl%3Dus&usg=AFQjCNFLcNSQwJ7TE-imag3PZq8IDJjqlA&sig2=WsBMba7CFgd5W7KyiMsnsA

The value of R varies between 100% (complete retention of the solute) and 0% (solute and solvent pass through the membrane freely).

Some commercial membranes have a very narrow pore size distribution and thus a sharp molecular mass cut-off. Others have a wider pore size distribution and therefore a more diffuse cut-off.

For more on this topic, please use the following link:

http://www.separationprocesses.com/Membrane/MT_Chp03c1.htm

Hoping this will be helpful,

Rafik

In addition charged moieties around and within the pores can interact with charged groups on the compound - depends on the nature of the membrane and the nature of the compound

You should also consider that BSA is a compact, globular protein, whereas dextran is a polymer of sugar. The difference in shape can also affect the ability to pass through the ultrafiltration membrane.

BSA monomer with Mw 67000 has an approximate hydrodynamic radius of less than 4nm. Dextrane 150000 has an approximate hydrodynamic radius of more than 10nm.  So your dextrane should be much larger than the BSA monomer. It may be that the BSA preparation contains aggregated components such that the average size is much larger than the expected monomer alone, i.e. that there are oligomeric components present. Preparation in a different buffer, or pre-filtration with a good filter may make the results more consistent.

http://www.materials-talks.com/blog/2014/09/16/faq-molecular-weight-estimate-from-dls/

Since you saw dextran passing through the ultrafiltration membrane despite its having too large a molecular weight to do so, a likely explanation is that the sample contained a substantial portion of low molecular weight material.