小中大
That means the membrane cut-off is 3000. There is a formular to convert the cut-off to real molecular weigh. I forgot the formula, but 3000 cut-off is already very small. If I am not wrong, it converts to MW around several hundrads (not accurate here, sorry).
Yeah, you are right. If the MW higher than, (let's use 3000 to represent here), 3000, then it will not go through the membrane but left in the residue part. Small molecules, like sodium Na+, will go through the membrane to the filtrate part. So that, your sample is concentrated and at the same time desalted. If you choose membrane cut-off, let's say, 100000, than some proteins with small MW will also go through the membrane. That means, if you know your target protein's MW, you can choose a suitable membrane to separate the protein you want form others, isn't it? But of course, the shape of a molecule may also effect the filtration efficiency. If a molecule with large MW, theriatically, it will stay in the solution. But, if its shape is rod, than, maybe some of them will also go through the membrane. So, I suggest that you could use the smallest one. I used 3000s one before, almost everything (I mean proteins) stays in the solution.
Are we clear here? Good luck.
