小中大Millipore's products are easy to use, but it intends to be blocked if you are using raw cell extracts or supernatant of culture media especially you want to collect small MW proteins. I once used a MW cuting 3000's, but I can only concentrate my samples 2-3 times before it was totally blocked.
Basicly, it is a centrifuge tube with a membrane, a little bit like PCR purification kits, except protein is not binding with the membrane. The use of membrane is to separate the residue molecular from the filtrate molecular by molecular weight. You can choose different memberane by the MW cutting (from 3000 to 100000) hence concentrate your target compounds in the sample. It can also be used to desalt and so on.
