Microcon Centrifugal Filter Units From Millipore

Filtration is a basic laboratory technique that allows the researcher to isolate biological material, remove impurities, sterilize, desalt, and dialyze as well as concentrate very dilute macromolecular samples. In the past, this process has been quite time consuming, especially dialysis! The Microcon centrifugal ultrafiltration filter devices from Amicon (Millipore) more than halve the time needed to accomplish any one of these tasks. Millipore offers several ultrafiltration membranes with various nominal molecular weight cut-offs (MWCO) or nominal molecular weight limits (NMWL). The Microcon filters allow very small molecules to be separated from larger molecules. Molecules ranging from 10 ³-10⁶Da pass easily through the membrane pores while molecules larger than the MWCO of the filter unit do not pass through the membrane pores.

To help you choose the best filter unit for your particular task, remember the “RULE OF 2”. To retain a molecule such as IgG (~150 KDa) you will need a membrane with a cut off that is at least 2 times smaller (i.e. 50 MWCO/NMWL) than the molecule. While this does not seem to be a hard and fast rule, it is best to be conservative. For example, we have successfully recovered IgG using a 100 MWCO membrane without loss of sample. The Microcon filters are especially useful for concentrating double-stranded RNA or DNA. We use the YM-30 microcon to concentrate 100-200 bp dsRNA or DNA (e.g. PCR products for sequencing) in 6-8 minutes, as compared to an hour or two (to overnight) following standard salt precipitation protocols. Some individuals claim that “inadvertently” spinning down to dryness will result in tremendous sample loss. The fact of the matter is that you can still easily recover your sample by adding a little bit of buffer and dilute detergent to the filter, then inverting the filter into a collection tube and centrifuging the sample into the tube.

The Microcon filters allow you to dialyze and concentrate in 10-15 mins on a standard bench top centrifuge. This is much more convenient than doing at least three buffer changes and overnight dialysis at 4ºC. We have also used two filters with different MWCO limits in a single spin to fractionate our sample. You will simply need a larger vessel to hold your 1.5 ml (cap-less) collection tube and the two filters one on top of the other. Using 30 MWCO and 100 MWCO filters, you can isolate molecules larger than 30 KDa, but less than 100 KDa in one spin.

The only drawback is cost. Students who are not familiar with Microcon filter units often discard the filters after a single use. They can be used at least twice (if not three times) to concentrate a large volume (of the same sample and buffer). Although the volume limit for the Microcon filter receptacles is 500 µl, the same filter can be used to concentrate 1.5 ml of microsample. We have not tried using the filters more than two or three times since there is some danger of warping the membrane and losing your sample. Another lab reportedly washed the microcons in order to use them again with a completely different sample and buffer. I am not so sure our experiments are that expendable.