A situation that is frequently encountered in life science research is a protein solution that is too dilute to allow further downstream analysis. This solution could either be a complex mixture, such as an extract, or a purified protein. The problem is how best to concentrate the proteins in the solution without losing most of your sample or losing crucial activity. One remedy for this problem is to pass the solution through a membrane filter that possesses a defined molecular weight cutoff (MWCO). The idea is to retain the desired protein(s) on one side of the membrane while allowing all others, including most of the solvent, to pass through. The result is a solution of considerably smaller volume but with an elevated protein concentration.
One company that is relatively new to the field of filtration-based concentration is Vivascience (Hannover, Germany). Vivascience offers a wide variety of centrifugal, pressurized, and tangential concentrators. We have used their centrifugal and tangential flow concentrators for the past 2 years and are very satisfied with them. The centrifugal concentrators are available in sizes that cover just about any volume between 0.5 and 100 ml. The 2-3 ml concentrators are available with filtration membranes of polyethersulfone (PES), cellulose triacetate (CTA), or regenerated cellulose (RC). The 0.5 ml size comes with PES or CTA, and the others, covering volumes from 2 to 100 ml, come with PES membranes. We have found PES to be quite adequate for work with complex mixtures, like extracts, although the recovery of purified proteins may vary depending on the membrane used. MWCOs available are 5,000 to 100,000 (for 0.5 to 20 ml) and 5,000 to 1 million for the 20-100 ml size.
Positives for the centrifugal concentrators include very low dead stop and hold-up volumes (30 µl and <10 µl, respectively, for the 2-6 ml size) and speed. We have found the Vivascience units to be much quicker, in general, than some of the other brands. However, the speed of concentration is very solution-dependent. For example, concentration of a very dilute solution of a purified protein may be complete in 5 minutes whereas the same volume of a solution of many different proteins (like urine) may take much longer. Our practice, no matter what the solution is we are concentrating, is to check it after 5 min of centrifugation to get an idea of the rate of concentration. Spinning all the way to dead stop seems to result in very poor recovery.
For larger volumes, Vivascience offers tangential flow concentrators in 2 sizes, recommended for volumes of 100 ml up to 5 L or more. A peristaltic pump is required to pump the solution through the concentrators. The units are available with PES or RC membranes and with MWCOs of 5,000 to 100,000. These are excellent for working with large volumes of dilute solutions, such as cell culture media. We have used a pair of the smaller size (VivaFlow 50), connected in series, to concentrate 2 L of insect cell medium with very good results. Although the units are sold as disposables, they are priced a bit higher than most disposable items. Fortunately, they can be cleaned and reused.
Michael Campa, Ph.D.
Asst. Research Professor of Radiology
Duke University Medical Center