Amicon® Ultra-4 Centrifugal Filter Units From Millipore

The Amicon® Ultra-4 Centrifugal Filter Devices made by Millipore Corporation, USA are provided in packs of 8, 24 or 96 filter device units. The filter units are comprised of a standard, graduated 15 ml falcon tube and a built-in, detachable filter unit. The Ultra-4 series is designed for working with volumes of up to 4 ml. The filter units in the tubes are fitted with Ultracel® membranes which are made of regenerated cellulose. Five different membranes are available which differ in the amounts of total sample volume they can process and sample recovery. I only have experience using the Ultracel-3 membrane fitted devices which result in up to ~99% sample recovery.

I use the Amicon® filter devices primarily for concentrating crude protein extracts from different plant tissues or cell cultures. I am concerned with adjusting the concentrations of my samples to standardize further analyses which require fixed protein amounts, although the filter devices can be used as efficiently for work with dilute samples. They have been designed for the concentration of biological samples which may contain a whole range of macromolecules, including proteins and nucleic acids. The filter devices are also suitable for de-salting and buffer exchange steps because of their compatibility with a wide range of chemicals, particularly buffers which are commonly used to re-suspend biological macromolecules.

The filter devices work perfectly well in both swinging bucket and fixed-angle rotor centrifuges, although the manufacturers recommend the swinging bucket type. Different speeds have been suggested for macromolecules of various molecular weights. For example, spinning at 7500 x g for about 15 minutes in a 35 degree fixed-angle rotor centrifuge can efficiently and quite accurately concentrate a 1.5 mg/ml crude plant protein sample to 5 mg/ml. Anybody deciding to use this product should calculate how much protein they have, how much they want and then do some simple math to determine the final volume that’s pipetted into the filter device and how much will be retained. This retentate is the concentrated protein sample that can easily be recollected using a 200 ul tip. This is especially convenient since the filter unit is designed to accommodate regular 200 ul tips. Design features like this add to the reliability of the product because a crucial factor in concentrating biological samples is ensuring that the collected sample contains everything you’re after.

While using these filter devices, it is important to be careful about a few things. First, the spin speed and times selected should be for the type of centrifuge you intend to use. Second, if you intend to spin some samples down and then head off for lunch, you should remember that if the membrane dries up, although you will not lose your macromolecule of interest since it would not go through the cellulose membrane, it will be very hard to reconstitute. All in all, I’ve been very satisfied with using these devices. In a busy week I can go through maybe 20 or 30 of these and I’ve never had a problem to date!