Applications of Ultrafiltration

Although dialysis is still used occasionally as a purification tool, it has been largely replaced by gel filtration adn ultrafiltration techniques. The major disadvantage of dialysis that is overcome by the newer method is that it may take several days of dialysis to attain a suitable separation. The other methods require 1-2 hours or less.
Ultrafiltration involves the separation of molecular species on the basis of size, shape, and/or charge. The solution to be separated is forced through a membrane under teh influence of high pressure or centrifual force. Membranes may be chosen for optimum flow rate, molecular specificity, and molecular weight coutoff. Two applications of membrane filtration are obvious:
1) desalting buffers or other solutions and
2) clarification of turbid solutions by removal of micron- or submicron- sized particles. Other applications are discussed below.
Membrane filters are divided into two major classes, depth and screen. Depth filters, which may be composed of paper, cotton, or fiberglass, function by trapping particles primarily within the "depths" of the filter matrix. The interior of these filters is a random arrangement of fiber material forming tiny channels. Particles that are larger than the passages are retained in the filter by entrapment in the matrix. Since they are thick, depth filters have a high load capacity, retaining particles both on the surface and within teh matrix. In adddition, they have relatively high flow rates, are inert to most solvents, and are inexpensive. Howeverm they have several disadvantages, including
1) ill-defined and variable pore sizes due to a random matrix,
2) exxtensive absorption and loss of liquid filtrate, and
3) loss of filter fragments that contaminate the filtrate.

Many of these disadvantages are overcome by screen filters, which have uniform pore size. The screen filters function by retaining particles on their surfaces rather than within the matrix. The most widely used screen-type filters are composed of polycarbonate and cellulose esters (cellulose nitrate adn acetate). Membrane filters of these materials can be manufactured with a predetermined and accurately controlled pore size. Filters are available with a mean pore size ranging from 0.025 to 15 um. These filters clog more readily than do depth filters and require suction, pressure, or centrifugal force for liquid flow. A typical flow rate for the commonly used 0.45 -um membrane is 57 mL min-1 cm-2 at 10 psi. Clogging can be reduced by combining depth and screen filters. The depth filter serves as a "prefilter" to remove particles that would rapidly clog the screen filter.

Ultrafiltration devices are available for macroseparations (up to 50 L) or for microseparations (milli- to microliters). For solutions larger than a few milliliters, gas-pressurized cells or suction-filter devices are used. For concentration and purification of samples in the milli- to microliter range, disposable filters are available. These devices, often called microconcentrators, offer the user simplicity, time saving, and high recovery. The sample is placed in a reservoir above the membrane and centrifuged in a fixed-angle rotator. The time and centrifugal force required depend on the membrane, with spin times varying from 30 minutes to 2 hours and forces from 1000 x g to 7500 x g. These are available from a variety of sources, including Schleicher and Schuell, Bio-Rad, Pierce, Amicon, and Millipore.
The principles behind ultrafiltration are sometimes misunderstood. The nomenclature implies that separation are the result of physical trapping of the particles and molecules by teh filter. With polycarbonate and fiberglass filters, separations are made primarily on teh basis of physical size. Other filters (cellulose nitrate, polyvinylidene fluoride, and to a lesser extent cellulose acetate) trap particles that cannot pass through the pores, but also retain macromolecules by adsorption. In particular, these materials have protein and nucleic acid bingin properties. Each type of membrance displays a different affinity for various molecules. For protein, the relative binding affinity is polyvinylidene fluoride > cellulose  nitrate > cellulose acetate. We can expect to see many applications of the "affinity membranes" in the future as the various membrane surface chemistries of macromolecules and quantitative binding assays.