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Product Focus: Filters

Filtration is the mechanical separation, based on particle size, of materials from fluids by using a porous membrane.

Filtration is the mechanical separation, based on particle size, of materials from fluids by using a porous membrane. Most laboratory filtrations involve the removal of solids from liquids; gas filtration based on molecular weight is common in industrial processes but less so in the lab. Although filtration appears simple conceptually, the physics and mathematics underlying it are complex and still yield interesting insights after hundreds of years of study.

Polymeric membranes are by far the most common in analytical lab settings for water-based filtration. Plastics, including Teflon, nylon, polyethersulfone, and polyvinylidene fluoride, provide almost limitless opportunities for pore size and surface chemistry and for designing pore channels for specific separations.

Important considerations for lab filtration include the following:

  • Throughput – how much fluid can pass through individual filters before they clog, or how many samples can be run per unit of time
  • Chemical compatibility with the process fluid
  • Time/cost of filter or sample preparation
  • Ability to validate filtration operations, particularly for regulated industries

Filtration most commonly involves the size-based isolation of insoluble particles from a liquid medium that may be aqueous or organic. Many lab filtrations, particularly those involving nonviscous organic solvents, occur spontaneously under the gravitational pressure of the fluid against the filter medium. An example is the removal of drying agents from hexane. But for many aqueous filtrations, operators apply pressure above the filter medium or create a vacuum below it to force fluid through.

Simplistically, filtration works because particles larger than the filter medium’s nominal pore size are retained while smaller ones pass through. In practice the situation is more complex. The filter medium—e.g., paper, polymeric membrane, or glass frit—may appear flat but is actually three-dimensional, with pores resembling complex channels more than smooth chutes. As filtration proceeds, particles accumulate at the pore surface and within the filtration medium, blocking access to the pores. In preparative filtrations, the filtrate—the solids removed from the suspension—form a cake atop the filter medium. As a result, greater pressure must be applied to push fluid through. Eventually the filter may clog completely. This phenomenon, quite common in industrial filtration, also arises in the laboratory.

Filtration is distinct from sieving, which involves the straightforward separation of solids from other solids on the basis of physical dimensions. Channel effects play no role in sieving.

Analytical filtration

Although filtration is most often thought of as a preparative operation, analytical filtration of suspended solids in water is a major application in environmental laboratories. Manufacturers that produce wastewater perform this test routinely to assure regulators that their effluent meets safety specifications.

The technique involves passing a standard volume of liquid through a preweighed filter, drying the filter, and reweighing. The difference in mass corresponds to the concentration of undissolved particles per unit volume of wastewater.

EPA methods are quite specific as to how the filters and samples are prepared and handled before the analysis. A typical preparation involves rinsing the filter with a specified volume of deionized water, drying at 105ºC, cooling, and weighing on a balance to within 0.1 mg. Handling before the weighing step is specified as well.

Another interesting analytical application of filtration is metal digestion, a process for quantifying metals that uses acid and heating to convert soluble metals into insoluble particles, which may then be isolated on an appropriate filter membrane.

Preparation of analytical filter membranes consumes a significant amount of operator time, says Joe Boyd, a technical specialist at Environmental Express (Mt. Pleasant, SC), which manufactures filters and other equipment for environmental labs. Customers therefore increasingly demand filters that are pre-treated, -weighed, -dried, and ready to use. “All of our weighing and prepping is carried out robotically,” Boyd tells Lab Manager Magazine, and for environmental applications the filters are strictly single-use.

Filtration is used routinely to prepare samples for injection into HPLC and GC, to prevent particles from clogging columns. The most common form of chromatographic sample filtration, according to Larry Scheer, senior product manager at Pall Life Sciences (Ann Arbor, MI), involves syringe filters. These small, disposable membranes attach to the end of a syringe via a Luer lock and are quite popular in regulated industries.

Filtration is also used to prepare HPLC mobile phases, for example, organics such as acetonitrile, aqueous buffers, or mixtures. “Operators are looking to remove non-dissolved particulates to protect the instrument and column and cut down on maintenance,” Scheer says. A 47 mm polymeric disk will typically process up to two liters of mobile phase. Customers interested in this application are most often concerned with chemical compatibility, particularly of leachables and extractables. “You don’t want your filter contributing to the chromatogram,” he notes.

Angelo DePalma holds a Ph.D. in organic chemistry and has worked in the pharmaceutical industry. You can reach him at angelo@adepalma.com.