Filter Media

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities

Filter Media

Filters should have high collection efficiencies (i.e., >99%) for particles over a wide range of sizes.

Many cellulose ester (acetate, nitrate, or mixed ester) or glass-fiber filters meet these requirements and are

commonly available. Other filters with reasonably high collection efficiency may be used if required for

special applications or assay methods. Selection of a filter type generally involves compromises between

filter efficiency, flow resistance, and requirements imposed by the desired assay method.

The specific ations of a filter medium often include pore size and filter efficiency. Pore size is

determined by filtration of a liquid; the particle size at which the collection efficiency is 95% in water is

given as the effective pore size. Filtration efficiency for particles in air, however, is dramatically different.

Aerodynamic effects make the collection efficiency dependent on the face velocity through the filter.

Airborne particles of aerodynamic size equal to the pore-size rating of a filter are usually collected with

high efficiency (>99%). Smaller particles may also be collected efficiently; however, some sizes may

substantially penetrate the filter. Particles in the range 0.1- to 1.0 micron diameter are most likely to

penetrate a filter. Many manufacturers use dioctylphthalate (DOP) to produce an aerosol of particles 0.3

micron in diameter for testing filter efficiency, following a procedure such as ASTM D 2986-71. Thus, if a

filter is rated for efficiency by DOP retention, collection of other particle sizes will be more efficient.

Collection efficiency is also increased by higher flow rate for particles >0.1 micron.

Cellulose ester membrane filters have interconnecting pores of uniform size. They typically produce

a higher resistance to flow than glass-fiber filters and collect most particles near the surface of the filter.

Glass-fiber filters are made of a mat of randomly oriented glass fibers. They have lower flow

resistance than most membrane filters, but trap an appreciable fraction of the particles within the filter

mat. This interferes with detection of alpha radiation from the filter.

Cellulose filters are often used for air sampling. They have moderate flow resistance, but

relatively poor collection efficiency. Their use may be justified in some situations, but only with the

recognition that efficiency for certain particle sizes may be low. Generally, if analytical and sample -

handling requirements allow, glass-fiber or cellulose-ester membrane filters are a better choice than

cellulose filters.

Each type of filter has inherent advantages and disadvantages. The higher flow resistance of membrane

filters may overtax the capabilities of older models of some PAS pumps although membrane filters can be

used successfully with many of the new models of pumps. Glass-fiber filters should be substituted if a

significant pressure drop occurs with the sampler being utilized.

The surface-collection properties of membrane filters can be an advantage when sampling for alpha

and weak beta-emitting materials. Deposition of particles on the surface minimizes energy absorption by

the filter medium. This is especially important for alpha spectrometry, where the energy spectrum is

substantially degraded. Membrane filters are also advantageous if the assay procedure involves ashing or

dissolution of the filters, but they are relatively fragile.

Filter Holders

Criteria for filter holders are simple, but critical. For the collection of large-volume air samples,

filter holders should be open-face such that sample air is drawn directly onto the filter surface from the

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