Centrifugal samplers: the Reuter centrifugal sampler. (Tools for Environmental Health).

With centrifugal samplers, the principle of collection is centrifugation. It involves the creation of a vortex in which particles with sufficient inertia leave the airstreams to impact upon a collection surface such as a semisolid medium. The most frequent example of such a device is the Reuter centrifugal air sampler. Air is drawn into the sampler by an impeller housed inside an open, shallow drum. The air is then accelerated by centrifugal force toward the inner wall of the drum. Lining the inner wall is a plastic strip supporting a thin layer or agar medium onto which airborne particles are impacted. These strips are subsequently removed and incubated. The motor for the impeller is battery operated, and the whole sampler unit is small enough to be hand-held.

Some concern has been expressed about effective flow rate and collection efficiency Air enters and leaves the sampler through a single opening; therefore, flow rate quantification is theoretical. The sampling rate is calculated from the impeller revolution value of 4,092 rpm, yielding an effective sampling rate of 40 L/min. Studies have shown, however, that the effective sampling rate may be nearer 100 L/min under certain conditions. While these centrifugal samplers collect larger particles (15 [mu]m) quite efficiently less than 10 percent of particles under 2 [mu]m in diameter are deposited. Therefore, comparisons to the stacked-sieve, slit-to-agar, and liquid impinger samplers cannot be made. A newly redesigned Reuter sampler allows separate entry and exit airflow; which may improve performance hut also provides an actual flow rate and can more closely approximate isokinetic conditions.

If different groups of organisms are to be studied, which necessitates the inoculation of different media, repeat samples must be taken at each sample point. This process increases the sampling time and may result in errors if rapid and large variations occur in the air. The distinct advantage, however, is of ease of use. Also, if the sampling protocol is properly defined, it can be used as its own control, and the need for volumetric sampling becomes moot. Rapid comparisons of qualitative microorganism profiles in the intra- and extramural air can be made in order to determine the proper sampling strategy with quantitative instrumentation. This makes the Reuter centrifugal sampler an ideal, rapid, and versatile prescreening tool. We reported favorably on the Biotest centrifugal sampler in this column back in the October 1997 issue of the Journal and plan an update in 2003.