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Impactors. (Tools for Environmental Health).

Inertial forces are responsible for impaction action. The impactor is basically a jet, under which there is an agar impaction surface. Particle-laden air sucked through the jet is directed at the collection agar surface so that particles impact onto it. In a cascade impactor, particles are captured by inertial force. Particles impact in cascading order: Larger particles are captured first, while those that are smaller remain airborne and follow the streamlines, each successive stage having increased air velocity through the corresponding jet. This type of sampler is the one most commonly used in clean-room air sampling. Several types are designed to serve various applications.

Stacked Sieve Samplers

The stacked-sieve impactor sampler consists of a series of six stages, each composed of a plate with 200 holes. Each plate is held above a petri dish containing nutrient agar, with successive plates having smaller holes. At constant flow (28.3 L/min), the largest particles impact on the first impaction stage, whereas the smallest ones impact on the last stage. Unless sampling times are short or the relative humidity high, the areas of nutrient agar directly under each hole of a stage can rapidly dry. Fastidious microorganisms consequently may fail to grow on the upper stages where there is desiccation of the agar, but may grow on the lower stages because the relative humidity of air increases as it passes through the sampler. Covering the agar with a water-evaporation retardant or increasing the water content of the agar can reduce this problem. The precision with which airborne microbes are captured depends on the collection surface. The agar-containing petri dishes need to be level and filled precisely with the recommended amount of agar to give the correct plate-to-agar-surface distance. Using plastic petri dishes may give rise to an electrostatic effect, but it can be argued that the small problems of wall losses can be offset by the practicalities of use. The major advantage of a multiple-stage cascade impactor is that it can provide data on particle size.

There is a two-stage version of the impact sampler. It separates viable particles into "respirable" and "nonrespirable" fractions. The upper stage corresponds roughly to deposition on stages 1 and 2 of the six-stage version, and the lower stage to deposition on stages 3 through 6. Side-by-side comparisons with the six-stage version, however, revealed an approximately 40 percent lower recovery rate of bacteria with the two-stage sampling device, except when the device was sampling dilute concentrations of particles larger than 1 [micro]m. Using the correction factors provided by the manufacturer when completing the count can minimize this difference in efficiency.

There are several variants of the sieve sampler, including one-stage devices. While these units will not provide particle size distribution, they are efficient collectors across the entire size spectrum. They have the advantage of being rapid and consistent but should be used only as a validation tool, once the scope of the bioburden is known.

A particular exception is a unique single-stage, 219- or 487-hole microbiological impactor air-sampling device: the Surface-Air System (SAS) sampler. It is a self-contained, programmable, battery-operated portable unit that has seen many enhancements over the past several years. Numerous industries have adopted it as a standard, and the SAS is the only sampler mentioned by name in Microbiological Evaluation of Clean Rooms and Other Controlled Environments, the 1998 USP 23-NF 24 Eighth Supplement, Section 1116. The basic SAS sampler uses inexpensive contact plates (RODAC) and is traceable to the National Institute of Standards and Technology The enhancements and accessories made for the basic model allow for sampling in many different environments--and under many different environmental conditions. This flexibility makes the SAS sampler a versatile and excellent screening tool for airborne microbes. For instance, the latest unit, the high-volume SAS Super 180 BW and its accessories, is specifically designed fo r the detection of biological-warfare agents. We will be presenting a thorough review of this device in a future column.

Slit-to-Agar Samplers

In slit-to-agar samplers, air is drawn through a narrow slit, accelerated, and directed toward the surface of a petri dish containing agar media. The collection surface is often placed on a rotating turntable. Airborne microorganisms impact onto the agar surface and are separated spatially by the plate's rotation, thereby providing an analysis based on time. This process allows for the effect of mechanical, human, or animal movement on the bioburden of air. Slit-to-agar samplers yield sampling plates that are easy to interpret because the colonies on the collection plates can be read very much like the divisions on the face of a clock. There is a fixed sampling rate of 50 L/min; the rotational speeds can be varied. Slit-to-agar samplers are efficient collectors of particles larger than 0.5 [mu]m.
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Article Details
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Author:Balsamo, James J., Jr.
Publication:Journal of Environmental Health
Article Type:Brief Article
Date:Jun 1, 2002
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