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KANG AND FRANK

bc maintained sterile until sampling, as ihey can contrib-uie to contamination.

Fil (radon methods. Fillers arc włdcly used for aerosol sampling duc to Iow cost and simplicity of opcration. The air filtration apparalus consists of ccllulosc fibcr, sodium alginate. glass fibcr, gelatin membranę filier (GMF, porę sizc 3 pm) or synthetic membranę fillers (porę sizc 0.45 or 0.22 pm) mounted in an appropriate holder and eon-nectcd to a vacuum source through a flow ratę controller (e.g. critical orifice). Aftcr a fiber filier is used. the whole filier or a scciion of it is agilated in a suitable liquid until the particles arc unifomnly dispersed. Aliquots of the suspension arc then assayed by appropriate bacteriological techniqucs. Membranę fillers can either be treated similar to fibcr fillers or dircctly placcd on on agar surface and ineubated.

Gelatin membranę filtration method. The gelatin membranę is water soluble so ihat it can easily be diluted for plating or be sotubilized on top of a nuiricnt medium rcsulting in bacieria colonies that are easily counted. However, this hygroscopic property causes difficulties in sainpling due to swelling of the membranę when the rclative humidity is over 90% (64). The large number of porcs present in these membranes allows a large volume of air to be samplcd during a short limę (2.7 L of air/min/cm^?/500 mm water column).

Limitations. Filtration melhods are good for enumerating mold or bactenal spores. They may not be effective for counting vegetative cells because of the stresś of celi dehydration produced during sampling (26). The shorter sampling times used in gelatin membranę filtration may reduce this stress.

Centrifugal melhods. Centrifugal fotce can bc used to propcl aerosol particles onto a collection surface. When the aerosol is spun in a circular path at high velocity. the suspended particles impact on the collccting surface by a force proportional to the particle's ve!ociiy and mass. Centrifugal samplers do not generale high vclocity jet fiow during sampling, so less stress is imposcd on airbome microbes as comparcd to impingement and impaction melhods. Centrifugal samplen; arc simplc and easy to operate and may be less expcnsivc than impactor types. Generally. centrifugal samplers can rapidly sample a high volume of air rcsulting in morę rcpresentativc sampling.

Limitations. Some deviccs may not generate sufficient centrifugal force to propcl smali pariiclcs onto the collcc-tion surface. The rccovcry efficicncy of these samplers depends on the particie sizc bcing samplcd and the umnuni of centrifugal force generated.

Oiotest Reuter centrifugal air sanipfrr. The Reuter cen* trifugal air samplcr (RCS samplcr, Biotcst Diagnostics Co.) is battery operated, portahle. light in wcight (2.5 Ib.) and

convenient to use (Fig. 3). A plastic strip containing a culture medium lines the impeller drum. Air from a dis-tance of at least 40 cm is sucked into the sampler by mcans of an impeller. Air enters the impeller drum eon-ccntncally from a conkal sampling area. 1( is set in rotation. and the aerosol impacted by centrifugal force onto the agar surface. Air then leavcs the sampling drum in a spiral outside the conc of entering air. After the sample has bcen taken, Ihc agar slrips are ineubated and the colonies counted. The sampler has a self-limer for sampling from 30 s to 8 min. The actual sampling ratę is 280 L/min. Howcvcr, the manufacturer has published an cffcctivc sampling ratę or separation volume of 40 L/min for 4 pm particles, a value derivcd from an attempt to reconcile ihe actual number of viable particles collccted from an air sample with meas-urcments involving airflow dircction, air velocity and available collecting surface area. Clark et al. (J8) indi-cated the cffcctive sampling volume of tlie RCS sampler will vary widely depending on aerosol particie size. Conse-qucntly, the results obtained by using this sampler musi be interpreted with considerable caution. Macher and First (49) measured the collection efficicncy of RCS sampler and found improved efficiency with incrcasing particie size. Particles larger than 15 pm are almost 100% collecied, those in the 4 to 6 pm rangę are collected at 55 - 75% efficiency and particles smallcr than 1 pm pass through the sampler without significant retemion. Ałthough RCS samplcr docs not accurately estimate total viable particie concentration. Placcncia and Oxborrow (58) recommended this sampler for good manufacturing practices investiga-tions. These investigators found that the RCS sampler will collect morę viablc particles than a slii sampler and it could detect the difference in the environmental quality of cach medical device manufacturing facility tested. In addition, the RCS sampler effectively detects variOtis types of microorganisms (58).

Electrostatic precipitotion melhods

Aerosol particles can be ionized and collected on cithcr a positively or negatively chargcd surface. Electrostatic precipilators cmploy various solid collection surfaces such as agar or glass. During ionizalion of the air sample, oxidcs

Figurę 3. The Hiatesl RCS sampler.

JOUHNAt. Oh hOOI) rKOih.CnON. VOL. 52. JUJ.Y 19X9