Slit impactor

The most common collection methods rely on impaction, the same process as described in Section 5.5 for nonbioaerosols. Slit impactors impact particles directly onto a culture medium. For bacteria and fungal spores, the culture medium, called agar, is a semisolid material containing water and nutrients that foster the growth of the viable particles that are collected. For viruses, cell or tissue culture media are used. Typically, the agar fills a 100-mm or 150-mm disposable petri dish, called a culture plate, that is slowly rotated under the slit to provide a history of bioaerosol concentration. Rotating slit impactors have flow rates of 28-50 L/min and cutoff diameters of about 0.5 pm. 

An adjustable-speed, rotating slit impactor has a total deposition area of 50 cm for one full rotation. For how long should a sample be taken to get the desired colony surface density if bioaerosol concentration is 1000 cfii/m Assume the sample flow rate is 28 L/min, and that sampling takes place for one full rotation. 

This dimensionless parameter is used to describe impactor behavior. For impactors with rectangular openings, W is the slit width for circular openings W represents the diameter of the impactor opening. Thus the Stokes number is the ratio of the stop distance to the impactor opening half-width. 

However, more meaningful data can be obtained if force rather than gravity is used to collect airborne particles. A stream of air can be directed onto the surface of a nutrient agar plate (impaction slit sampler) or bubbled through an appropriate buffer or culture medium (liquid impingement). Various commercial impactor samplers are available. Filtration sampling, where the air is passed through a porous membrane, which is then cultured, can also be used. 

To minimize the particle bounce off effect, collection surfaces should also be selected carefully. Common types of impaction surfaces include membrane, fiberglass, silver membrane, Teflon and Nuclepore filter, and brass and stainless steel shim stock. Table 2.2 shows an example of the effect of selection of collection surface on the wall losses (Newton et al., 1990). In Table 2.2, the test aerosols are droplets of 1% CsCl plus 1% uranine. Three types of cascade impactors were used, including Mercer, Sierra Radial Slit Jet (SRSJ), and Lovelace Multi-Jet (LMJ). The occurrence of particle bouncing may be indicated by the presence of excess mass on the back-up filter. 

Rotating Media Slit-to-Agar Impactors Slit-to-Agar Biological Air Sampler ... 

Grinshpun SA, Adhikari A, Cho S-H, Kim K-Y, Lee T, Reponen T (2007) A small change in the design of a slit bioaerosol impactor significantly improves its collection characteristics. J Environ Monit 9 855... 

Bergman W, Lochner JS, Sawyer S, Milanovich F, Mariella R Jr (2005) High air flow, low pressure drop, bio-aerosol collector using a multi-slit virtual impactor. J Aerosol Sci 36 619... 

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