Prudence dictates that a check of cassette integrity and the seal area on each filter should be made after sampling to ensure that the cassette was properly assembled; otherwise, the sample may underestimate the actual exposure. At least two approaches to eliminating the sealing problem with press-fitted cassettes have been taken. One was to assemble the cassette using a press. Pressing the cassette together by hand often produces misalignment of the cassette parts, resulting in bypass leakage. Frazee and Tironi designed a mechanical press that held the two cassette pieces in proper alignment, while applying just enough pressure to effect a seal, but not so much as to cause cracking of the plastic [56]. This press was designed to allow motion of the cassette pieces to compress to a certain distance. A commercial pneumatic press (Accu-Press, Omega Specialty, Chelmsford MA) used a selected pressure to compress the cassette components. For additional information on bypass leakage and bypass leak test procedures see Chapter N: Aerosol Sampling: Minimizing Particle Loss from Bypass Leakage. The second approach was to redesign the cassette to provide a more positive filter seal [57]. In a well-designed sampler, opening the seal should not cause tearing and loss of the filter or collection medium during removal from the sampler. The 37-mm closed-face cassette is usually sealed with tape or shrink bands around the outside. There is a common misconception that this seal prevents bypass leakage in the cassette. These external seals primarily cover the joint between the cassette components to prevent deposited particles on the external surface of the cassette from contaminating the sample during filter removal. The tape or shrink band also aids in holding the cassette together and preventing external air leakage. However, Puskar, et al. [58] found that even by using this precaution, a significant amount of dust was found downstream of the filter. The authors hypothesized that this dust was deposited during filter removal. Three other commercial samplers, the IOM (figure 2b), the CIS, and the coal mine dust sampler (MSA, Inc, Pittsburgh) use a cartridge to hold the filter. For the first two samplers, an external threaded cover applies pressure to the cartridge to ensure a good seal around the filter. This prevents twisting at the filter surface while creating positive, even contact around the filter edge. 5. ELECTROSTATIC LOSSES Most aerosol particles generated in workplaces have electrostatic charge levels considerably higher than the minimum level [60]. Minimum charge (Bolzmann equilibrium) levels are usually achieved after particles are suspended in the atmosphere for an hour or so because of the presence of naturally occurring ions. When freshly generated particles are sampled in an electric field, as when a sampler is highly charged [61], the particle trajectories may be modified to such an extent that the particles are inefficiently sampled. No electrostatically induced particle motion occurs when either the particle charge or electric field during sampling is zero; when both the sampler and particle are highly charged, particle acceleration is much greater than that caused by gravity, inertia, diffusion or other mechanisms. Samplers can achieve a high charge level when they are electrically insulated from ground and are triboelectrically charged (i.e., by contacting or rubbing against other surfaces); this sampler charging, as well as particle charging, tends to occur more frequently at low (<20% RH) humidity levels. Certain plastic materials, such as polycarbonate, polytetrafluoroethylene,
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