polyvinyl chloride (PVC), and polystyrene readily retain high charge levels; others, such as Tygon®, retain relatively little charge [62]. The PVC/polystyrene copolymer used in the 37 mm closed-face cassette is an excellent electrical insulator and can retain high charge levels on its surface. These charges can be incorporated in the bulk plastic during manufacture or accumulated on the surface by handling or contact with other objects; the charge levels and polarity are highly localized and variable. Such samplers can exhibit particle losses to the internal walls of the cassette and negative sampling biases [63]. Non-conductive plastic asbestos samplers were shown to produce large negative biases and variable results [61, 63, 64]. Conductive samplers have demonstrably lower losses when sampling charged particles. Metal samplers obviously have high conductivity. Samples collected using nylon cyclones were shown to exhibit higher variability [65, 66] and negative biases [66] when sampling charged dusts. However, the degree of conductivity required is not high; as long as charges can move over the sampler surface and reach equilibrium in seconds, the effect of charges transferred to the sampler is likely to be minimized. Materials with this low level conductivity (surface resistivity < 108 ohms/square) are often termed “static-dissipative.” Graphite-loaded plastics were developed that have adequate conductivity to distribute charges over the surface of the cassette (e.g., the 25-mm asbestos sampler). A simple test to ensure adequate conductivity of these samplers can be performed by attaching a good quality multimeter at any two points on the sampler surface. Resistance readings in the range of tens of megohms or less indicate sufficient conductivity for sampling purposes. Some metals are coated with a thin, non-conductive layer, e.g., anodized aluminum. These coatings may retain a surface charge, but this charge will induce an opposite charge in the conductive layer beneath the surface, effectively canceling out the field produced by the surface charge. Recent measurements at NIOSH using a non-contacting electrostatic voltmeter (Model 300, Trek Inc. Medina NY) indicated that no significant external field (< 50 volts) could be produced near an anodized surface by rubbing the surface with various plastics or other materials. Plastic or cellulose-based materials rubbed in a similar manner produced electrostatic potentials measured in the hundreds to thousands of volts. Thus, metals with a thin, insulating surface layer are not likely to produce significant external fields that would affect aerosol sampling. A further electrostatic problem not specifically associated with the cassette is the use of filters made of highly nonconductive materials, such as PVC, polytetrafluoroethylene, or polycarbonate. In addition to having desirable chemical properties, these filters have the advantage of not absorbing water from atmosphere, leading to improved weight stability [6769]. However, these filters can retain a high electrostatic charge level, resulting in non-uniform particle deposition and even repulsion of particles from the filter surface. Such filters are also more difficult to handle during weighing because of charge effects. Even filters that are normally more conductive, such as cellulose-based filters, can become non-conductive and exhibit non-uniform particle deposition and particle losses at very low humidity levels (< 10% RH) [70]. A treatment was developed to make filters more conductive without significantly affecting weighing accuracy or moisture absorption [71]. In one study, it was found that applying this treatment to the filter decreased particle losses from 14% to 2% [72]. Anti-static sprays are available that leave a temporary static-dissipative coating on surfaces.
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