filter/cartridge are analyzed. The air stream entering the cassette is surrounded by the cartridge and any deposition on the walls of the cartridge is retained for analysis. The IOM sampler for inhalable dust uses this approach by having the cartridge form the inlet of the sampler (Figure 2b). This approach also has been used in the in-line cassette of the coal mine dust personal sampling unit (MSA, Pittsburgh PA) where an aluminum foil cover is crimped onto the filter. A similar cartridge was designed for the 37 mm closed-face cassette in measurements of pharmaceutical dust [80]. Another cartridge made of “static dissipative” plastic is commercially available (Accu-Cap, Omega Specialty Instruments, Chelmsford MA). However, it is important that cartridge material be compatible with the analytical method. For instance, the plastic material used in the first version of the IOM sampler cartridge (Figure 2b, SKC, Eighty Four PA) was found to absorb milligrams of water over periods of days, making the accuracy of gravimetric measurements problematic [81-83]. Demange et al. more recently demonstrated significantly improved agreement between inhalable sampling using the IOM sampler and the 37-mm cassette by including all deposits inside the cassette [23]. This suggests that the accuracy and precision of the 37-mm cassette can be improved by including internal sampler deposits by wiping or washing, or by using an internal cartridge such as the Accu-Cap. 8. COLLECTION MEDIA AND ANALYTICAL ISSUES The sampling medium should be compatible with the analytical method. Some analytical methods require specific filter properties. For instance, atomic absorption and inductively coupled plasma analyses typically require complete ashing of the filter material; organic compound analyses require that no reaction or adsorption of the compounds occur at the filter surface. Several studies have dealt with gravimetric stability of filters and recommended specific procedures [67-69, 84, 85]. Generally, plastic materials that do not absorb water (polycarbonate, polyvinyl chloride, polytetrafluoroethylene) are more weight stable than natural cellulose-based materials; uncoated glass fiber filters also may absorb water [67-69, 84, 85]. Note, however, that the more weight stable materials also tend to be more highly charged, resulting in more charged particle repulsion and deposit non-uniformity. When a plastic (Tyvek®) backup pad is crimped into a cartridge together with a filter, the weight stability of the cartridge may suffer [86]. To improve the weight stability of coal mine dust sampler cartridges, stainless steel backup pads have been used by MSHA. The IOM sampler can be purchased with either a plastic or a stainless steel cartridge. The plastic cartridge has been shown to exhibit poor weight stability and should not be used for gravimetric analysis [81, 83]. Lawless and Rodes investigated the use of modern electronic balances to determine factors affecting the accuracy of gravimetric measurements and found that balance stability, balance leveling, vibration and thermal drafts, electrostatic charge reduction, positioning of the filter in the balance so that the filter did not hang over the edge of the pan, and temperature and humidity control were all important in achieving accurate results [87]. Although not strictly a problem with the collection medium, the sampler construction material should not outgas vapors that can condense on the collection medium and affect the analysis. Early (circa 1970) versions of the closed-face cassette were made of a plastic called “tenite,” which resulted in weight gain of the filter over time. This currently does not appear to be a problem.
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