Whenever a strong temperature gradient exists between two adjacent surfaces, particles tend to be deposited on the colder of these surfaces. Collecting aerosols by this means is termed thermal precipitation, and several commercial devices are available.
Because thermal forces are so weak, a large temperature difference must be maintained in a small area, and the air flow rate between the two surfaces must be low in order not to destroy the temperature gradient and to permit particles to be deposited before moving out of the collection area. As a result of these requirements, most devices use a heated wire as the source of the temperature differential and deposit a narrow ribbon of particles on the cold surface. Airflows are small, on the order of 10 to 25 ml per minute. At such low rates, the amount of material collected is normally insufficient for chemical analysis or weight determinations but is ample for examination by optical or electron microscopy.
Collection for particle size studies is the principal use for thermal precipitation units, and they are well suited to collecting samples for such investigations. Because the collecting forces are gentle, the particles are deposited unchanged. The microscopic examination gives information that can be translated into data concerning the number of particles and their morphological characteristics. The use of a small grid suitable for insertion into an electron microscope is also convenient as the collecting surface; this use eliminates additional manipulation of the sample prior to examination by electron microscopy.
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