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Passive samplers for gases use the motion of the gas molecules themselves to furnish the energy for the transfer of gases from the ambient air to a suitable collecting medium. There is no need for pumps, flow regulators, batteries, or other power supplies. Thus the passive sampler can be made much smaller, lighter, less expensive, and much simpler to use and maintain than active samplers. However, passive samplers cannot be used to sample particulates, which, at the particle sizes encountered in industrial or community air, have coefficients of diffusion that are many orders of magnitude lower than those for gas molecules.
All passive samplers depend on Fick's first law of diffusion, which, paraphrased, states that the sampling rate is directly proportional to the coefficients of diffusion of the gas being sampled, the cross-sectional area of the diffusion path, and the concentration of the gas; it is inversely proportional to the length of the diffusion path.
There are two major types of diffusion samplers currently described in the literature. One type uses an air column as the diffusion barrier. Since the coefficient of diffusion for a gas in air is a constant that can be calculated independently, the area and length of the diffusion path, that is, the sampler geometry, can be chosen to give a suitable sampling rate. The other type uses a thin membrane as the diffusion barrier. At the present state of the art, permeation constants for gases through specific membrane materials cannot be calculated from the data available in the literature. Therefore, the permeation devices currently in use depend on empirical calibration of a specific membrane and a specific absorbent for each gas.
A wide variety of sampling rates can be obtained by using passive samplers, but each sampler operates at a fixed rate. The question of which sampling rates or ranges are the most desirable deserves serious attention by industrial hygienists.
gas sampling, organics, toxic organics, passive samplers, diffusion samplers, permeation samplers, personal gas dosimeters, sampling rate determinants, atmosphere
Professor of environmental medicine, New York University Medical Center, Institute of Environmental Medicine, New York, N.Y.