Abstract:
Passive monitors have been popular since they were introduced to the field of industrial hygiene air sampling in the 1970s. The use of passive air monitors has historically included the assumption that the sorbent is 100 percent efficient and that reverse diffusion does not occur. However, several investigators have reported losses attributed to reverse diffusion. In a theoretical analysis, Bartley predicted that these losses would lead to a characteristic distribution of methyl chloroform between sorbent layers of a dual stage passive monitor exposed to a single pulse depending on the timing of the pulse. Bartley derived a relationship based on the Langmuir isotherm intended to allow accurate determination of the ambient concentration regardless of the distribution. However, manufacturers have characterized monitor performance based on geometry instead of performing the isotherm experiments necessary for Bartley's calculation. The purpose of this study was to investigate Bartley's predictions. Dual stage charcoal monitors were exposed to single-pulses of methyl chloroform for two hours within an eight-hour sampling period. The methyl chloroform concentration was held constant in a series of experimental runs where the starting time of the pulse was varied. Additional experimental runs were conducted that allowed comparison of the distribution based on pulse timing at the higher pulse concentration level (p< 0.0001), but not at the lower level (p=0.19). The TWA concentrations calculated with the manufacturer's method did not differ from Bartley's method (p=0.8805). The results indicate that, although the masses collected on the sorbent layers do depend on the timing of an exposure pulse, there was no advantage to the use of Bartley's calculation method compared to the manufacturer's method based simply on the geometry of the monitor under the conditions of this study.