RESPONSE CHARACTERISTICS OF THE DIFFUSIVE SAMPLER AT FLUCTUATING VAPOR CONCENTRATIONS

Abstract

An unsteady state model for calculating the collecting characteristics of organic vapors in the diffusive sampler is proposed using Fick's law and a mass balance equation. The results of a theoretical model suggest that when the vapor concentration in the ambient air rises suddenly from zero to a certain concentration, C0, a larger mass of the vapors should move into the sampler than will move under the steady state conditions because a greater vapor concentration gradient is formed in the sampler. On the other hand, when the vapor concentration changes from C0 to zero, the concentration profile becomes convex and the vapors that exist near the entrance move out of the sampler. When the above two phenomena occur consequently, that is, the vapor concentration changes rectangularly, the collected amount of the vapor becomes equal to that of the steady state condition. In the case of sinusoidal and triangular vapor concentration changes, the collected amount of the vapor also becomes the same as that of the steady state when the time passes 1.4tR (tR: average residence time at the steady state) after the end of the sampling. This theoretical result is supported by an experimental study. There are no significant differences observed between the measured collected amount of vapor at intermittent loading and the calculated one using a conventional steady state model although the collected amount at continuous loading is lower.