Modelling Permeation Passive Sampling

Abstract

Understanding the theory behind passive sampling, as well as factors influencing its accuracy, is crucial for proper planning and application of passive sampling methods. The Waterloo Membrane Sampler (WMS) is a permeation air passive sampler that is used for determining the Time Weighted Average (TWA) concentrations of Volatile Organic Compounds (VOCs) in air and soil gas. Determination of the TWA concentrations has been based on the zero-sink assumption, according to which the adsorbent of the sampler efficiently removes analytes permeating through the membrane leaving negligible concentrations at the barrier-sorbent interface. In this thesis, a dynamic model is presented to simulate the sampling process in the WMS. The model equations were solved numerically using MATLAB. The calculated uptake rates were successfully compared to the experimental data. The model predicted that resistance to mass transfer within the sorbent bed may develop during sampling. This resistance needs to be taken into consideration when significant. Therefore, the applicability of the zero-sink assumption depends on the significance of this resistance and, hence, on the properties of the analyte-adsorbent pair, as well as the concentration level and the sampling time. The model presented in this thesis provides the tool to evaluate this effect in a given sampling scenario, allowing optimization of the sampling method. Alternatively, the TWA concentration of the sampled analyte can be calculated using a method that accounts for this effect, as demonstrated in the thesis. An extension of the model that evaluates the post-sampling/storage period of analytes in the WMS is also presented. It was proven both theoretically and experimentally that the amounts of analytes retained in the PDMS membrane are negligible after sampling; therefore, analyzing the sorbent is sufficient to quantitatively determine the sampled amounts. The experimental evaluation also showed that the amounts of analytes found in the sorbent were stable over up to three-weeks of storage at room temperature. Additionally, the effect of intraparticle resistance to mass transfer within the sorbent bed was evaluated. The aim of this evaluation was to extend the applicability of the model to include the case of adsorbents with porous particles. This evaluation is followed by comprehensive sensitivity analysis using two types of adsorbents with different properties and adsorption strengths. The purpose of this analysis was to detect the influential parameters that have major control over the model output, the uptake rate, and to optimize the model parameters. Finally, the effect of air face velocity on the uptake rate of the WMS was added to the model, so that the resistance to mass transfer in the air boundary layer is taken into consideration. The work presented in this thesis provides better understanding of the sampling process in permeation passive samplers similar to the WMS. This understanding permits correct application of the sampler in environmental analysis.