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.
Cite this version of the work
Faten Salim
(2019).
Modelling Permeation Passive Sampling. UWSpace.
http://hdl.handle.net/10012/14645
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