Theory

The passive sampling devices described here consist of a hydrophobic solid receiving medium enclosed in an air-filled semi-permeable polyethylene membrane (Figs. 5.1 and 5.2).

stir bar PDMS coating stir bar PDMS coating

silicone tubing

Fig. 5.1. Diagram of the MESCOs type A1 (stir bar) and A2 (silicone tubing).

silicone tubing

Fig. 5.1. Diagram of the MESCOs type A1 (stir bar) and A2 (silicone tubing).

Type B1 Type B2

Fig. 5.2. Diagram of the MESCOs type B1 (stir bar) and B2 (silicone rod).

Type B1 Type B2

Fig. 5.2. Diagram of the MESCOs type B1 (stir bar) and B2 (silicone rod).

Huckins et al. [6] developed a theoretical model describing the uptake kinetics of organic compounds in water by passive samplers consisting of a triolein-filled polyethylene membrane. In a laboratory study investigating polychlorinated biphenyls (PCBs) in air, Petty et al. [7] demonstrated that this model was also applicable to air sampling. Vrana et al. [1] gave an overview of the theoretical aspects concerning the uptake of organic compounds in water by passive samplers consisting of a hydrophobic phase enclosed in a semi-permeable membrane based on the model mentioned above. This model is also applicable for the MESCOs of the type A and B. In the initial uptake phase (when the uptake is linear or integrative), the correlation between the concentration of the analyte in the receiving organic phase CS and the concentration of the analyte in the gaseous phase CAir is described by the following equation:

where CS(0) is the concentration of analyte in the receiving organic phase at time t — 0, Vs is the volume of the receiving organic phase, kO is the mass transfer coefficient, AS is the membrane surface area, a is the pore area of the membrane as a fraction of the total membrane area and t is the time.

Equation (5.1) can be rewritten as ms — ms(0) + CAirRst (5.2)

where ms is the amount of analyte in the receiving organic phase, mS(0) is the amount of analyte in the receiving organic phase at t — 0 and Rs is the sampling rate of the passive sampler:

According to Eq. (5.2) the sampling rate RS can be determined in laboratory experiments at constant analyte concentrations CAir. With regard to environmental air monitoring, the term CAir represents the TWA concentration over the exposure time period. When the calibration parameter RS is known, CAir can be estimated from mS, the amount of analyte received by the sampler:

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