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Rs/Ksw V s

Figure 8.3 illustrates most of the barriers to chemical uptake by the POCIS sorbent. In this example, we assume WBL control of uptake rates. Mass transfer of analytes into POCIS sorbents include the following steps: movement of HpOC solutes from the bulk water and diffusion through the WBL (note: the biofilm is not considered in this narrative), diffusion through the water-filled membrane pores and through the membrane matrix, and finally, diffusion through any WBLs associated with the inside surface of the membrane and sorbent

Bulk

Sorbent

Water Aqueous Boundary Layer Membrane ^

Bulk

Sorbent

Water Aqueous Boundary Layer Membrane ^

Fig. 8.3. Representation of the barriers to chemical uptake into the POCIS. The deltas (d) represent the effective thickness of each region and the associated subscripts denote the region. The line shows the chemical concentration gradient in the water through each barrier.

Fig. 8.3. Representation of the barriers to chemical uptake into the POCIS. The deltas (d) represent the effective thickness of each region and the associated subscripts denote the region. The line shows the chemical concentration gradient in the water through each barrier.

particles. The resistance to mass transfer of any biofilm and the WBLs associated with the inside of the membrane and sorbent particles are not considered in Fig. 8.3. The resistance to mass transfer for all steps or barriers are assumed to be additive. Therefore, a fractional reduction in the resistance of any barrier will result in some increase in chemical uptake rate. However, for the same percentage reduction in individual barrier resistances, the greatest increase in sampling rate is achieved when the change occurs in the rate-limiting barrier (e.g., turbulent thinning of the WBL).

Mass transfer through the microporous PES membrane used in the POCIS may follow a biphasic pathway with solute transport through both the water-filled pores and the polymer matrix. Assuming steady state and no biofouling, the flux, or mass per unit time, of solutes through the membrane can be described as a weighted average of both water-filled pores and the polymer matrix pathways.

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