Introduction

Contaminant uptake by passive sampling devices (PSDs) can be seen as a multi-stage transport process. To illustrate the basic steps involved, we will first discuss contaminant uptake by a PSD that consists of a central sorption phase, surrounded by a membrane. For this exercise, we assume that the sampler is biofouled, and is contained within a protective cage (Fig. 7.1). Coming from the surrounding waters, analytes first have to enter the protective cage, where the motion of water may be reduced relative to the water outside the cage. Close to the biofouling layer, convective transport of analyte molecules is reduced more and more, until all transport takes place by molecular diffusion within the water boundary layer (WBL). When ventilating organisms are present, diffusion may be amended with convective currents that are set up by the organisms. After diffusion through the membrane, analytes are finally sorbed by the central sorption phase. This general picture may differ from case to case. For example, protective cages and biofouling layers may be absent, the membrane may act as the final sorption phase (e.g. various types of solid-phase microextraction devices (SPMEs), and low-density polyethylene (LDPE) and polydimethylsiloxane (PDMS) strip samplers), or the sampler may be equipped with additional phases between the membrane and the central phase (e.g. membrane-enclosed sorptive coating (MESCO) and Chemcatcher samplers).

A variety of models has been used over the past 15 years to better understand the kinetics of contaminant transfer to passive samplers. These models are essential for understanding how the amounts of absorbed contaminants relate to ambient concentrations, as well as for

Comprehensive Analytical Chemistry 48

R. Greenwood, G. Mills and B. Vrana (Editors)

Volume 48 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)48007-7

© 2007 Elsevier B.V. All rights reserved. 141

Fig. 7.1. Schematic representation of concentration profiles in a dual-phase PSD with exterior biofilm (i.e. the right half of a symmetrical sampler, or the whole cross section of a sampler with an impermeable boundary located to the left of the central phase). Dashed lines indicate how the effective thickness of the respective phases may be estimated (see Sections 7.5 and 7.6).

Fig. 7.1. Schematic representation of concentration profiles in a dual-phase PSD with exterior biofilm (i.e. the right half of a symmetrical sampler, or the whole cross section of a sampler with an impermeable boundary located to the left of the central phase). Dashed lines indicate how the effective thickness of the respective phases may be estimated (see Sections 7.5 and 7.6).

the design and evaluation of calibration experiments. Models differ in the number of phases and simplifying assumptions that are taken into consideration, such as the existence of (pseudo-) steady-state conditions, the presence or absence of linear concentration gradients within the membrane phase, the way in which transport within the WBL is modelled and whether or not the aqueous concentration is constant during the sampler exposure.

In the next sections, we will introduce the basic concepts and models used in the literature on passive samplers for the special case of trio-lein-containing semipermeable membrane devices (SPMDs). These can easily be extended to samplers with more or with less sorption phases. Then we will discuss the transport of chemicals through the various phases constituting PSDs. Finally, we will discuss the implications of these models for designing and evaluating calibration studies.

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