Semipermeable membrane devices (SPMDs) are passive monitors that are being increasingly used by monitoring agencies and wastewater dischargers to measure the contents of lipophilic organic chemicals that may adversely affect water quality. Passive sampling devices can monitor most 75% of the organic pollutants included in the EU Water Framework Directive (WFD) priority pollutant list as well as many other compounds. Furthermore, applications and the theory underlying the use of SPMDs have been described in over 200 peer-reviewed scientific publications during the last two decades, making them the most comprehensively studied type of passive sampler for semivolatile organic pollutants in water.

The most frequently asked questions regarding the use of SPMDs for water monitoring are the following. What compartments of the environment do the SPMD extracts represent? Have lowest environmental concentrations of concern (Cc) been established for the compounds of interest? How many SPMDs are needed to detect these concentrations? What are the ranges of, and the most suitable, SPMD exposure times? What additional information should be collected about the site to enhance the interpretation of SPMD results? What quality control (QC) measures are needed for SPMD sampling? Are SPMD calibration data available for the compounds of interest? What constitutes good SPMD practice in terms of storage, transportation, deployment, retrieval and analytical procedures?

This chapter addresses these and other questions related to the field application of SPMDs (many of which are also relevant to other types of

Comprehensive Analytical Chemistry 48

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

Volume 48 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)48014-4

© 2007 Elsevier B.V. All rights reserved. 311

passive water samplers). In so doing, we aim to provide a sound understanding of the applicability and limitations of SPMDs for obtaining reliable monitoring data.

14.1.1 SPMD rationale and applicability

Standard, commercially available SPMDs consist of a layflat 92 x 2.5 cm virgin, low-density, 75-90 mm thick, low-density polyethylene (LDPE) tube containing 1 mL of pure (>95%, > 99% or > 99% and cleaned) pure triolein (Fig. 14.1). The last of these three types is mainly used for toxicity testing. The surface area-to-volume ratio (A/V) is about 90 cm2 mL-1 of SPMD (membrane plus triolein) or about 460 cm2 mL-1 of triolein. A device with these dimensions weighs approximately 4.50 g, of which triolein accounts for about 20%. However, an SPMD of any length with an A/V ratio of about 460 cm2 mL-1 of triolein, a lipid-to-membrane mass ratio of approximately 0.25, and a 70-95 mm wall thickness is considered to be a standard SPMD. In order for an SPMD to function correctly, nearly 100% of the targeted compounds taken up should partition into the device and no additives with adsorptive properties should be present in it.

Standard, commercially available SPMDs are modeled on an original United States Geological Service (U.S.G.S.) design (Fig. 14.1) [1]. Use of SPMDs with a standard design and quality ensures that published SPMD sampling rate calibration data are applicable for estimating ambient water concentrations of analytes. Use of standard SPMDs also allows data obtained in different studies to be validly compared, since such SPMDs are used globally in most applications [2]. Recently, alternative device called the polar organic chemical integrative sampler (POCIS) has been developed for sampling polar compounds. In the USA, these devices are called ''Aquasence-P'' and in Europe ''ExposMeter Hydrophilic'' samplers.

Standard SPMDs are designed to sequester and concentrate bio-available dissolved aqueous-phase hydrophobic organic contaminants (HOCs) with 3<log Kow<8 and molecular weights of approximately <600 Da such as polyaromatic hydrocarbons (PAHs), non-polar pesticides, polychlorinated biphenyls (PCBs), polychlorinated naphthalenes, polychlorinated dibenzofurans, polychlorinated dibenzodioxins, poly-brominated diphenyl ethers, polychlorinated benzenes and alkyl phenols (nonyl phenols).

HOCs in the water are carried to the SPMD by convection or eddy diffusion. Molecular diffusion is the dominant transport process in the

Fig. 14.1. A standard SPMD and commercially available stainless steel deployment spider.

layer extending a few millimeters from the surface of the membrane. HOC molecules of appropriate sizes move through the transient membrane pores and accumulate in the triolein and partly in the polymer itself.

SPMDs are not suitable for sampling ionic species such as metal ions, ionized forms of organic acids or polar organic chemicals. The suitability of SPMDs for monitoring chlorinated phenols is an issue that is frequently raised by wastewater dischargers. The environmental pH determines the ratio of ionized to neutral species for such compounds as chlorinated phenols and, thus the capacity of SPMDs to sample them since only the neutral species are available for accumulation. Table 14.1 shows the applicability of SPMDs to target compounds included in the EU WFD priority pollutant list [3].

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