often fails to address other factors that are equally—or quite likely more—important in evaluating a passive sampling design. The real value of a field validation is that it allows the developer to experience the process that users will need to go through in deploying the devices in a real-world situation. These include storage, refrigeration and transport issues; deployment and retrieval processes at the site; clean- ing up a sampler after deployment; assessment of degradation of the sampler surface housing; and extraction, clean-up and analysis of a contaminated passive sampler extract. Laboratory studies seldom ad- dress these issues, which may indeed prove to be crucial in adoption of a technology. In situ validation also obviates the requirement for maintaining an exposure concentration in the laboratory. However, here the problem becomes independently validating the exposure concentration, partic- ularly for hydrophobic compounds distributed in both the dissolved and particulate associated phases. The purpose of this section is to review briefly the methods that have been employed in the limited simulta- neous active and passive field deployments to date. It covers both high- volume sampling for hydrophobic organics and high-frequency grab sampling for hydrophilics and inorganics. 15.4.1 High-volume solid-phase extraction Solid-phase extraction (SPE) has become the preferred method for separation of organics from water in recent decades. This is due to the widespread availability of SPE phases with an affinity for a broad spectrum of compounds; lower solvent requirements (compared with liquid–liquid extraction), relatively high recovery rates and ease of use. Although SPE is typically performed under vacuum in the laboratory, a number of mechanical systems have been developed for its application in the field, in particular for extracting hydrophobics from large vol- umes of water. Several devices such as the Infiltrex, and Kiel in situ pump are available commercially, but the basic elements of these sys- tems may be put together by any laboratory. They are hosing, filter(s), sorbent column(s), pump, power supply, flow meter and an optional electronic controller. 15.4.1.1 Pumping systems Typically, water pumps are designed to be placed such that they are ‘‘pushing’’ water through the greatest resistance; however, it is desir- able to prevent contact between the mechanical components of the Techniques for quantitatively evaluating aquatic passive sampling devices 339 TABLE 15.3 High-volume solid-phase extraction configurations Media Adsorbent Filter Flow rate (mL min À1 ) Column Volume extracted (L) Refs. Cut-off (mm) Diameter (cm) Arctic Isolute Env+ GF/F 200 mg 20 [56] High elevation lake water XAD-2 and Speedisks s GF/F 200 50 [38] Mountain lake XAD-2 1.0 14.2 300 75 g 100 [48] Lake Superior XAD-2 GF/F 29.3 o600 75 g [37] Lake Michican XAD-2 0.7 29.3 1000 (filter), 250 (column) 65 [45] Seawater XAD-2 and PUF GF/C, 0.5 14.2 100–1900 PUF: 5cm 30.5 cm; XAD: 2.7 cm  20 cm 42–1000 [35] Ohio River XAD-2 140 14 1600 2  75 g in parallel 1000 [53] B.S. Stephens and J.F. Mu¨ller 342 Milli-Q and seawater XAD-2 and C 18 Empore TM 0.7 GF/F 400 (XAD-2), 50 (C 18 ) 50 g XAD-2, 90- mm diam. disk 50 (XAD), 10 (C 18 ) [49] Ocean outfalls, California XAD-2 Not filtered 50–200 37-cm long, 2.5- cm diam. [51] Reef platform and seawater basins XAD-2 Gelman AE GFF 14.9 500 100 mL 221–435 [36] Aluminium reduction plant discharge PUF 0.7 GF/F 14.2 1000 30-mm diam., 45- mm long 30–40 [39] Oceanic XAD-2 Not filtered o300 300 mm long., 22- mm diam. 150–400 [52] San Francisco Estuary XAD-2 1 1400 250 g and 2  75 g 400 [41] Deep Atlantic XAD-2 GF/F 5 bed volumes [50] Techniques for quantitatively evaluating aquatic passive sampling devices 343 TABLE 15.5 Some simultaneous passive and grab field validations Location Type Sites Passive samples a Grab samples a Exposure duration Comments Refs. Menai Straits, UK DGT 1 6 6 5–6 h [27] Ephraim Island, Australia DGT 13 24 24 Up to 72 h Numerous deployment periods and sampling regimes. [65] Gold Coast Broadwater, Australia DGT 8 1/1 6/2 24 h/72 h Entailed two field exposures. Grab samples were composited for second. [66] Thames Tideway POCIS 7 [16] Constructed Wetlands, Missouri POCIS 3 1 1 28 d Only single water samples taken at each of five sites. [17] Portsmouth Harbour Empore TM 2 2 10 14 d [33] a Not counting replicates. Techniques for quantitatively evaluating aquatic passive sampling devices 345 Chapter 16 Theory and applications of DGT measurements in soils and sediments William Davison, Hao Zhang and Kent W. Warnken 16.1 INTRODUCTION The technique of diffusive gradients in thin-films (DGT) was first used for the measurement of trace metals in sea-water [1]. However, within a year it was used to measure trace metals in sediments at high spatial resolution [2]. Its use in sediments was a natural extension of the technique of diffusive equilibration in thin-films (DET), which had been developed a few years earlier [3]. With DET, a strip of hydrogel, which typically comprises 95% water, is held in a plastic supporting probe, which is inserted into the sediment. Solutes equilibrate between the pore-water of the sediment and the water of the hydrogel. After a typ- ical equilibration time of 24 h, the probe is removed and the solutes in the gel are back-equilibrated and analysed [4]. Initially, DET was used for the measurement of solutes present at relatively high concentra- tions, including Fe and Mn [5], major anions [6] and major cations [7], as analysis of eluent solutions from small volumes of gel was challeng- ing. The continued improvement in analytical techniques, particularly inductively coupled plasma mass spectrometry (ICP-MS), made it pos- sible to measure trace metals by DET in some studies [8–11], but care must be taken to verify that binding of trace components to the hy- drogel does not bias the results [12]. In DGT, a layer of binding agent is introduced behind the diffusive layer of hydrogel. This allows trace solutes such as metals to accumu- late progressively with time, greatly improving the detection limits compared to DET. However, the basis of the technique is fundamen- tally changed from the simple equilibration of DET to a dynamic meas- urement of a flux of the solute. DGT perturbs the environment into which it is introduced by removing solute. The subsequent analysis Comprehensive Analytical Chemistry 48 R. Greenwood, G. Mills and B. Vrana (Editors) Volume 48 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)48016-8 r 2007 Elsevier B.V. All rights reserved. 353 . bed volumes [50] Techniques for quantitatively evaluating aquatic passive sampling devices 343 TABLE 15.5 Some simultaneous passive and grab field validations Location Type Sites Passive samples a Grab samples a Exposure duration Comments. desir- able to prevent contact between the mechanical components of the Techniques for quantitatively evaluating aquatic passive sampling devices 339 TABLE 15.3 High-volume solid-phase extraction. the limited simulta- neous active and passive field deployments to date. It covers both high- volume sampling for hydrophobic organics and high-frequency grab sampling for hydrophilics and inorganics. 15.4.1