Passive Sampling Techniques Episode 11 docx

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 sampl

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

339

TABLE 15.3

High-volume solid-phase extraction configurations

extracted (L)

Refs Cut-off

(mm)

Diameter (cm)

Env+

High elevation

lake water

XAD-2 and

250 (column)

PUF

GF/C, 0.5

5 cm  30.5 cm;

XAD:

2.7 cm  20 cm

parallel

EmporeTM Ocean outfalls,

California

filtered

2.5-cm diam.

[51] Reef platform

and seawater

basins

AE GFF

Aluminium

reduction plant

discharge

45-mm long

filtered

22-mm diam.

San Francisco

Estuary

volumes

[50]

TABLE 15.5

Some simultaneous passive and grab field validations

Grab

Exposure duration

Menai Straits,

UK

Ephraim Island,

Australia

deployment periods and sampling regimes.

[65]

Gold Coast

Broadwater,

Australia

exposures Grab samples were composited for second.

[66]

Constructed

Wetlands,

Missouri

samples taken at each of five sites.

[17]

Portsmouth

Harbour

a Not counting replicates.

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