Passive Sampling Techniques Episode 9 ppt

Over a 72 h deployment period, the mass of Cd and Cu in two rivers increased linearly with time and matched the total dissolved concentrations.. They made the simplifying assumption that

Concentrations of Cd, Cu and Mn measuredin situ by DGT in river-and lake-waters was first compared with the total dissolved concentra-tions measured in filtered samples in four Australian waters[25] Over

a 72 h deployment period, the mass of Cd and Cu in two rivers increased linearly with time and matched the total dissolved concentrations This indicated that biofouling was not a concern and that concentrations of strong complexing ligands were negligible in these waters However, in two other rivers, the fraction of Cu and Cd measured by DGT was only 30% and 50% of the total dissolved concentration DGT was used to obtain an in situ depth-profile of Mn in a stratified estuary, which showed a pronounced concentration maximum associated with redox-associated, reductive mobilization

Zhang and Davison [26] were able to obtain more quantitative speciation information They made the simplifying assumption that natural waters can be regarded as having two classes of compounds that can be measured by DGT (a) labile inorganic species, including the free ion, of total concentrationCiwith a mean diffusion coefficientDiand (b) labile organic species of total concentration Co with a mean diffusion coefficientDo This assumption is reasonable in humic-rich fresh-waters where the metal speciation is dominated by complexes with fulvic acid The mass of metal (MDGT) accumulated by a DGT device is then the sum of the contributions from both labile inorganic (Mi) and organic complexes (Mo)

According to Fick’s first law of diffusion, the inorganic and organic contributions toMDGTare given by Eqs (11.18) and (11.19), respectively

Mi ¼DiCiAt

Mo¼DoCoAt

Combining Eqs (11.17)–(11.19) gives

MDGT¼ðDiCiþDoCoÞAt

It can be rearranged to

MDGTDg

DiAt ¼Ciþ

Do

Di

Chapter 12

Use of ceramic dosimeters in water monitoring

Hansjo¨rg Weiß, Kristin Schirmer, Stephanie Bopp and

Peter Grathwohl

12.1 INTRODUCTION

Passive sampling with ceramic dosimeters allows for time-integrated monitoring of dissolved chemicals in ground and surface water The purely diffusion controlled device is based on a porous ceramic mem-brane This membrane is in the shape of a tube The ceramic tube functions as a diffusion barrier and at the same time serves as a con-tainer to hold a solid sorbent The latter can be selected according to compounds of interest and time scale needed for monitoring The sor-bents are required to have a high affinity and capacity for the uptake of the chemicals of concern combined with an easy extraction at high analyte extraction recovery rates As long as such sorbents can be found, ceramic dosimeters fit any analytical need

Diffusive transport of chemicals across the ceramic membrane at steady state can be described by Fick’s first law Thus, the accumulated mass of a chemical at the end of an exposure period can be used to calculate the time-weighted average (TWA) concentration at which this chemical was present over the entire sampling time Based on this, the ceramic dosimeter allows for quantification of chemical concentrations over extended periods, without the need for calibration or frequent snapshot sampling

The idea of the ceramic dosimeter was first conceived by Grathwohl [1] and by now, a number of laboratory experiments as well as explo-rations in the field have proven the suitability of ceramic dosimeters for time-integrated, long-term monitoring Applications to date include the sampling of polycyclic aromatic hydrocarbons (PAHs) using Amberlite IRA-743 sorbent (available from Sigma-Aldrich) as solid receiving Comprehensive Analytical Chemistry 48

R Greenwood, G Mills and B Vrana (Editors)

Volume 48 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)48012-0

TABLE 12.1

Minimum sampling times required to reach detection limits for selected chemicals in the ceramic dosimetera

Minimum mass

(mg)b

Assumed aqueous

concentration

Predicted required sampling times

a Calculations are based on Eqs (12.3) and (12.4) with T ¼ 101C, m ¼ 2, e ¼ 0.305, Dx ¼ 0.15 cm, A ¼ 8.4 cm2.

b Minimum analyte mass detectable in the ceramic dosimeter by the method described in Bopp et al [2] The extraction volume in this method is

30 mL, which could be significantly reduced to improve sensitivity if needed Detection limit is set to three times the detection limit of standard instrumental analytical methods.

c Trichloroethylene.

Chapter 13

Passive diffusion samplers to monitor volatile organic compounds in ground-water

Don A Vroblesky

13.1 INTRODUCTION

Diffusion samplers (DSs) have been used since at least the 1990s to sample volatile organic compounds (VOCs) in ground-water [1,2] DSs can be advantageous for sampling VOCs in ground-water primarily because they have the potential to reduce costs substantially compared with pumping approaches to well sampling The depth-specific charac-teristic of the samples can also be advantageous in certain investigations

In general, the types of DSs used to examine VOCs in ground-water can

be divided into sorption devices and equilibrium DSs Sorptive devices are discussed briefly in the following paragraph; however, this chapter concentrates on equilibrium DSs

Sorption devices for measuring VOC concentrations in ground-water typically consist of a semi-permeable membrane enclosing a sorptive medium, such as hydrophobic carbonaceous resins or polymeric resins

In this type of sampler, dissolved VOCs partition into a vapor phase in order to cross the hydrophobic membrane (examples include low-density polyethylene (LDPE) and Gore-Texs1) to the internal sorption material

in an air space A simple example consists of an air-filled glass vial containing a wire coated with activated carbon The vial is enclosed in a plastic zip-lock bag and buried in the bottom sediment in a pond This type of inexpensive DS has been used successfully to map the zone of VOC-contaminated ground-water beneath a tidal pond where the con-taminated ground-water was discharging to surface-water [1] Because sorptive-type samplers continue to sorb analytes until the sorptive

1

The use of trade names does not imply endorsement by the U.S Geological Survey.

Comprehensive Analytical Chemistry 48

R Greenwood, G Mills and B Vrana (Editors)

Volume 48 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)48013-2

Fig 13.2 Passive vapor diffusion (PVD) sampler showing (A) an empty vial, (B) a completed sampler consisting of the open vial enclosed in two poly-ethylene bags and attached to a survey flag, and (C) a crimp cap.

Fig 13.1 Typical water-filled polyethylene diffusion bag (PDB) samplers used

in wells, including (A) a diffusion bag with polyethylene protective mesh, (B) a diffusion bag without mesh, and (C) a bag and mesh attached to a bailer bottom.