Accuracy of the Diffusive Gradients in Thin-Films Technique: Diffusive Boundary Layer and Effective Sampling Area Considerations

Department of Environmental Science, Lancaster Environment Center, Lancaster University, Lancaster LA1-4YQ, United Kingdom.
Analytical Chemistry (Impact Factor: 5.64). 06/2006; 78(11):3780-7. DOI: 10.1021/ac060139d
Source: PubMed


When using the diffusive gradients in thin-films (DGT) technique in well-stirred solutions, the diffusive boundary layer has generally been ignored on the assumption that it is negligibly thin compared to the total thickness of delta g, i.e., the sum of the thickness of the prefilter and diffusive gel. Deployment of devices with different diffusive layer thicknesses showed that the thickness of the DBL was approximately 0.23 mm in moderate to well-stirred solutions, but substantially thicker in poorly or unstirred solutions. Measurement of the distribution of Cd in the DGT resin gel at high spatial resolution (100 microm) using laser ablation inductively coupled plasma mass spectrometry showed that the effective sampling window had a larger diameter (2.20 cm) than the geometric diameter of the exposure window (2.00 cm). Lateral diffusion in the gel, which had previously been neglected, therefore increased the effective surface area of the device by approximately 20%. The concentrations measured by DGT agreed well with the known concentrations in standard solutions for all diffusion layer thicknesses, when the effective area and the appropriate diffusive boundary layer (DBL) were used. The extent of the error associated with neglecting the DBL and using the geometric window area depends on the gel layer thickness and the true thickness of the DBL, as determined by the deployment geometry and flow regime. When DGT measurements were made in well-stirred solutions using a 0.80-mm diffusive gel, the effect of neglecting the DBL and using the inappropriate geometric area offset each other, with the error being <+/-10%. For precise measurements, and especially work involving speciation or kinetic measurements, where DGT devices with different diffusive gel layer thicknesses are deployed, it is necessary to use the effective area and the appropriate DBL thickness in the full DGT equation, which allows for the use of layer-specific diffusion coefficients.

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    • "For elements with one or two reactive species only, such as some oxyanions, the estimated concentration can be interpreted directly [7]. The diffusive boundary layer (DBL), a layer of quiescent water at all surfaces through which mass transport occurs only by diffusion, also needs to be determined for accurate DGT measurements as it can vary considerably (0.023-0.065 cm) [11] [12] [13]. "
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    ABSTRACT: A systematic comparison of the diffusion coefficients of cations (Al, Cd, Co, Cu, Mn, Ni, Pb, Zn) and oxyanions (Al, As, Mo, Sb, V, W) in open (ODL) and restricted (RDL) diffusive layers used by the DGT technique was undertaken. Diffusion coefficients were measured using both the diffusion cell (Dcell) method at pH 4.00 and the DGT time-series (DDGT) method at pH 4.01 and 7.04 (pH 8.30 was used instead of 7.04 for Al) using the Chelex-Metsorb mixed binding layer. The performance of Chelex-Metsorb as a new DGT binding layer for Al uptake was also evaluated for the first time. Reasonable agreement was observed between Dcell and DDGT measurements for both ODL and RDL, except for V and W. The ratios of Dcell/DDGT for V of 0.44 and 0.39, and for W of 0.66 and 0.63 with ODL and RDL respectively, were much lower due to the formation of a high proportion of polyoxometalate species at the higher concentrations required with the Dcell measurements. This is the first time that D values have been reported for several oxyanions using RDL. Except for Al at pH 8.30 with ODL, all DDGT measurements were retarded relative to diffusion coefficients in water (DW) for both diffusive hydrogels. Diffusion in RDL was further retarded compared with ODL, for all elements (0.66-0.78) with both methods. However, the degree of retardation observed changed for cations and anions at each pH. At pH 7.04 cations had a slightly higher DDGT and oxyanions had a slightly lower DDGT than at pH 4.01 for both ODL and RDL. It is proposed that this is due to partial formation of acrylic acid functional groups (pKa ≈4.5), which would be fully deprotonated at pH 7.04 (negative) and mostly protonated at pH 4.01 (neutral). As Al changes from being cationic at pH 4.01 to anionic at pH 8.30 the results were more complex. Copyright © 2015 Elsevier B.V. All rights reserved.
    Full-text · Article · Aug 2015 · Analytica chimica acta
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    • "The accurate determination of metal concentrations using DGT depends on the selection of appropriate diffusive gel thickness (Wu et al., 2011; Warnken et al., 2006). For thin diffusive gels, the effect of diffusive boundary layer between porewater and the DGT probe may result in negligible error, while metal concentrations in porewater may be overestimated if thick diffusive gels are adopted since excess metals could be resupplied from the solid phase. "

    Full-text · Article · Apr 2015 · Journal of Environmental Sciences
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    • "The effective area of the section through which diffusion occurs has been reported to be somehow larger than the nominal area due to lateral diffusion; that is, diffusion occurs in three dimensions [6, 7]. Warnken et al. report that the radius of the effective diffusion window is 1.02 ± 0.024 cm and also note that the gel disc diameter had shrunk on average 0.12 cm (n = 6) during drying prior to determination of the radius [16]. No estimate on uncertainty was given for this measure, so a 0.05 cm uncertainty was assumed based on the number of significant figures reported, and a rectangular distribution was selected due to the lack of information on the measurement. "
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    ABSTRACT: Diffusion-based passive samplers are increasingly used for water quality monitoring. While the overall method robustness and reproducibility for passive samplers in water are widely reported, there has been a lack of a detailed description of uncertainty sources. In this paper an uncertainty budget for the determination of fully labile Cu in water using a DGT passive sampler is presented. Uncertainty from the estimation of effective cross-sectional diffusion area and the instrumental determination of accumulated mass of analyte are the most significant sources of uncertainty, while uncertainties from contamination and the estimation of diffusion coefficient are negligible. The results presented highlight issues with passive samplers which are important to address if overall method uncertainty is to be reduced and effective strategies to reduce overall method uncertainty are presented.
    Full-text · Article · Sep 2014 · International Journal of Analytical Chemistry
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