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

Abstract

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.

Predicting availability of mineral elements to plants with the DGT technique: a review of experimental data and interpretation by modelling

“…Procedures are available to correct for non-negligible depletion in solution if necessary. [62] If c DGT is smaller than the bulk concentration for a well stirred solution, it indicates the presence of complexes that are non-labile on the DGT timescale. The DGT technique can thus serve as a dynamic speciation tool, with the ratio of c DGT to total solution concentration allowing evaluation of the dissociation kinetics of complexes in solution, and has indeed been used for that purpose in recent years.…”

Fractionation of iron species and iron isotopes in the Baltic Sea euphotic zone

“…The resulting Fe blank from HNO 3 was routinely tested and has not been found to represent a significant contribution to the overall blank levels. Concentrations of Fe collected by the DGT samplers ([Fe] DGT ) were calculated assuming a 0.23 mm diffusive boundary layer (DBL), which is reasonable to expect in moderate to well mixed waters (Warnken et al, 2006;Garmo et al, 2006). The accumulation area was assumed to be equal to 3.8 cm 2 (Warnken et al, 2006).…”

“…Concentrations of Fe collected by the DGT samplers ([Fe] DGT ) were calculated assuming a 0.23 mm diffusive boundary layer (DBL), which is reasonable to expect in moderate to well mixed waters (Warnken et al, 2006;Garmo et al, 2006). The accumulation area was assumed to be equal to 3.8 cm 2 (Warnken et al, 2006). Diffusion coefficients in the gels were supplied by DGT Research Inc (www.DGTresearch.com).…”

The labile metal concentrations in Sava River water assessed by diffusive gradients in thin films

“…As pointed out by Warnken et al (2006), for monitoring that involves in situ DGT field deployments, if absolute accuracy is not a prime concern, it is acceptable to ignore the diffusive boundary layer. However, lower discrepancy of the calculated concentrations (C DGT ) from the true values in the surface water will be achieved if standard DGT devices with diffusive gels of 0.8 mm are used.…”

“…Influence of the diffusive layer thickness on metal accumulation Although, according to Zhang and Davison (1995) and Gimpel et al (2001), the DBL can be neglected in the fast flowing waters, a number of subsequent studies have emphasized a possibility of its substantial contribution to the diffusive layer thickness, either in association to the flow velocity or to the biofouling on the surface of DGTs (Alfaro-De la Torre et al, 2000;Zhang and Davison, 2000;Webb and Keough, 2002;Garmo et al, 2006;Warnken et al, 2006). Furthermore, if metal is present in water in the form of a kinetically inert complex, the mass of a metal accumulated on Chelex resin in DGT device will be reduced.…”