Accuracy of the Diffusive Gradients in Thin-Films Technique: Diffusive Boundary Layer and Effective Sampling Area Considerations
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- 作者：Kent W. Warnken ; Hao Zhang ; and William Davison
- 刊名：Analytical Chemistry
- 出版年：2006
- 出版时间：June 1, 2006
- 年：2006
- 卷：78
- 期：11
- 页码：3780 - 3787
- 全文大小：174K
- 年卷期：v.78,no.11(June 1, 2006)
- ISSN：1520-6882
文摘
When using the diffusive gradients in thin-films (DGT)technique in well-stirred solutions, the diffusive boundarylayer has generally been ignored on the assumption thatit is negligibly thin compared to the total thickness of 
g,i.e., the sum of the thickness of the prefilter and diffusivegel. Deployment of devices with different diffusive layerthicknesses showed that the thickness of the DBL was~0.23 mm in moderate to well-stirred solutions, butsubstantially thicker in poorly or unstirred solutions.Measurement of the distribution of Cd in the DGT resingel at high spatial resolution (100 
m) using laser ablationinductively coupled plasma mass spectrometry showedthat the effective sampling window had a larger diameter(2.20 cm) than the geometric diameter of the exposurewindow (2.00 cm). Lateral diffusion in the gel, which hadpreviously been neglected, therefore increased the effective surface area of the device by ~20%. The concentrations measured by DGT agreed well with the knownconcentrations in standard solutions for all diffusion layerthicknesses, when the effective area and the appropriatediffusive boundary layer (DBL) were used. The extent ofthe error associated with neglecting the DBL and usingthe geometric window area depends on the gel layerthickness and the true thickness of the DBL, as determined by the deployment geometry and flow regime.When DGT measurements were made in well-stirredsolutions using a 0.80-mm diffusive gel, the effect ofneglecting the DBL and using the inappropriate geometricarea offset each other, with the error being <±10%. Forprecise measurements, and especially work involvingspeciation or kinetic measurements, where DGT deviceswith different diffusive gel layer thicknesses are deployed,it is necessary to use the effective area and the appropriateDBL thickness in the full DGT equation, which allows forthe use of layer-specific diffusion coefficients.
g,i.e., the sum of the thickness of the prefilter and diffusivegel. Deployment of devices with different diffusive layerthicknesses showed that the thickness of the DBL was~0.23 mm in moderate to well-stirred solutions, butsubstantially thicker in poorly or unstirred solutions.Measurement of the distribution of Cd in the DGT resingel at high spatial resolution (100 m) using laser ablationinductively coupled plasma mass spectrometry showedthat the effective sampling window had a larger diameter(2.20 cm) than the geometric diameter of the exposurewindow (2.00 cm). Lateral diffusion in the gel, which hadpreviously been neglected, therefore increased the effective surface area of the device by ~20%. The concentrations measured by DGT agreed well with the knownconcentrations in standard solutions for all diffusion layerthicknesses, when the effective area and the appropriatediffusive boundary layer (DBL) were used. The extent ofthe error associated with neglecting the DBL and usingthe geometric window area depends on the gel layerthickness and the true thickness of the DBL, as determined by the deployment geometry and flow regime.When DGT measurements were made in well-stirredsolutions using a 0.80-mm diffusive gel, the effect ofneglecting the DBL and using the inappropriate geometricarea offset each other, with the error being <±10%. Forprecise measurements, and especially work involvingspeciation or kinetic measurements, where DGT deviceswith different diffusive gel layer thicknesses are deployed,it is necessary to use the effective area and the appropriateDBL thickness in the full DGT equation, which allows forthe use of layer-specific diffusion coefficients.