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Accuracy of the diffusive gradients in thin-films technique: diffusive boundary layer and effective sampling area
Kent W Warnken1, Hao Zhang, William Davison
1Department of Environmental Science, Lancaster Environment Center, Lancaster University, Lancaster LA1-4YQ, United Kingdom. k.warnken@lancaster.ac.uk
Analytical Chemistry
|June 2, 2006
Summary
The diffusive boundary layer (DBL) in diffusive gradients in thin-films (DGT) measurements is crucial for accurate metal ion concentration determination. Neglecting the DBL and using geometric area can lead to errors, especially in unstirred solutions.
Area of Science:
- Environmental Science
- Analytical Chemistry
- Geochemistry
Background:
- The diffusive boundary layer (DBL) in diffusive gradients in thin-films (DGT) technique is often assumed negligible.
- This assumption may lead to inaccuracies in metal ion concentration measurements.
Purpose of the Study:
- To investigate the significance of the DBL and lateral diffusion in DGT measurements.
- To determine the effective surface area of DGT devices and the thickness of the DBL under various conditions.
Main Methods:
- Deployment of DGT devices with varying diffusive gel thicknesses.
- Measurement of Cadmium (Cd) distribution in DGT gels using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).
- Analysis of DBL thickness in different solution stirring conditions.
Main Results:
- DBL thickness was ~0.23 mm in well-stirred solutions, significantly thicker in unstirred conditions.
- Lateral diffusion increased the effective surface area of DGT devices by ~20%.
- Accurate concentrations were obtained when using effective area and appropriate DBL thickness.
Conclusions:
- The DBL and effective surface area significantly impact DGT measurement accuracy.
- Precise speciation and kinetic measurements require incorporating effective area and DBL thickness into the DGT equation.
- Accurate DGT analysis necessitates accounting for DBL and lateral diffusion effects, particularly in unstirred or poorly stirred environments.