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Donnan effects in metal speciation analysis by DET/DGT
Lee P Yezek1, Pascal L R van der Veeken, Herman P van Leeuwen
1Department of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703HB Wageningen, The Netherlands.
Environmental Science & Technology
|January 30, 2009
Summary
Polyacrylamide gels used in analytical techniques like DET and DGT may possess a structural charge. This charge affects metal speciation analysis, particularly at low ionic strengths, necessitating combined DET and DGT interpretation for accuracy.
Area of Science:
- Analytical Chemistry
- Environmental Science
- Materials Science
Background:
- Polyacrylamide gels are widely used in analytical techniques such as Diffusional Equilibration in Thin Film (DET) and Diffusive Gradient in Thin Film (DGT).
- These techniques often assume a chemically inert and uncharged gel matrix.
- Recent findings indicate nonideal behavior in low ionic strength solutions, suggesting a finite structural charge within the gel matrix.
Purpose of the Study:
- To investigate the impact of fixed charged sites within polyacrylamide gel matrices on metal speciation analysis.
- To explore the ramifications of gel matrix charge on DET and DGT techniques.
- To assess the influence of Donnan partitioning on metal ion concentrations.
Main Methods:
- Theoretical exploration of the effects of charged gel matrices on analytical techniques.
- Analysis of metal speciation under varying ionic strengths.
- Comparison of results from DET and DGT under conditions influenced by gel charge.
Main Results:
- A finite structural charge in the gel layer leads to different DET equilibrium concentrations compared to the bulk sample due to Donnan partitioning.
- The steady-state diffusion of ions in DGT is influenced by gel charge, with the net effect dependent on speciation details.
- Significant deviations from ideal behavior are observed at low ionic strengths.
Conclusions:
- The presence of structural charge in polyacrylamide gels significantly impacts metal speciation analysis by DET and DGT.
- Quantitative interpretation of DGT data in submillimolar ionic strength solutions is improved by combining it with DET.
- Accounting for Donnan partitioning is crucial for accurate metal speciation analysis when using these techniques in low ionic strength environments.
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