Warnken Kent W, Zhang Hao, Davison William
Department of Environmental Science, Lancaster Environment Center, Lancaster University, Lancaster LA1-4YQ, United Kingdom.
Anal Chem. 2006 Jun 1;78(11):3780-7. doi: 10.1021/ac060139d.
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.
在充分搅拌的溶液中使用薄膜扩散梯度（DGT）技术时，扩散边界层通常被忽略，这是基于这样的假设：与δg的总厚度（即预过滤器和扩散凝胶的厚度之和）相比，其厚度可忽略不计。部署具有不同扩散层厚度的装置表明，在适度搅拌至充分搅拌的溶液中，扩散边界层（DBL）的厚度约为0.23毫米，但在搅拌不良或未搅拌的溶液中要厚得多。使用激光烧蚀电感耦合等离子体质谱法在高空间分辨率（100微米）下测量DGT树脂凝胶中镉的分布表明，有效采样窗口的直径（2.20厘米）大于暴露窗口的几何直径（2.00厘米）。因此，凝胶中的横向扩散（此前一直被忽略）使装置的有效表面积增加了约20%。当使用有效面积和适当的扩散边界层（DBL）时，对于所有扩散层厚度，DGT测量的浓度与标准溶液中的已知浓度吻合良好。与忽略DBL并使用几何窗口面积相关的误差程度取决于凝胶层厚度和DBL的真实厚度，这由部署几何形状和流动状态决定。当在充分搅拌的溶液中使用0.80毫米的扩散凝胶进行DGT测量时，忽略DBL并使用不适当的几何面积的影响相互抵消，误差<±10%。对于精确测量，尤其是涉及形态分析或动力学测量的工作（其中部署了具有不同扩散凝胶层厚度的DGT装置），有必要在完整的DGT方程中使用有效面积和适当的DBL厚度，这允许使用特定层的扩散系数。
