电容
化学
电导
导纳
机械
等效电路
电导率
等效串联电阻
毛细管作用
电极
分析化学(期刊)
振幅
频率响应
电压
凝聚态物理
光学
复合材料
物理
材料科学
电阻抗
电气工程
量子力学
色谱法
工程类
物理化学
作者
Min Zhang,Brian N. Stamos,Natchanon Amornthammarong,Purnendu Κ. Dasgupta
出处
期刊:Analytical Chemistry
[American Chemical Society]
日期:2014-11-13
卷期号:86 (23): 11538-11546
被引量:44
摘要
Techniques that have been variously termed oscillometric detection or (capacitively coupled) contactless conductivity detection (C4D) are known actually to respond to the admittance. It is not often appreciated that the frequency range (f) over which such systems respond (quasi)linearly with the cell conductance decreases acutely with increasing cell resistance. Guidance on optimum operating conditions for high cell resistance, such as for very small capillaries/channels and/or solutions of low specific conductance (σ), is scant. It is specially necessary in this case to take the capacitance of the solution into account. At high frequencies and low σ values, much of the current passes through the solution behaving as a capacitor and the capacitance is not very dependent on the exact solution specific conductance, resulting in poor, zero, or even negative response. We investigated, both theoretically and experimentally, capillaries with inner radii of 5–160 μm and σ ≈ 1–1400 μS/cm, resulting in cell resistances of 51 GΩ to 176 kΩ. A 400-element discrete model was used to simulate the behavior. As model inputs, both the wall capacitance and the stray capacitance were measured. The solution and leakage capacitances were estimated from extant models. The model output was compared to the measured response of the detection system over broad ranges of f and σ. Other parameters studied include capillary material and wall thickness, electrode spacing and length, Faraday shield thickness, excitation wave forms, and amplitude. The simulations show good qualitative agreement with experimental results and correctly predict the negative response behavior observed under certain conditions. We provide optimum frequencies for different operating conditions.
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