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 of inner radii 5‐160 μm and σ = ~1‐1400 μS/cm, resulting in cell resistances of 51 GΩ‐176 kΩ. A 400‐element discrete model was used for simulating 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.
Figure. Detection cell configuration (a) and equivalent circuit (b). 1, capillary; 2, grounded metal box; 3, electrode casted by Woods metal alloy; 4, crimp‐snap connectors; 5, BNC connector; 6, grounded Faraday shield; 7, adhesive paper tape for insulation; Re, segmented solution resistors; Rcell, resistors between the electrodes gap; CW, wall capacitor; CS, stray capacitor; CL, leakage capacitor; Caq, aqueous solution capacitor. In circuit b, only 12 units of Re and CW were shown to simplify the drawing; 400 segments were used in the model.
(1) Zhang, M.; Stamos, B.N.; Amornthammarong, N.; Dasgupta, P.K. Anal. Chem. 2014, 86, 11538-11546.