Capillary Scale Admittance Detection - Analytical Chemistry (ACS

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Article pubs.acs.org/ac

Capillary Scale Admittance Detection Min Zhang, Brian N. Stamos, Natchanon Amornthammarong,† and Purnendu K. Dasgupta* Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065, United States S Supporting Information *

ABSTRACT: 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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divided into a multitude of RC elements in a network,7 with an additional leakage capacitance from the coupling of a Faraday shield, if used between the electrodes, to the solution. The effect of the shield has been theoretically and experimentally examined.11 Others investigated the effects of electrode width, spacing, and excitation frequency on detector performance21 and the effect of wall thickness at constant inner diameter (ID).22 The apparent superiority of thinner walls has been noted; this disappears at higher specific conductances (σ). The DEM correctly predicted the overshooting artifact with abrupt conductivity transitions.12 The effects of frequency, voltage, cell geometry, and electronic components, simulated with the simple model, was compared with experimental data.23,24 Another two companion papers expansively covered C4D behavior using both models.25,26 A high-resolution DEM (but not the simple model) corresponded well with experimental data; merits of lock-in detection were also demonstrated.27 Although many have shown that the simple model fails in many details, it continues to be used.28−30 Given such extensive extant knowledge, it may seem superfluous to add more to this. Actually, available guidance for optimum operating conditions is scant. Operating frequencies, for example, range from 250 Hz31 to 1.25

lectrical properties of a medium have long been probed through an insulating wall using alternating excitation; the term “oscillometry” has been in use since before 1950. The benchmark instrument, the Sargent Model V Oscillometer, operated at a frequency ( f) of 5 MHz and could detect a dielectric constant change of 0.003 units.1 A major application was monitoring conductometric titrations. 2 The 1980s witnessed the first such detectors for capillary scale isotachophoresis, using four wire−electrode arrangements,3,4 and an oscillometric flow-through detector that measured permittivity or conductivity.5 Spurred by increasing needs in capillary electrophoresis (CE), Zemann et al.6 and da Silva and do Lago7 independently introduced such a detector for CE with two tubular/ring electrodes placed/painted on the capillary a small distance apart. They respectively used f = 20−40 kHz and 600 kHz. Zemann et al. first coined the term capacitively coupled contactless conductivity detection (originally abbreviated as CCCD and later, more commonly, as C4D). Many C4D improvements have been proposed;8−13 there are several recent reviews;14−16 Kubáň and Hauser in particular have made sustained contributions through both original work and reviews.17−20 C4D simulations assume either (a) the simple model, which involves just one or two capacitors (wall capacitance) in series with the solution resistance and a capacitor in parallel (interelectrode/stray capacitance)5 or (b) the discrete element model (DEM), wherein the first two elements above are © 2014 American Chemical Society

Received: August 29, 2014 Accepted: October 29, 2014 Published: October 29, 2014 11538

dx.doi.org/10.1021/ac503245a | Anal. Chem. 2014, 86, 11538−11546

Analytical Chemistry

Article

Figure 1. (a) Detection cell configuration [legend: 1, capillary; 2, grounded metal box; 3, electrode cast by Woods metal alloy; 4, crimp-snap connectors; 5, BNC connector; 6, grounded Faraday shield; and 7, adhesive paper tape for insulation]. (b) Equivalent circuit [legend: Re, segmented solution resistance; Rcell, interelectrode resistance; CW, wall capacitance; CS, stray capacitance; CL, leakage capacitance; and Caq, aqueous solution capacitance]. In panel b, only 12 units of Re and CW were shown to simplify the drawing; 400 segments were used in the model.

MHz22 (available commercial detectors permit operations in the 38−612 and 50−1200 kHz range). We have been interested in (suppressed) open tubular ion chromatography (OTIC),32−34 preferably in columns of