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Admittance detector for high impedance systems: design and applications

journal contribution
posted on 2023-05-18, 14:31 authored by Zhang, M, Stamos, BN, Dasgupta, PK
We describe an admittance detector for high impedance systems (small capillary bore and/or low solution specific conductance). Operation in the low frequency range (≤1 kHz, much lower than most relevant publications) provides optimum response to conductance changes in capillaries ≤20 μm in bore. The detector design was based on studies described in a preceding companion paper (Zhang, M.; Stamos, B. N.; Amornthammarong, N.; Dasgupta, P. K. Anal. Chem. 2014, 86, DOI 10.1021/ac503245a.). The highest S/N for detecting 100 μM KCl (5.5 μM peak concentration, ∼0.8 μS/cm) injected into water flowing through a capillary of 7.5 μm inner radius (r) was observed at 500–750 Hz. A low bias current operational amplifier in the transimpedance configuration permitted high gain (1 V/nA) to measure pA–nA level currents in the detection cell. Aside from an oscillator, an offset-capable RMS-DC converter formed the complete detection circuitry. Limits of detection (LODs) of KCl scaled inversely with the capillary cross section and were 2.1 and 0.32 μM injected KCl for r = 1 and 2.5 μm capillaries, respectively. When used as a detector on an r = 8 μm bore poly(methyl methacrylate) capillary in a split effluent stream from a suppressed ion chromatograph, the LOD was 27 nM bromide (Vex 22 V p-p), compared to 14 nM observed with a commercial bipolar pulse macroscale conductivity detector with an actively thermostated cell. We also show applications of the detector in electrophoresis in capillaries with r = 1 and 2.5 μm. Efficient heat dissipation permits high concentrations of the background electrolyte and sensitive detection because of efficient electrostacking.


Publication title

Analytical Chemistry










School of Natural Sciences


Amer Chemical Soc

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1155 16Th St, Nw, Washington, USA, Dc, 20036

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© 2014 American Chemical Society

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