Admittance Detector for High Impedance Systems: Design and Applications

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 mu m 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 mu M KCl (5.5 mu M peak concentration, similar to 0.8 mu S/cm) injected into water flowing through a capillary of 7.5 mu m 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 mu M injected KCl for r = 1 and 2.5 mu m capillaries, respectively. When used as a detector on an r = 8 mu m bore poly(methyl methacrylate) capillary in a split effluent stream from a suppressed ion chromatograph, the LOD was 27 nM bromide (V-ex 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 mu m. Efficient heat dissipation permits high concentrations of the background electrolyte and sensitive detection because of efficient electrostacking.