Reduction of the Impedance of a Contactless Conductivity Detector for Microchip Capillary Electrophoresis: Compensation of the Electrode Impedance by Addition of a Series Inductance from a Piezoelectric Quartz Crystal

摘要

A low-impedance capacitively coupled contactless conductivity detector (LIC~(4)D) for microchip capillary electrophoresis was reported. The LIC~(4)D was the series combination of a piezoelectric quartz crystal (PQC) resonator with a capacitively coupled contactless conductivity detector (C~(4)D) outside on the microchip lid. The electrode impedance in the LIC~(4)D was reduced because the capacitive impedance from the wall capacitance was compensated by the inductive impedance from the PQC. The operation frequency of the LIC~(4)D was set at the resonant frequency of the series combination of a PQC with a C~(4)D, wherein a minimum in the total impedance was obtained. It was shown that the sensitivity of LIC~(4)D was much higher than that of C~(4)D itself, especially in the microchip with a thick lid. Under the experimental conditions, the signal-to-noise ratios of the LIC~(4)D were improved by approx20-50 times over those of the C~(4)D. Reproducible separations of a mixture of inorganic cations (K~(+), Na~(+), Li~(+)) were demonstrated. After a digital filter treatment by the fast Fourier transform algorithm, the detection limits were 0.38, 0.49, and 1.6 (mu)M for K~(+) in the LI C~(4)D with the microchip lid thickness of 0.20, 0.40, and 1.0 mm, respectively.