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

A low-impedance capacitively coupled contactless conductivity detector ((LICD)-D-4) for microchip capillary electrophoresis was reported. The (LICD)-D-4 was the series combination of a piezoelectric quartz crystal (PQC) resonator with a capacitively coupled contactless conductivity detector ((CD)-D-4) outside on the microchip lid. The electrode impedance in the (LICD)-D-4 was reduced because the capacitive impedance from the wall capacitance was compensated by the inductive impedance from the PQC. The operation frequency of the (LICD)-D-4 was set at the resonant frequency of the series combination of a PQC with a (CD)-D-4, wherein a minimum in the total impedance was obtained. It was shown that the sensitivity of (LICD)-D-4 was much higher than that of (CD)-D-4 itself, especially in the microchip with a thick lid. Under the experimental conditions, the signal-to-noise ratios of the (LICD)-D-4 were improved by similar to 20-50 times over those of the (CD)-D-4. 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 (LICD)-D-4 with the microchip lid thickness of 0.20, 0.40, and 1.0 mm, respectively.