AN OSCILLOMETRIC DETECTOR FOR CAPILLARY-ELECTROPHORESIS

An oscillometric detector for capillary electrophoresis (CE) has been described. Two 2-mm silver rings separated by 1 mm were painted over t he polyimide coating of a fused-silica capillary (75-mu m i.d. and 360 -mu m o.d.) and used as electrodes for oscillometric measurements. A f unction generator was used to apply a sinusoidal signal over one of th e electrodes; the other one was connected to a current-to-voltage conv erter. The rectified signal is proportional to the admittance of the c ell, which is a function of the inner solution conductivity in the reg ion of the electrodes. Electropherograms of alkaline and alkaline-eart h cations showed good signal-to-noise ratio. For typical electrophoret ic conditions, the limit of detection for lithium was 1.5 mu M, and th ere was good linearity (R = 0.998 for eight data points) up to 2 mM. I ndirect conductivity detection of quaternary ammonium salts was achiev ed by using potassium acetate running buffer, showing results similar to those from conventional conductometric detectors. Despite the cell length (5 mm), good resolution was obtained in the electropherograms. Equivalent electrical circuits were proposed for the cell. The most si mplified model comprises a resistor-capacitor couple in parallel with another capacitor. The resistor stands for the inner solution resistiv ity, the series capacitor stands for the fused-silica wall dielectric properties in the region between the electrodes and the solution, and the parallel capacitor stands for the leakage through the wall and edg e capacitance effects.