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 the polyimide coating of a fused-silica capillary (75-μm i.d. and 360-μm o.d.) and used as electrodes for oscillometric measurements. A function generator was used to apply a sinusoidal signal over one of the electrodes; the other one was connected to a current-to-voltage converter. The rectified signal is proportional to the admittance of the cell, which is a function of the inner solution conductivity in the region of the electrodes. Electropherograms of alkaline and alkaline-earth cations showed good signal-to-noise ratio. For typical electrophoretic conditions, the limit of detection for lithium was 1.5 μM, and there was good linearity ( R = 0.998 for eight data points) up to 2 mM. Indirect conductivity detection of quaternary ammonium salts was achieved 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 simplified model comprises a resistor−capacitor couple in parallel with another capacitor. The resistor stands for the inner solution resistivity, 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 edge capacitance effects.