Analyzing the response of a contactless conductivity detector in capillary electrophoresis by a resonant method

We report a resonant method to measure the wall capacitance (Cw) and solution resistance (RS) in a capacitively coupled contactless conductivity detector (C4D). Under the typical operating conditions in capillary electrophoresis (I.D. 50 μm, O.D. 360 μm, electrode length of 4 mm, electrode gap of 1 mm, frequency of 200 kHz), the values of Cw measured in 1 and 20 mmol/L NaCl solutions are 2.8 and 32 fF, which are only 1.1% and 12% of prediction by the equation in references, respectively. The value of RS is less than the prediction in solutions with κ < 0.02 S/m. The response current of C4D is due to the change in Cw because the total impedance of a C4D is composed mainly by the impedance from Cw.