Expanded electrical model of a contactless conductivity detector: development and verification.

A theoretical model of the contactless conductivity detector (CCD) has been developed consisting of a network of resistors and capacitors. The output of the model is compared to experimental results and to the output of a simpler model. Experimentally, a lock-in amplifier is added to the detection scheme of the contactless conductivity detector to provide a more sensitive method of signal isolation. The detector is assembled on a printed circuit board with the electrodes in a co-axial configuration. The electrodes are chosen to allow for use with fused silica capillaries in capillary electrophoresis. The use of a lock-in amplifier in place of a previous rectification/filtering circuit allows for an approximate 10-fold improvement in S/N. The detector shows a linear response to changes in excitation voltage and to changes in analyte concentration. Mass limits of detection of 60, 63, and 50 fg are determined for the inorganic cations potassium, sodium, and lithium, respectively (for a signal three times the level of the rms noise).