Sensitivity Improvement of C4D Detectors by Adjusting Sinusoidal Integration With a Potentiometer

Capacitively coupled non-contact conductivity detectors (C4D) are a versatile and universal type of detector because they can detect cationic, anionic, fluorophore, non-fluorophore, fluorescent, or non-fluorescent compounds and can be adapted to capillary electrophoresis, liquid chromatography, or ion chromatography systems. In this study, a capacitive pseudo-bridge is adapted to a C4D detector to facilitate the adjustment to optimal levels of sensitivity using a potentiometer, which is adjusted by a screw that divides its resistance to perform a sinusoidal integration. When the C4D is manufactured, different capacitance values are obtained for the sensor due to variations in diameter, length, and void space, as well as a change in the resistivity of the mobile phase. All these changes can be adjusted using a potentiometer and an oscilloscope to visualize the optimum integration that occurs when a phase change is observed. To demonstrate the sensitivity of the C4D, analysis by High Performance Liquid Chromatography (HPLC), flow injection analysis (FIA), and ion chromatography (IC) produced a limit of detection of $0.98~\mu $ g/L for fluorides, $0.4~\mu $ g/L for sodium chloride, and $20~\mu $ g/L for fluorides, respectively. This novel adjustment of a potentiometer in a circuit of C4D detectors has not been proposed before for this kind of detector. A cosine output of the operational amplifier gives the maximum sensitivity of the capacitance to voltage converter.