As can be seen in figure 1, two ring electrodes are separated by a short distance along a capillary. An alternating voltage is applied to the first electrode.
The wall acts as a capacitor so the field is coupled to the solution inside, it travels through the resistance represented by the solution, and is then coupled again capacitively through the wall to the second electrode where it is picked up, amplified and rectified.
Under the right circumstances (as the wall capacitive reactance remains the same) the overall current can be linearly related to the solution specific conductance (σ) over a small range of σ. Commonly recognized advantages of C4D are its simplicity and low cost, its ability to monitor the solution electrical properties noninvasively on-column, the ability to respond to all charged species, and freedom from electrode fouling. Presently, several commercial C4D detectors are available and are widely accepted.