The thermal conductivity detector, or TCD, measures the thermal conductivity of the surrounding gas using thermistors arranged in a Wheatstone bridge circuit.

A Wheatstone bridge involves balancing four resistances in a bridge configuration to determine an unknown resistance by tuning the voltage to zero.

In TCD, pure carrier gas passes through the reference arm, while the column effluent passes through the other arm of the circuit.

When the column effluent contains the carrier gas alone, the circuit is balanced.

The appearance of sample vapors unbalances the bridge, resulting in a chromatographic signal.

In a flame ionization detector or FID, the column effluent vapors are mixed with hydrogen gas and burned in the air to produce a flame rich in electrons and ions.

When a potential is applied across the flame, a small current is generated, which is amplified to obtain the analytical signal.

In an electron capture detector or ECD, a beta emitter ionizes the mobile phase and produces electrons that generate an electric current between the electrodes.

When solutes with electronegative groups capture electrons, the decrease in the current serves as the signal.