Thermal conductivity is a robust method for determining the concentrations of inert gases in a binary mixture. The thermal conductivity detector is a universal detector, and the method can be used where the identity of the constituents of the binary gas is known. The pure carrier gas is passed over an electrically heated resistance filament and the carrier gas with the eluted impurity is passed over a second, heated resistance filament. The gases cool the heated filaments, changing their resistance, where the amount of heat lost is a function of the thermal conductivity of the gas. The heat loss at each filament will be different due to the presence of the impurity in the carrier gas. The heated filaments are configured as a Wheatstone bridge. The output of the Wheatstone bridge will be dependent on the difference in resistance of the filaments, and provides a measure of the concentration of the impurity.