Carrier gas and detector sensitivity

To be effective, the carrier gas must fulfill two requirements. First, it cannot be adsorbed at the coolant temperature second, it must possess a thermal conductivity sufficiently different from that of the adsorbate that small concentration changes can be detected. 

To understand the effect of the carrier gas on the response of the thermal conductivity detector, consider the steady-state condition that 

When the gas composition is altered due to adsorption or desorption, the value of k changes by Ak which in turn alters the filament temperature by Aff. Under the new conditions equation (15.2) can be rewritten as 

Equating the right-hand side of equations (15.2) and (15.3) gives 

By neglecting the term AkAtf, a second order effect, equation (15.4) rearranges to 

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