Final answer:
The two preferred carrier gases for a thermal conductivity detector are Helium and Hydrogen due to their high thermal conductivity, which is crucial for the detector's operation.
Step-by-step explanation:
The two carrier gases of choice for use with a thermal conductivity detector are Helium and Hydrogen. These gases are preferred because of their high thermal conductivity properties, which are essential for the operation of a thermal conductivity detector. A thermal conductivity detector measures the ability of a gas to conduct heat, with changes in thermal conductivity indicating the presence of different substances in a gas sample.
A thermal conductivity detector operates based on the principle that when an electrically-heated wire or filament is exposed to a carrier gas, the cooling effect on the filament is dependent on the thermal conductivity of the gas mixture. Helium and hydrogen are ideal choices due to their high thermal conductivities, which means they can readily absorb heat from the filament, leading to more significant and detectable changes when analytes are introduced into the carrier gas stream.