Topics - Specific Heat
The specific heat of a substance is classically defined as the amount of heat needed to raise one gram of a substance one degree Centigrade. Its importance in chromatography is mostly associated with the response of the thermal conductivity detector. The thermal conductivity detector consists of a filament heated by a small electric current and situated in the exit gas from a gas chromatography column. As vapor from an eluted solute passes the filament the thermal conductivity and the specific heat of the gas changes and the, thus, the heat loss also changes. As a result, the temperature of the filament changes and thus, its electrical resistance also changes. The filament is situated in one arm of a Wheatstone Bridge which becomes unbalanced and the voltage across the bridge is fed to a suitable amplifier and recording device. Originally it was though that the detector responded only to changes in the thermal conductivity of the gas but it is now clear that the major effect is due to the change in specific heat. Specific heat has some minor impact on temperature control of ovens and in temperature programming but its major significance in chromatography is the part it plays in the response of the thermal conductivity detector.