Galvanic Applied Sciences AccuChrome MA2963, Operation Manual

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Revision 4 108 12/15/2018
11.4 How are the Components Detected and Quantified?
The purpose of a detector is to monitor the carrier gas as it emerges from the column and to
generate a signal in response to variation in its composition due to eluted components. A
Thermal Conductivity Detector (TCD) is used in the ACCUCHROME Gas Chromatograph.
The Thermal Conductivity Detector consists of four spiral wound filament wires supported
inside cavities in a metal block. A constant DC current is applied to the filaments, which are
arranged in a Wheatstone bridge configuration. When pure carrier and reference gas are
flowing across the filaments, the heat loss, and thus filament temperature, is constant. This
consistent filament temperature produces a constant filament resistance. The currents in the
electronic bridge can be balanced to produce a zero signal level as a reference.
When a specific component enters the TCD with the carrier, the heat dissipated from the
filaments on the measured side changes. The amount of change is dependent on the
thermal conductivity of the gas, which is different for every component in the sample. This
change in heat dissipation causes a change in electrical resistance, which leads to an
imbalance in the electronic bridge. The resulting electrical signal is then used in conjunction
with a Response Factor to measure the concentration of the component. Figure 8-3 shows
an example of the TCD filament configuration.
Figure 11-3: Design of a Thermal Conductivity Detector