Teledyne GC-Pro/TCD - Gas Chromatograph - Manual (Page 28)

Operational Theory GC-Pro TCD
Teledyne Analytical Instruments 14
2.3.3 Gas Separation System
The carrier gas pressure and column temperature are the critical
parameters in the separation process. A heater is used in column housing
and the temperature controlled by the Temperature PCB. A constant
carrier gas flow is provided by a pressure regulator on the front panel set
to a specific pressure with an inlet carrier gas supply of 80 psig at the
rear panel. Thus, a stable flow is achieved by maintaining a constant
pressure across restrictors upstream from the cell. Actual carrier gas
flow rate through the column, which is crucial for separation retention
time, will depend on the carrier gas pressure and the type of column
used for each application. It is important that all temperature and
pressure settings are set to the values listed in the Addendum and Testing
Results section of Appendix C.
2.4 Detector Cell
The thermal conductivity sensor contains two chambers, one for the
reference gas of known conductivity (which is of the same as the carrier
gas) and one for the sample gas which is driven by the carrier gas. Each
chamber contains a pair of heated filaments. Depending on its thermal
conductivity, each of the gases conducts a quantity of heat away from the
filaments in its chamber. See Figure 2-3(a).
The resistance of the filaments depends on their temperature. These
filaments are parts of the two legs of a bridge circuit that unbalances if
the resistances of its two legs do not match. See Figure 2-1(b).
Figure 2-3: Thermal Conductivity Cell Operating Principle