Thermo Scientific FOCUS GC, Instruction Manual, Page: 172 / 356

Chapter 9
Thermal Conductivity Detector (TCD)
TCD Overview
172 Instruction Manual
TCD Overview
The TCD is sensitive to any compound having thermal conductivity other than
that of the carrier gas used. The TCD is a universal type detector. It has a broad
range of uses in the analysis of permanent gases and other organic or inorganic
compounds for which the Flame Ionization Detector (FID) is practically non-
sensitive, such as CO
2
, CS
2
, H
2
O, H
2
, and N
2
.
While the FID is more sensitive to most organics, the simplicity of the TCD often
makes it the preferred detector when analyte concentrations are high enough. The
TCD typically requires only one type of gas, such as helium. The FID requires up
to four.
The TCD consists of a stainless steel block containing two filaments (generally
tungsten/rhenium filaments) which have the same electrical resistance. The block
is housed in an aluminum case that accommodates the heating elements and the
temperature sensor.
NOTE TCD with polyimide coated filaments is optionally available for the analysis of very
aggressive gas matrices.
The filaments are electrically connected to a Wheatstone bridge. Two gas flows, a
reference flow and an analytical flow, enter the TCD cell, pass across the
filaments, and vent to the atmosphere. Figure 9-1 shows the filaments and gas
flows.