VICI TCD2 - Manual (Page 5)

1
Introduction
Description and Operating Principle
The Thermal Conductivity Detector (TCD) has been one of the most popular
GC detectors since the 1950’s, second perhaps only to the Flame Ionization
Detector (FID). The principal of operation is based on the relative change in
the thermal conductivity of the gas passing across the detector filament as
components elute from the column. Heat is lost continuously by the filament
through the carrier gas to the cell wall of the detector. By measuring the
amount of current required to maintain a constant filament temperature as
gases of varying thermal conductivities cross the filament, a chromato-
graphic signal is produced. This process is nondestructive of the sample and
is concentration dependent.
The Valco Microvolume TCD is unique in its implementation. Since changes
in conductivity are measured only by the change in current required to keep
the filament at a constant temperature, each of the two filaments can be
operated independently without referencing these changes to a matched
filament with reference gas. This constant temperature provides longer
filament life and safeguards it from the extremely high temperatures and
oxidation which can occur with high concentrations of oxidative or corrosive
components. Optional signal referencing is provided to minimize background
variables such as column bleed and temperature programming.
Figure 1: Unique dual filament design
Cell volume has been minimized to accommodate capillary column
chromatography and optimize the sensitivity of the detector at low flow
rates. (Carrier flow rates of 1 - 10 mL/min are recommended for best sensi-
tivity.) Thermal stability is maintained in the detector cell to within 0.010°C
drift, giving the detector a stable, noise-free signal.
The Model TCD2 is a stand-alone system which can be easily added to
any chromatograph. It consists of a detector and a control module which
incorporates the electrometer and temperature controls. The detector cell
includes two separate filaments, capable of independent or referenced
(differential) operation. Output signal is provided as 0-1 and 0-10 volt
attenuated for chart recorders and 0-1 and 0-10 volts unattenuated for
integrators and data systems.
FILAMENT
A
FILAMENT
B
A
FLOW
INLET
B
FLOW
INLET
OUTLET OUTLET