1
Getting Familiar with your EA IsoLink IRMS System for CNSOH
Detection System Description
18
EA IsoLink IRMS System for CNSOH Operating Manual
Thermo Scientific
Detection System Description
It consists of a thermal conductivity detector (TCD) sensitive to any substance with thermal
conductivity other than that of the carrier gas used.
The detector consists of a stainless steel block provided with two pairs of filaments (generally of
tungsten/rhenium) having the same electrical resistance. The detector is housed in a thermally
insulated metal block (detector oven) and maintained at constant temperature.
The two pairs of filaments are electrically connected according to a Wheatstone bridge circuit powered
at constant voltage. The first pair of filaments is fed with pure carrier gas (reference channel), whereas
the second pair is fed with the gas flowing from the reactor (analytical channel). When the bridge is
powered, the filaments heat at a temperature (resistance) that is a function of the thermal conductivity
of the gas feeding the filaments. The reference channel is exposed only to pure carrier gas, whereas the
analytical channel is exposed to the reactor effluents (carrier gas + sample).
When pure carrier gas flows through both the reference and the analytical channels, a constant
temperature gradient is established between the elements and the detector walls, and the Wheatstone
bridge is balanced, namely there is no output signal. As a component is eluted, a change in heat transfer
occurs, with consequent variation of the filaments temperature. Since electrical resistance is a function
of temperature, the bridge unbalances and the detector generates a signal proportional to the difference
in thermal conductivity between the eluted component and the carrier gas. The output signal is then
sent to the data acquisition board.
Electrical Compartment
It is located on the right part of the instrument, and it is accessible by removing the right side cover.
Behind the electrical compartment, there is the Connections Panel. For more details, see the section
The electrical compartment, shown in
, comprises:
• Low voltage compartment
• High voltage compartment
• EFC electronic flow controller for gas regulation
Note
The filaments are powered constantly at 5 V and are electrically protected if their
temperature exceeds 220 °C (Safety Cut Off ).
WARNING
DO NOT OPEN the electrical compartment because there are no user serviceable
parts inside. Any operation inside the compartment must be carried out by authorized and trained
Thermo Fisher Scientific personnel.
