Thermo Scientific FOCUS GC, Instruction Manual, Page: 174 / 356

Chapter 9
Thermal Conductivity Detector (TCD)
TCD Gas Supplies
174 Instruction Manual
TCD Gas Supplies
The TCD detector requires the same gas whether for the measure channel (carrier
plus make-up gas) and the reference channel (reference gas).
Helium is the recommended carrier gas due to its high thermal conductivity and
chemical inertness. Low conductivity gases (argon, nitrogen) are used for special
analytical requirements. With special precautions, you can also use hydrogen as
the carrier and detector gas.
WARNING! Hydrogen is a potentially dangerous gas. Refer to Using Hydrogen for safety
information.
Table 9-1 contains information about the thermal conductivity of several gases.
Detector gas flow rates are factory preset to 30 mL/min for reference and to
27 mL/min for make-up. To modify the detector flow setting refer to Chapter 3.
To ensure the correct regulation of detector gases a minimum line pressure of 4
bar (400 kPa, 60 psi) must be provided to the instrument.
WARNING! The TCD DGFC has a pressure sensor that generates an error signal in case the gas
line pressure goes below 1 bar (100 kPa, 14 psi). In this case the TCD filament power
is automatically switched off.
Table 9-1. Thermal Conductivity of Gases
Gas
Thermal Conductivity (
λ
x10
7
) at 0 ºC
where
λ
=Cal/cm x sec. x ºC
Hydrogen 4130
Helium 3363
Methane 720
Oxygen 583
Nitrogen 580
Carbon Oxide 540
Argon 406
Carbon Dioxide 343