Detectors 5
Advanced Operation Manual 179
compounds may attack the filament. The immediate symptom
is a permanent change in detector sensitivity due to a
change in filament resistance.
If possible, such compounds should be avoided. If this is not
possible, the filament may have to be replaced frequently.
Changing the TCD polarity during a run
Negative polarity On inverts the peak so the integrator or
ChemStation can measure it. Negative polarity can be a run
table entry; see “Run Time Programming” on page 12.
Detecting hydrogen with the TCD using helium carrier gas
Hydrogen is the only element with thermal conductivity
greater than helium, and mixtures of small amounts of
hydrogen (<20%) in helium at moderate temperatures exhibit
thermal conductivities less than either component alone. If
you are analyzing for hydrogen with helium carrier gas, a
hydrogen peak may appear as positive, negative, or as a split
peak.
There are two solutions to this problem:
• Use nitrogen or argon- methane as carrier gas. This
eliminates problems inherent with using helium as carrier,
but causes reduced sensitivity to components other than
hydrogen.
• Operate the detector at higher temperatures—from 200 °C
to 300 °C.
You can find the correct detector operating temperature by
analyzing a known range of hydrogen concentrations,
increasing the operating temperature until the hydrogen peak
exhibits normal shape and is always in the same direction
(negative relative to normal response to air or propane)
regardless of concentration. This temperature also ensures
high sensitivity and linear dynamic range.
Because hydrogen peaks are negative, you must turn
negative polarity on at appropriate times so the peak
appears positive.
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