76 Advanced User Guide
5 Detectors
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 Agilent
data system 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.
Setting parameters for the TCD
1 Press [Front Det] or [Back Det].
2 Set the detector temperature. Do not set higher than the
maximum temperature allowed for the column because part of
the column passes through the heated block and into the cell.
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