Detector Systems
Thermal conductivity detector (TCD)
Note that TCD response becomes relatively flat (insensitive) to reference gas flow rates equal to, or somewhat greater than, flow rate through the column.
Analyzing for hydrogen, special considerations
Only H has thermal conductivity greater than He. However, binary mixtures of small amounts of H (< 20%) in He at moderate temperatures exhibit thermal conductivities less than either component alone.
Thus, depending on concentration and temperature, a H peak may appear as positive, negative, a split peak, or as a W when using He as carrier. The phenomenon disappears at higher temperatures.
Note: Alternatively, N or Ar may be used as carrier when analyzing for
H; this eliminates problems inherent in using He as carrier, but detector sensitivity is substantially reduced toward components other than H .
To ensure normal peak shape in H analyses with He as carrier, it is advisable to operate the detector at relatively high temperature (between 200 and 300 C).
A specific detector operating temperature is found by analyzing a known range of H concentrations, increasing operating temperature until the
Hpeak 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 good linear dynamic range.
Since an H peak is negative, detector polarity must be inverted at appropriate times so the peak appears positive to a connected integrator or chart recorder.
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