HP 5890 Series II, Reference Manual

Page: 148 / 244

Detector Systems
Thermal conductivity detector (TCD)
148
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
1
has thermal conductivity greater than He. However, binary
mixtures of small amounts of H
1
(< 20%) in He at moderate temperatures
exhibit thermal conductivities less than either component alone.
Thus, depending on concentration and temperature, a H
1
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
1
or Ar may be used as carrier when analyzing for
H
1
; this eliminates problems inherent in using He as carrier, but detector
sensitivity is substantially reduced toward components other than H
1
.
To ensure normal peak shape in H
1
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
1
concentrations, increasing operating temperature until the
H
1
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 good
linear dynamic range.
Since an H
1
peak is negative, detector polarity must be inverted at
appropriate times so the peak appears positive to a connected integrator
or chart recorder.