Toconvert the FID detector to NPD
detector:
1) The FlOand NPDdetectors are almost identical.
The detector
body
ismounted on a heated aluminum
block on the right hand side of the GC oven.
The NPDbody is slightly different from the FlO in
that the NPD flame jet does not protrudeas far into
the detector
body
as it does on the FlO. This allows
the NPD thermionic bead to be positioned directly in
front of the jet. Remove the FlO body from the
heated aluminum block and replace
it
with the NPD
body.
2) The photo at right shows the FIDINPDdetector
body
and both the
FrO
ignitor and
NPD thermionic
bead side by side for comparison. Both the FlO igni–
tor and NPD thermionic bead are inserted into the
detector body from the bottom. The ignitor is in–
serted until the tip of the ignitor is just below the tip of
the flame jet. while the NPDbead is inserted until the
heated part of the bead is directly in front of the
flame jet. For NPD operation, the samplemolecules
must collide with the bead in order to be ionized and
detected.
3) The gas flows to the NPDdetector are different
than the FlO gas flows. The NPO hydrogen flow is
normally about 3mUminwhile the FlO hydrogen flow
is about 25mllmin. To obtain this lower H2 flow rate,
an additional restrietor coil is attached to the hydro–
gen bulkhead fitting immediately below thedetector
body. With this additional restrictor coil in place, 10
psi hydrgen pressurewill result in a flow rate of about
3ml/min. The NPOair flow rate is typically about 100
mllmin, but this flow rate can be achieved by simply
reducing the air pressure from 8 psi to about 3 psi.
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