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Jun 2001
Detectors
Agilent 6890 Gas Chromatograph Service Manual
Nitrogen Phosphorus Detector (NPD) 320
Correcting NPD hardware problems
The baseline drifts (upward) significantly during an oven program
• If the oven temperature is increasing dramatically during a run (for exam-
ple, from 50 to 350°C) a change of between 10 and 15 pA is normal. Howev-
er, if you suspect that the baseline drift is excessive, heat the inlets and
oven to a temperature above 300°C for 60 minutes to eliminate excess
baseline drift during oven programs.
• Verify that the detector insulation is not cracked or damaged.
High detector baseline of GC at room temperature
• Moisture in the detector can cause the baseline to be at 10s or even 100s
of pA when the detector is at a low (such as room) temperature. Set the
detector temperature to 150°C with the detector gases on. The baseline
should drop below 1 pA in approximately 10 minutes.
The signal baseline does not fall below 3 pA when the bead voltage is 0
• The ceramic insulators may be dirty. The insulators must be very clean
for NPD performance to be satisfactory.
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