Agilent Technologies 6890 Series, Руководство по обслуживанию, Страница: 238 / 867

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Jun 2001
Detectors
Agilent 6890 Gas Chromatograph Service Manual
Nitrogen Phosphorus Detector (NPD) 320
Correcting NPD hardware problems
Correcting NPD hardware problems
No detector response to injected sample
• A solvent peak has extinguished the hydrogen/air plasma. Increase the
bead voltage. Run the detector at a higher offset (for example, 40 to 50
pA), or use makeup gas at a flow rate of 5 mL/min.
• Check that hydrogen is flowing to the detector. The flow rate should be
between 1.0 and 5.5 mL/min.
• The bead is not activated. Look through the vent hole on the detector lid
to see if the bead is glowing orange. If not, check that there is enough
current reaching the bead. Check the detector background signal. Reduce
the bead voltage to zero to establish a reference level, then look for a
sudden sharp increase in output as the bead voltage increases, which
indicates that plasma ignition occurred. If 4 V are being supplied to the
bead but it is not igniting, the bead is probably burned out. Replace the
bead.
• The bead power cable is bad.
• A contaminated upper ceramic insulator will cause a high offset (2 to 15
pA or more) when the bead voltage is off. This directly affects sensitivity.
Replace the ceramic insulator. Minimize the problem by running the detec-
tor hot (320 to 335°C).
No baseline; output signal exceeds 8 million
• The electrometer ribbon cable is not attached to the PC board properly.
Be sure to turn the GC off before reattaching the cable! If the signal does
not drop to a normal level (<3 pA), replace the electrometer.
• The collector is shorted to the detector housing. Check the insulators.
Baseline level is 0.0
• Broken electrometer