Agilent Technologies 6890 Series Service Manual page 239

320 Nitrogen Phosphorus Detector (NPD)
Correcting NPD hardware problems
Baseline does not recover after solvent peak
• Create a time table that turns hydrogen off at the time of injection. When
the solvent has passed through the detector, restore the hydrogen flow to
the operating level. The NPD will usually recover rapidly to a stable base-
line.
Add makeup gas at a flow rate of 5 mL/min.
A large 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).
Adjust offset does not function properly (it is either too high or too low
by hundreds of pA)
• A flame is burning at the top of the jet. If the hydrogen flow is too high,
the flame at the tip of the jet will continue burning. Turn off the hydrogen
flow completely and decrease the flow rate. The hydrogen flow should
never be higher than 4.0 mL/min.
Large solvent signal with very small NPD signal
• Check the hydrogen flow rate. If it is too high, a flame could be burning
at the tip of the jet. Turn off the hydrogen flow completely and decrease
the flow rate. The hydrogen flow should never be higher than 4.0 mL/min.
• The collector may be contaminated. Change the collector and ceramic
insulators.
Peak tailing
• Verify that a good liner and column are being used.
• Some polar compounds tail due to contact with the metal collector. The
optional extended jets are recommended.
• Some compounds cause peak tailing, especially those containing phos-
phorus. The optional black ceramic bead is recommended for phosphorus.
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Detectors
Agilent 6890 Gas Chromatograph Service Manual
Jun 2001