Table of Contents
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Chapter 13
Maintaining a Nitrogen Phosphorus Detector
NPD Troubleshooting
Sensitivity
The main cause of changes in sensitivity is related to the Thermionic source. Its
gradual depletion results in a drop of sensitivity, that can be compensated by
increasing the source current. Take into account that increasing current results in a
shorter source life.
Loss of sensitivity are also related to source contamination, due to high boiling
sample compounds not completely eliminated. Shift of detector temperature can
reduce sensitivity.
Gases
The gases normally used for NPD are reported in Table 13-2. Consistent flows
are necessary to maintain a constant and stable sensitivity.
Nitrogen is preferred over helium as make-up gas due to its lower thermal
conductivity. Using nitrogen, the source requires a lower heating current.
NOTE
When a packed column is used, make-up gas generally is not necessary.
WARNING! It is the customer's responsibility to ensure compliance with all local safety
regulations concerning gas supplies.
Hydrogen is a potentially dangerous gas. Mixed with air it may give rise to an
explosive mixture. The use of hydrogen requires the operator's extreme caution due
to the risks involved. For further details concerning hydrogen, please refer to
Hydrogen
264
Carrier gas
Capillary Columns
Helium
Nitrogen
Hydrogen
on page xvii.
Table 13-2. Gases used for NPD
Packed Columns
Helium
Nitrogen
Maintenance and Troubleshooting Manual
NPD Troubleshooting
Make-up gas
Detector Gas
Gas
Nitrogen
Hydrogen +
Air
Helium
Using
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