HP 5890 Series II Plus Reference Manual page 133

Detector Systems
Nitrogen-phosphorus detector (NPD)
Residual silanizing reagents from derivatization, and/or bleed from
silicone columns, may coat the active element with silicon dioxide. This
decreases ionization efficiency, reducing sensitivity.
If silanizing is necessary, remove excess reagent before injection. Silicone
columns should be well conditioned and loaded less than 5%.
Active element lifetime
Lifetime of the active element is reduced by the silicon dioxide coating,
described above, and by irreversible loss of rubidium salt.
Rubidium loss is caused by overheating the active element, particularly if
element power is on when gas flows, particularly carrier, are interrupted.
The detector must be turned off or element power reduced to zero when
changing columns, replacing septa, and/or replacing gas cylinders. Power
to the element with gas flow off can destroy an element within a few
minutes.
Humidity also affects element lifetime adversely: keep the detector warm
(100 to 150 C) when not in use. A collector removed from an NPD for an
extended period of time, or spare collectors, should be stored in a
desiccator.
Element lifetime is extended by using the lowest element power possible,
consistent with maintaining sufficient detector sensitivity and selectivity
for the particular analyses.
Generally, sensitivity and selectivity toward nitrogen decreases first as
the element ages. Phosphorus response is affected less.
It is good practice to have a spare collector available for replacement
purposes. A recoating kit is available (Part No. 5080•8872)to rejuvenate
the active element in an old collector.
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