HP 5890 Series II Plus Reference Manual page 134

Detector Systems
Nitrogen-phosphorus detector (NPD)
Both detector baseline and sensitivity change with carrier flow rate due
to change in temperature of the active element. This is the reason for the
baseline drift in pressure•controlledinlet systems (capillary inlets) when
temperature•programmingthe column. The amount of change in the
detector response is proportional to the ratio of the total column flow
change (temperature sensitive) to the makeup gas flow (not temperature
sensitive), i.e., total column flow change divided by makeup gas flow.
Element power should be adjusted after any change in carrier flow rate.
Septa should be checked for leaks daily or replaced regularly. An upward
baseline shift at injection is often due to a leaky septum.
Turn the detector off to avoid damaging the active element before
Caution
changing septa.
When the detector is first turned on, its sensitivity and signal level
change slowly over several hours. Therefore, for applications requiring
very stable operation, leave the detector on overnight, lowering oven
temperature to prevent contaminating the active element with column
bleed.
To preserve the active element lifetime, it is good practice to reduce the
power to the active element whenever the detector will not be operated
for extended periods of time (such as over the weekend). To determine the
proper amount of power reduction, plot the normal offset and note the
displayed zero value (20-30 would be in the normal range). Then reduce
the power setting slightly until the displayed zero value (offset) just goes
to zero or to a value close to zero (lower than 5 picoamps). In this way the
temperature of the active element will be lowered such that there will be
little loss of rubidium but the active element will still be kept hot enough
to prevent contamination (condensation) while in standby.
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