HP 5890 Series II Plus Reference Manual page 130

Detector Systems
Nitrogen-phosphorus detector (NPD)
H and air are required, but at flows significantly less than those for an
FID. Normal FID•typeionizations are therefore minimal, so response to
compounds not containing nitrogen or phosphorus is reduced. Thus, the
detector is both sensitive to and selective toward only compounds
containing nitrogen and/or phosphorus.
The electrical power for heating the active element is supplied through a
toroidal transformer located inside the NPD detector cover. The toroidal
transformer secondary winding is connected directly to the
collector/active element assembly. The electrical heating current passes
directly through the small platinum wire that is also used to position the
active element inside the collector.
The active element of the NPD operates in a very delicate thermal
balance that is dependent on several different variables. The magnitude
of the response of the NPD is a function of the temperature of the active
element and of the active zone around the active element itself. Because
of this temperature dependence, the output of the detector is very
sensitive to anything that affects the temperature of this active zone.
Some of the important variables and their effects are listed below.
1. Increasing detector temperature. This increases the active element
temperature and the response.
2. Increasing electrical power to the active element. This increases
active element temperature and increases the response.
3. Increasing hydrogen flow. This increases the active element
temperature as well as increasing the size of the active zone around
the active element; both effects will result in increased response.
4. Increasing air flow to the detector. Normally this cools the active
element slightly and decreases the response. (The overall effect is
much less than the hydrogen flow effects.) Increasing the air flow also
decreases the residence time of a given peak in the active zone of the
active element and decreases response.
5. Increasing the carrier gas flow. This cools the active zone slightly,
decreasing response. This also decreases the residence time of a
component in the active zone and decreases response.
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