NPD detector, Agilent 6890
The nitrogenophosphorusdetector uses a jet and collector similar to
the FID; however, the collector contains a small alumina cylinder
coated with a rubidium salt (the active element) which is heated
electrically. In the presence of this thermionic source, nitrogen and
phosphorus containing organic molecules are efficiently ionized. Ions
are collected, and the resulting current is measured.
H2 and air are required, but at flows significantly less than those
for an FID. Normal FID type ionizations 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.
- Increasing detector temperature. This increases the active element
temperature and the response.
- Increasing electrical power to the
active element. This increases active element temperature and
increases the response.
- 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
- 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.
- 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.
Other gas flow effects of too high flow rates of the hydrogen may allow a true
flame to exist around the active element. This would overheat the
active element severely and destroy the specific response. Too low
flow rates of air tend to quench the background response of the active
element, and this results in a reoequilibrationtime that is too long
to establish proper background response (negative solvent peaks
killing the active element).