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 increased response.
- 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).