Gas Chromatography coupled with the Nitrogen Phosphorous Detector makes it possible to analyze organic compounds containing nitrogen or phosphorous.
The NPD is a selective ionizing detector which uses a hydrogen/air flame but whose flow rate in hydrogen is much lower than in the FID. This prevents a flame from being maintained at the tip of the jet but allows the formation of cold plasma.
The principle is based on the variation of the electrical condutibility of the flame. Unlike the FID, the NPD contains a bead of rubidium or alkali cesium chloride mounted in an array of silica between the jet and the polarization electrode.
This bead is fused on a platinum wire, heated by a coil and emits electrons by thermionic emissions which are collected at the level of the anode and provide the basic current.
When a component containing nitrogen or phosphorous leaves the column, the partially burnt nitrogen and phosphorus compounds are adsorbed on the surface of the bead. The quantity of electrons emitted increases as well as the current put together at the collecting electrode which corresponds to the response of the detector.