Due to its selective and sensitive response to nitrogen and/or phosphorous containing compounds, the system is commonly used to detect pesticides, herbicides, and drugs of abuse.
When this detector was invented by Karmen and Giuffrida in 1964 it was known as the alkali flame ionization detector (AFID) because it consisted of an FID to which was added a bead of an alkali metal salt. As it has continued to evolve, its name has also changed and it has been known as a thermionic ionization detector (TID), a flame thermionic detector (FTD), a thermionic specific detector (TSD), etc. It is most commonly known today as an NPD detector as it responds well to both nitrogen and phosphorous organic compounds.
The NPD shows selectively higher sensitivity when an alkali metal salt is present in the vicinity of the flame. In its present configuration, a bead of rubidium or cesium salt is electrically heated in the region where the flame ionization occurs. While the mechanism is not well understood, the detector does show enhanced detectivity for phosphorous-, nitrogen- and some halogen-containing organic substances.