The Nitrogen-Phosphorus Detector responds to nitrogen-phosphorus compounds about 100,000 times more strongly than normal hydrocarbons. Due to this high degree of selectivity, the NPD is commonly used to detect pesticides, herbicides, and drugs of abuse.   
   
The NPD is similar in design to the FID, except that the hydrogen flow rate is reduced to about 3mLs/minute, and an electrically heated thermionic bead (NPD bead) is positioned near the orifice. Nitrogen or phosphorus containing molecules exiting the column collide with the hot bead and undergo a catalytic surface chemistry reaction. The resulting ions are attrtacted to a collector electrode amplified and output to the data system.