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The overall design of a nitrogen-phosphorus detector (NPD) is similar to a flame-ionization detector (FID). The major difference is that the hydrogen/air flame of the FID is replaced by a heated rubidium silicate bead in the NPD. The effluent from the GC column passes through the hot bead. The hot rubidium salt emits ions when nitrogen and phosphorus-containing compounds pass over it. The ions are collected on a collector above the heated bead to produce a current, similar to the FID.
Schematic of NPD
See the FID document.
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