Figure 4.20
Flame ionization detector (FID).Nitrogen/Phosphorus Detector
This is a more recent introduction which is (NPD), basically an FID, and can in fact be used as such, if
a selective response to phosphorus and/or nitrogen compounds is not required. Nitrogen and phosphorus
response is achieved through the incorporation of an electrically heated glass bead containing a
rubidium silicate which is positioned a few millimetres above the burner jet. By heating the bead to
600 – 800 °C and applying a negative potential (– 180 V) to it, a reaction cycle occurs involving the
vaporization, ionization and recapture of rubidium by the bead. During this recycling of rubidium,
which prevents its long-term depletion in the bead, an electron flow to the positive collector electrode
occurs. This flow or background current is enhanced when nitrogen or phosphorus compounds are
eluted, as they form radicals in the flame which participate in the reaction cycle and accelerate the rate
of rubidium recycling. Thus, nitrogen compounds are believed to form mainly cyanide radicals which
then abstract electrons from vaporized neutral rubidium atoms to form CN– ions. The CN– ions are
captured by the collector electrode, thereby increasing the detector current, whilst the Rb+ ions are
recaptured by the bead. The phosphorus response is thought to be due to similar processes involving
and species.The detector can be made to respond to phosphorus compounds only by earthing the jet, which is at a
negative potential for simultaneous nitrogen and phosphorus detection, and altering the flow rates of the
flame gases. If