Nitrogen–phosphorus detector

The Nitrogen Phosphorus Detector (NPD) is a form of gas chromatography in which thermal energy is used to ionize an analyte. With this method, nitrogen and phosphorus can be selectively detected with a sensitivity that is 10⁴ times greater than that for carbon. NP-Mode: A minimum concentration of hydrogen gas is used such that it is just below the minimum required for ignition. A rubidium or caesium bead, which is mounted over the nozzle, ignites the hydrogen (by acting catalytically), and entails the formation of a cold plasma. Excitation of the alkali metal results in ejection of electrons, which in turn is detected by current flow between an anode and cathode in the chamber. In the presence of nitrogen or phosphorus combustion products exiting the column, the work function of the metal is reduced, resulting in an increase in the current measured.

Since the alkali metal bead is over time consumed, it must be regularly replaced - a disadvantage compared to the related FID detector which lacks the selectivity for nitrogen and phosphorus.

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