The nitrogen–phosphorus detector (NPD) or thermionic specific detector (TSD) is a type of detector commonly used with gas chromatography, in which thermal energy is used to ionize an analyte. It is a type of flame thermionic detector (FTD), the other being the alkali flame-ionization detector (AFID). 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 concentration of hydrogen gas is used such that it is just below the minimum required for ignition. A rubidium or cesium bead, which is mounted over the nozzle, ignites the hydrogen (by acting catalytically), and forms a cold plasma. Excitation of the alkali metal results in ejection of electrons, which in turn are detected as a current flow between an anode and cathode in the chamber. As nitrogen or phosphorus analytes exit the column, they cause a reduction in the work function of the metal bead, resulting in an increase in current. Since the alkali metal bead is consumed over time, it must be replaced regularly .
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