The nitrogen–phosphorus detector (NPD) is also known as thermionic specific detector (TSD) is a 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 also known as AFD).
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 .
- Wolfgang Kleiböhmer (2001). Environmental Analysis. Elsevier. pp. 8–. ISBN 978-0-444-50021-2.
- Burgett, Charles A.; Smith, Douglas H.; Bente, H.Bryan (1977). "The nitrogen-phosphorus detector and its applications in gas chromatography". Journal of Chromatography A. 134 (1): 57–64. doi:10.1016/S0021-9673(00)82569-8. ISSN 0021-9673.
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