Theory of Operation
Inside theNPDdetectorbody,anelectricallyheated thermionicbead(NPDbead) ispositionedbetween
thejetorificeandthecollectorelectrode.Thebeadiscoatedwithanalkalimetalwhichpromotestheionization
ofcompounds thatcontainnitrogenorphosphorus.Hydrogenandairflowscreateahydrogenplasmaaround
thehotNPDbead.Whenmoleculescontainingnitrogenorphosphorusenter theplasmafrom thecolumnand
jetorifice, theyundergoacatalyticsurfacechemistryreaction,producing thermionicelectrons.Theresulting
ionsareattracted toapositivelychargedcollectorelectrode, thenamplifiedandoutput to thedatasystem.The
hydrogentoairratioistooleantosustainaflame,thereforeminimizinghydrocarbonionizationandcontributing
to theNPDdetector’sselectivity.
Stainless steel jet
Compressedair feeds
thehydrogen flame
Sample-ladencarriergas flowing
from thecolumn to the jet
Positivelychargedcollector
electrodedoublesas the
exhaustvent
ThermionicNPDbead
Column
Hydrogengas
Detectorviewport
Hydrogen
plasma
NPDheater
block
NPD jet
NPD bead (the tip is inside the
ionizationareaof thedetectorbody)
Electrical leads provide current to the NPD
bead tomaintain thehydrogen-airplasma
Detectorviewport
capnut
Positivelycharged
collectorelectrode
Electrical lead
toamplifier
Areawhere
ionization
takesplace
DETECTORS
Nitrogen/Phophorus Detector - NPD
