Theory of Operation
TheSRIHIDdetectoruses twoelectrodeswhichsupporta lowcurrentarc through theheliummake-up
gasflow. Theheliummoleculesbetweentheelectrodesareelevatedfromgroundstatetoformaheliumplasma
cloud. As theheliummoleculescollapseback togroundstate, theygiveoffaphoton. Thesamplemolecules
are ionizedwhen theycollidewith thesephotons. Allcompoundshavingan ionizationpotential lower than
17.7eVare ionizeduponcontactwithphotonsfrom theheliumcloud. The ionizedcomponentmoleculesare
thenattracted toacollectorelectrode,amplified,andoutput to thePeakSimpledatasystem.
DETECTORS
HID - Helium Ionization Detector
NOTE: If thearcelectrode iscoveredwithTeflon
TM
(translucent) insulation, itshould leave1mmof its tip
exposed. If theflatelectrode iscoveredwithceramic(white) insulation, then the tipshouldbeflushwith the
edgeof the insulationsleeve. Thereshouldbea1-2mmgapbetween thearcelectrodes,and thisgapshould
becentered in thearccross.
Sample-laden
carrier flow
Makeupgas
flow
Collector
electrode
Ground
electrode
Arcelectrode
Heliumplasmacloud
Purplearc
Photons in the
make-upgas
stream
Ionization reaction:
photonscolliding
withsample
molecules
