Asdiagrammedabove, thesampleenters theFIDflamefrom thecolumnwherehydrocarbonsare ionized
andcombusted. Electrons liberated in the ionizationarecollectedby theFIDcollectorelectrode. Abouthalf
thegaseffluent (carriergas+hydrogen+air+combustionproducts) flowsout through theFIDcollector
electrodewhichhasan internaldiameterof .040 (1mm.). The restrictioncausedby thesmallcollector i.d.
splits the flowofexhaustgasesso that theotherhalfof thegasespass through theDELCD. TheDELCD
consistsofasmall ceramic tubewhich isheated to1000ºC. In thecenterof theheated tube isaplatinum
thermocouplewhichmeasures the temperature and aDELCD collector electrodewhichmeasures the
conductivityofthegasesflowingthroughtheDELCD. Sincetheresponseisverydependentonthetemperature
, thecontrolcircuitmustmaintainthetemperaturewithinafractionofadegreeat1000ºC. ClO
2
-BrO
2
exhibits
extremelyhighconductivityat1000ºC. So theDELCDactuallyresponds to theClO
2
-BrO
2
concentrationof
thegases in theFIDexhaust. Becauseothermoleculesarenotdetected, theDELCD isalmostcompletely
selectiveforchlorineandbromine. Fluorineand iodinearenotwelldetected.
DETECTORS
FID/DELCD
