DETECTORS
Dry Electrolytic Conductivity Detector - DELCD
Theory of Operation
FIDamplifier
lead
FIDcollector
electrode
Column
effluent
Column
Hydrogen
supply
FID ignitor
FID
flame
FID jet
Air
supply
Teflon
TM
ferrule
FID flameport
DELCD
anode
DELCD
heater
DELCDceramic
electrodeprobe
DELCD
thermocouple /
cathode
Graphite
ferrule
DELCDcollector, thermocouple,
andheater filament leads
Graphite
ferrule
Exhaust
vent
Vespel
TM
ferrule
DELCD insulation
Platinum
thermo-
couple
and
collector
wires
Collector
assembly
terminal
TheDELCDconsistsofasmallceramic tube—
theDELCD reactor—heated to1000
o
C. Inside the
reactor, a platinum thermocouple measures the
detector temperature, and a nichrome collector
electrode measures the conductivity of the gases
flowing through theDELCD. Thedetectorresponse
isdependent upon its temperature. Therefore, the
controlcircuitmustmaintainthetemperature,withina
fractionofadegree, at1000
o
C.
When combined with the FID detector, the
DELCD ismountedon theFID exhaust. Column
effluententerstheFIDflamewherehydrocarbonsare
ionized and combusted. Electrons freed in the
ionizationprocessarecollectedby theFIDcollector
electrode,whichhas an internal diameter of 1mm
(0.040”). Due to itssmallI.D., thecollectorelectrode
actsasarestrictor,splitting theFIDexhaustgasesso
that it takesabouthalfof theflow,and theremainder
isdirected to theDELCD. TheFIDexhaust gases
consist of un-combusted hydrogen and oxygen,
nitrogen, andwater and carbon dioxide from the
combustionofhydrocarbons. Thereactionofchlorine
orbromineandhydrogen formsHCl andHBr, and
thereactionofchlorineorbromineandoxygenforms
ClO
2
andBrO
2
. TheDELCDdetects theoxidized
speciesof chlorineandbromine, suchasClO
2
and
BrO
2
. It doesnot detect theacidsHCl orHBr like
theconventionalwetELCD. In thehydrogen rich
effluentfromtheFIDflame, thechlorineandbromine
preferentiallyreactwithhydrogen(orthehydrogenin
water) tomakeHCl-HBr. Givenequalavailabilityof
hydrogenandoxygenmolecules,achlorineatom is
100 timesmore likely toreactwith thehydrogen than
theoxygen. Therefore,theFID/DELCDcombination
is100 times lesssensitive than theDELCDoperated
with theFIDoff. TheSRIFID/DELCD isoperable
as a combination detector, as anFID only, or as a
DELCDonly.
ADELCDonlydetectorreceivesthesampleladen
carriergasdirectly from thecolumnor fromanon-
destructivedetectoroutlet, likethePID. Itismounted
on theheaterblockon thecolumnovenwall so that
thecolumneffluent ismaintainedat a temperature
consistentwiththeanalysis. Thistypeofhighsensitivity
DELCDusesheliumornitrogencarriergasandair
make-upgas.
FID/DELCD
Combination
Detector
