DETECTORS
Dry Electrolytic Conductivity Detector - DELCD
General Operating Procedure
TheFID/DELCDcombinationdetector
canbeoperated in theComboMode, theHighSensitivity
Mode(DELCDonly),or theFIDonlymode.
ComboMode
In theComboMode, theDELCD is operated after theFID; theFID signal is usually connected to
Channel 1on thePeakSimpledata system,while theDELCD signal isonchannel 2or3. Eachdetector
amplifier isfactory labeledwith thedatachannel towhich it isconnected. TheDELCDresponse in thismode
isuseablefrom1 to1000nanogramswithaslightlyquadraticcalibrationcurve. EPAandotherregulations
allow theuseofdetectorswithnon-linear response if theoperatorcalibrateswithsufficientdatapoints to
accuratelymodel thedetectorresponsecurve. Therefore, theDELCDmayrequirea6pointcalibrationwhere
5pointcalibrationisnormallyrequired.
1. Set thehydrogenandairflowsfornormalFIDoperation:set thehydrogenflow to25mL/minand theair
flow to250mL/min. Thepressurerequiredforeachflow isprintedon therighthandsideof theGCchassis.
(
NOTE:
Ifyou’reusingabuilt-inaircompressor, lowlevelsofhalogenatedcompoundsinambientair—even
levelsbelow1ppm—cancause theDELCD to losesensitivity, and fluctuations in the leveloforganics in
ambientairmaycauseadditionalbaselinenoise. Toavoid this,useclean,dry tankair.)
2. Set theDELCD temperaturesetpoint to260byadjusting theappropriate trimpoton the topedgeof the
GC’s front control panel. The number 260 represents 1000
o
C; theDELCDwill heat to about 254 and
stabilize. Theendof theceramic tubewillglowbrightreddue to thehigh temperature.
3. In thismode, theFIDamplifier isnormallyoperatedonHIGHgainor, if thepeaks aremore than20
secondswideat thebase,onHIGHFILTEREDgainforamorequietbaseline.
4. TheDELCDamplifier isnormallyoperatedonLOWgain.
HighSensitivityMode
TheDELCDcanbeoperatedalone in thehighsensitivitymodebyeliminatinghydrogen. Withhydrogen
eliminated,oxygen in theairwillreactwith thechlorinatedandbrominatedmoleculesat1000
o
CtoformClO
2
andBrO
2
,whicharedetectedby theDELCD. Watermustalsobeeliminated;at thehigh temperatures inside
theDELCD,hydrogendisassociatesfrom theH
2
Omoleculeandbecomesavailableasareactant toformHCl
andHBr,which theDELCDwillnotdetect. TheDELCDresponsecurve isquadratic in thehighsensitivity
modeas in theFID/DELCDcombomode,butsensitivity is increasedby100 to1000 times. In thismode, the
DELCDcanperformmuch likeanECD,except that theDELCD ismoreselectiveforhalogensandblind to
oxygen.Whenpossible,quantitatebytheinternalstandardmethod,usingachlorinated/brominatedcompound
for the internal standardpeak. Although theDELCDwillnotbedamagedby largequantitiesofchlorine/
bromine, there isashort term lossof sensitivity forabout anhour following the injectionof1µLofpure
methylenechloride,forexample.
1. Remove thehydrogensupplyby turning itOFF, thendisconnecting itat theGC’s inletbulkheadon the left
handsideof the instrument.
2. Reduce theairflow to theDELCD to25mL/minby turning the theairpressure trimpotsetpointdown to1
or2psi. Anadditional24”restrictormadeof0.001”I.D. tubingwouldbeusefulforfinepressureadjustment.
3. Ifyou’reusingacapillarycolumn,pushthecolumnthroughtheFIDjetuntilitjustenterstheceramictubing
of theDELCD. Thiswill improvepeakshapeas thecolumneffluentwillbedischarged into the flowing
airstreamand immediatelyswept into theDELCDdetectorvolumeby theairmake-upgas. (Whenswitching
back to theFID/DELCDcombomode, remember topull thecolumnback into theFID jet.)
4. TheFIDcollectorelectrodeallowssomegas toescapefrom theFIDcombustionarea,which isundesirable
for thehighsensitivitymode. Remove theFIDcollectorelectrodeandreplace itwitha1/4”capfitting.
