1) TheDELCDcanbeoperated inahighsensitivitymodebyeliminating thehydrogenfrom thereactions
which leadup to thedetectionof theClO
2
-BrO
2
. Because thechlorine/bromineatomsprefer toreactwith
hydrogen toformnon-detectableHCl-Hbr, thanwithoxygen toformdetectableClO
2
-BrO
2
byafactorof
100-1000 to1,eliminating thehydrogen improves theDELCDsensitivitybyat least100 times. Water
mustalsobeeliminatedasat thehigh temperatures inside theDELCD,hydrogenbecomesdissassociated
from theH
2
Omoleculeandavailableasareactant. Inpractice, thismeans turningoff thehydrogenand
usingcleandry tankair(not thebuilt-inaircompressor).
2) RemovethehydrogensupplyfromtheGCbydisconnectingthehydrogensupplyattheGC’sinletbulkhead
on the lefthandsideof the instrument. Reduce theairflow to theDELCD to50ml/minby turning theair
pressuresetpointdown to1-2psi. Anadditionalairflowrestrictorconsistingof12”of .067 tubing(1/16’,
1.58mm)withan internaldiameterof .010(0.25mm)caneasilybeadded to theairsupply immediately
below thedetector toenable theflow tobecontrolledmorepreciselyathigherpressures. With theextra
restrictor installedapressuresetpointof10psiwilldeliveranair flowofapproximately50ml/min.
3) Ifusingacapillarycolumn,pushthecolumnthroughtheFIDjetuntilitjustenterstheceramictubingofthe
DELCD. Thiswillimprovethepeakshapesomewhatbecausethecolumneffluentwillbedischargedinto
theflowingairstreamandwillbe immediatelyswept into theDELCDdetectorvolume. Whenswitching
back toFID/DELCDcombomoderemember topull thecolumnback into theFID jet.
4) Remove theFIDcollectorelectrodeand replace itwitha1/4’cap fitting. TheFIDcollectorelectrode
allowssomegas toescapefrom theFIDcombustionarea,and this isnotdesirablewhenoperating in the
highsensitivitymode.
The DELCD chromatogram
shown at right illustrates the
response to 10 picograms
(1ul of 10PPB)ofTCE in the
highsenstivitymode.
Note that in high sensitivity
mode, there is some
response to the methanol
solvent.
Operating the FID/DELCD in the high sensitivity DELCD only mode
DETECTORS
FID/DELCD
Tenpicograms (10PPB)of
Trichloroethylene (TCE)
