Oneof fourTCD filaments
TCD filamentbridge
red/green
black/green
red/blue
black/blue
R S
S R
DETECTORS
Thermal Conductivity Detector - TCD
Theory of Operation
TheTCDdetectormeasures thedifference in thermalconductivity in thecarriergasflowand theanalyte
peaks. Everycompoundpossessessomedegreeof thermalconductivity,andmay thereforebemeasuredwith
aTCDdetector. Due to itshigh thermalconductivityandsafety,heliumcarrier ismostoftenusedwithTCD
detectors. However,othergasesmaybeusedsuchasnitrogen,argon,orhydrogen.
TheWheatstoneBridgecircuit
design in the SRI TCD uses four
general-purpose tungsten-rheniumfilamentsforsampleanalysis. Twoof
thefilamentsareexposedtothesample-ladencarriergasflowandprovide
theactualchromatographicsignal. Theother twofilamentsareprovided
withcleancarrier flow,enabling them tobeusedasabaseline reference
signal. When theeffluent from thecolumn flowsover the twosample
streamfilaments, thebridgecurrent is
unbalanced with respect to the
reference signal. This deflection is
translated intoananalogsignalwhich
issent to thedatasystemforanalysis.
Thefourpairsoffilamentleadsare
color-coded in two-colorunits; each
colorisusedontwodifferentleads. All
eightwiresareconnectedtothebridge
currentsupplyviafoursetscrew-type
terminalconnectorson the topcontrol
paneloftheGC. Silkscreenedlabelling
on thechassis indicateswhichcolor
wireconnects toeach terminal.
TheTCDdetectorblockisdivided
into twocellscontaining twofilamentseach. Onecellholds thereferencepairwhile theothercellholds the
samplepair. AllfourTCDfilamentsarephysically identicalexceptfor theircolor-coding. Thecarriergas is
plumbedso that isexits theElectronicPressureControllermodule,flows through thepolishingfilter, through
thereferencesideoftheTCDbridge, thenthroughtheinjectionporttothecolumn,andfromthecolumntothe
samplesideof theTCDbridge. After the flowpasses through thesamplecell, it isdirectedbackoutof the
TCDovenand into thecolumnoven through theTCDdetectoroutlet,where itmayberouted toasubsequent
detectoror tovent. All fourTCDdetector inlet/outlet tubesare1/16”stainlesssteel.
Simplificationof filament interconnection
Color-coded
filament leads
Referencegas in
REFERENCE
SAMPLE
Samplegas in
REFERENCE
SAMPLE
Red
Green
Red
Blue
Green
Black
Blue
Black
TCD carrier gas flow diagram
