DETECTORS
Electron Capture Detector - ECD
Theory of Operation
TheradioactiveNickel63sealed inside theECDdetectoremitselectrons(betaparticles)whichcollide
withand ionize themake-upgasmolecules(eithernitrogenorP5). Thisreactionformsastablecloudoffree
electrons in theECDdetectorcell. TheECDelectronicswork tomaintainaconstantcurrentequal to the
standingcurrent through theelectroncloudbyapplyingaperiodicpulse to theanodeandcathode. The
standingcurrentvalue isselectedby theoperator; thestandingcurrentvaluesets thepulse rate through the
ECDcell. Astandingcurrentvalueof300means that thedetectorelectronicswillmaintainaconstantcurrent
of0.3nanoamperes through theECDcellbyperiodicallypulsing. If thecurrentdropsbelow thesetstanding
currentvalue, thenumberofpulsespersecond increases tomaintain thestandingcurrent.
Whenelectronegativecompoundsenter theECD
cellfrom thecolumn, they immediatelycombinewith
someof the freeelectrons, temporarily reducing the
number remaining in theelectroncloud. When the
electron population is decreased, the pulse rate is
increased tomaintainaconstantcurrentequal to the
standing current. Thepulse rate is converted to an
analogoutput,which isacquiredby thePeakSimple
datasystem. Unlikeotherdetectorswhichmeasurean
increaseinsignalresponse,theECDdetectorelectronics
measurethepulserateneededtomaintainthestanding
current.
ExamplePulseTrains
Thepulse rate is increased in thepresenceof
electronegativecompounds
Detector electronicspulse tomaintain the
standingcurrent
ECDDetectorOperational Diagram
CATHODE
ANODE
ECD cell
Standing
current (equal
andopposite to
thecell current)
Integratingamplifier
Voltage-controlled
variable frequency
pulser
Offsetcontrol
Frequency /voltage
converter
Analogvoltageoutput
toPeakSimpledata
system
Sample-laden
carriergas
inlet
Gasoutlet
Makeupgas
inlet
Electroncloud
Nickel 63 foil
