DETECTORS
Flame Photometric Detector - FPD
Overview
TheFlamePhotometricDetectorissimilartotheFIDinthatthe
sampleexitstheanalyticalcolumnintoahydrogendiffusionflame.
Where theFIDmeasures ionsproducedbyorganiccompounds
duringcombustion, theFPDanalyzes thespectrumof lightemitted
by thecompoundsas they luminesce in the flame. Thedetector
chambermustbe light tightso thatonly lightfrom theflamewillbe
“seen”by thephotomultiplier tube(PMT)andanalyzed. TheFPD
usesasecondhydrogenflow topurge theopticalpathbetween the
PMTandthehydrogendiffusionflame. Thissecondhydrogenflow
helpstoaugmenttheFPDsensitivitybymaking
theflamehydrogenrich. TheFPDalsousesa
secondair flowdirectedacrossof the faceof
thePMT topreventheliumand/orhydrogen
molecules frompermeating thePMT’sglass
windowandcausingmalfunction. Thispurge
airisventedtoatmospherethroughashorttube,
coiled toprevent lightfromreaching thePMT.
TheFPDusesoneof twoavailablebandpass
filters to selectively detect compounds
containing sulfurorphosphorus. Compounds
containingphosphorusaredetectablewith the
526nmfilter,which isyellowononeside. The
394nmfilter(blueononeside)allowsdetection
of sulfur-containingcompounds. Whilenot
completelyselective, theFPDis100,000times
moresensitive tosulfurousandphosphorous
compounds than it is tohydrocarbons. Sulfur
compounds likeH
2
SorSO
2
canbedetected
down to about 200ppb; phosphorus
compounds canbe detecteddown to10ppb. Todetect
phosphorus and sulfur at the same time, theDual FPD,
featuring twoPMTs,maybeused. ThesingleordualFPD
may be equippedwith an FID collector electrode and
electrometerwhichwilldetect thehydrocarbonpeaksas
the PMTs are responding to the sulfur and phophorus
compounds. Becausethehydrogen-richflamerequiredfor
optimumsulfurandphosphorusdetection isnotoptimum
for thebesthydrocarbonresponse, theFID incombination
with theFPD is lesssensitive thanapureFIDresponse.
FPDdetectorequippedwithanFIDcollector
Exhaust
tube
Ignitor
FIDcollector
electrode
Bandpass
filterand
PMT
FIDcollector
electrode
PMT
housing
airpurge
outlet
Hydrogen&
airsupplies
FPD/FID
detector
Exhaust
capnut
