DETECTORS
FID - Flame Ionization Detector
In theSRIFID, thecarriergaseffluentfrom theGCcolumn ismixedwithhydrogen, thenrouted through
anunbreakablestainlesssteeljet. Thehydrogenmixsupportsadiffusionflameatthejet’stipwhichionizesthe
analytemolecules. Positiveandnegativeionsareproducedaseachsamplecomponentiselutedintotheflame.
Acollectorelectrodeattractsthenegativeionstotheelectrometeramplifier,producingananalogsignalforthe
datasystem input. Anelectrostatic field isgeneratedby thedifference inpotentialbetween thepositively
chargedcollectorelectrodeand thegroundedFID jet. Becauseof theelectrostatic field, thenegative ions
have toflow in thedirectionof thecollectorelectrode.
Theory of Operation
Theratioofair tohydrogen in thecombustionmixtureshouldbeapproximately10:1. If thecarrierflow is
higherthannormal, thecombustionratiomayneedtobeadjusted. FlowisuseradjustedthroughtheElectronic
PressureControllers (EPC); the ratesused togenerate testchromatogramsat the factoryareprintedon the
right sideof theGC in the flow ratechart. TheFID temperaturemustbehotenoughso thatcondensation
doesn’toccuranywhere in thesystem;150
o
C issufficientforvolatileanalytes;forsemi-volatiles,useahigher
temperature. Inadditiontousing theignitortolighttheflame, itmaybeleftonatanintermediatevoltagelevel
toprevent flameout (-750or7.5volts). The ignitor isverydurableandwill last a long time, evenathigh
temperatures.
TheFIDhydrogendiffusion flame
FIDdetectorschematic
