gas chromatographic detectors
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Flame Ionization DetectorMost common detector
Carbon atoms (C-C bonds) are burned in a hydrogen flame.
A small portion of carbon atoms are ionized (about in !"!!!)"
#he ions carry a charge from the flame to the $alls of the detector
$hich surrounds the flame.#he charge is electronically amplified
and sent to a recording de%ice.
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Flame Ionization Detector
%ery robust
and reliable.
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&itrogen 'hosphorus Detector
#he &'D uses a bead of a compound such as rubidium silicateabo%e a et of *. #he bead is heated by an electric current
forming a plasma. &itrogen and phosphorous react in the plasma
forming specific ions that carry a small current to the charged
collector. #he &'D is electronically similar to the FID" but sincethere is no flame" hydrocarbon ionization does not occur.
#he response to & is !+, ! greater than response to C
#he response to ' is !, ! .higher than response to C.
#he linear dynamic range is !
It is only fairly reliable since the bead burns up
o%er time causing drift in the signal.
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Flame 'hotometric Detector
#he (F'D) is an element specific detector
Commercial instruments are limited to the detection of ' and /
#he analytes are burned in a *flame causing electrons to
mo%e to an e0cited unstable state. 1hen the electrons return
to the ground state" they emit a specific $a%elength of light
*2 nm for ' and +3 nm for /
#hese $a%elengths are monitored by a photomultiplier" amplified"
and turned into an electrical signal.
#his detector is sensiti%e to !-3g
4inear dynamic range for ' of only !+, !.
For /" the response is non-linear.
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Flame 'hotometric Detector
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5lectron Capture Detector
#he 5CD uses a radioacti%e source such as &i2+$hich produces
6eta particles $hich react $ith the carrier gas producing free
electrons. #hese electrons flo$ to the anode producing an
electrical signal . 1hen electrophillic molecules are present"
they capture the free electrons" lo$ering the signal. #he amount
of lo$ering is proportional to the amount of analyte present.
It is sensiti%e do$n to !-but the dynamic range is only about !.
It7s an e0cellent detector for molecules containing an electronegati%e
group such as Cl or F etc. (or deri%itized molecules).
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5lectron Capture Detector
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Atomic 5mission Detector
8ne of the ne$est gas chromatography detectors9uite e0pensi%e compared to other :C detectors
#he strength of the A5D lies in the detector;s ability to
simultaneously determine elements. It uses micro$a%e energy
to e0cite helium molecules (carrier gas) $hich emit radiation
$hich brea
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Atomic 5mission Detector
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'hotoionization Detector
#he photoionization detector ('ID) uses a => lamp(0enon"
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'hotoionization Detector
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#hermal Conducti%ity Detector
Consists of an electrically-heated $ire or thermistor.
#he temperature of the sensing element depends on the thermalconducti%ity of the gas flo$ing around it. Changes in thermal
conducti%ity" such as $hen organic molecules displace some of
the carrier gas" cause a temperature rise in the element $hich is
sensed as a change in resistance.
#CD7s are often used to measure light$eight gasses or $ater(compounds for $hich the FID does not respond).
#he #CD is not as sensiti%e as other detectors but it is a
uni%ersal detector and is non-destructi%e.
Modern detectors called micro-#CD7s ha%e %ery small cell %olumes"
and ne$ electronics that produce much higher sensiti%ities and $ider
linear ranges. Due to its increased sensiti%ity" and the fact that it is a
uni%ersal non-destructi%e detector" it is again becoming more popular
for certain applications.
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#hermal Conducti%ity Detector
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#hermal Conducti%ity Detector
?epresentati%e #hermal Conducti%ity>alues" !! oC
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:C Detectors /ensiti%ities and ?anges