Gas chromatography

Dr. S. H. Burungale

1. GAS CHROMATOGRAPHY
Dr. S. H. Burungale
Head Department of Chemistry
Yashwantrao Chavan College of Science,
Karad-415124

2. GAS CHROMATOGRAPHY(GC)
Chromatographyisaphysicalmethodof
separationandanalysis.Contrasttochemical
methods,thismethodisveryquick,canbeused
withverysmallquantityofsampleandtheanalysis
canbedonewithhighaccuracyandprecision.

3. Inchromatography,thecomponentsofa
mixturearedistributedbetweentwophases:(1)
Stationaryphasehavinglargesurfacearea(a
poroussilicaorliquidcoatedoninertporoussolid)
and(2)mobilephase(agasorliquid)which
movescontinuouslyincontactwithstationary
phase.

4. Whenthemobilephaseisgas,itistermed
asgaschromatographyanddependingonthe
stationaryphaseused,itiseitherGas-solid
chromatography (GSC) or Gas-liquid
chromatography(GLC).

5. In,GCelutionmethodofdevelopmentis
used.Hereinertmobilephasegas,iskeptflowing
continuouslyincontactwithstationaryphasein
column.Thecomponentsofsampleeitheras
mixtureorpartiallyseparatedaretransportedfrom
oneendofthecolumntotheotherendofcolumn.

6. InGC,thestationaryphaseeitherfinely
dividedsolidorliquidcoatedoninertsolidistightly
packedinacolumnwithnarrowbore.Itis3to5
meterlong.Columniskeptinanovenmaintained
atconstanttemperature.Acarriergaswhichis
relativelyinertlikeN
2,Ar,H
2orHeisintroduced
throughoneendandiskeptconstantlyflowingin
thecolumn.
PrincipleofGCSeparation

7. Asampleisintroducedatapointwhichis
fewcentimetersawayfromentrancepointof
carriergas.Ifliquidmixtureofsampleisintroduced
itisinstantlyvaporisedinaheatedportand
insertedasasharpplug.

8. Whenthecomponentsofamixturecarriedby
carriergascomesincontactwithsolidstationary
phase,itgetsadsorbed.Adsorptionoccurs
accordingtotheFreundlichadsorptionisotherm;
x/m = KC
1/n
or
TheLangmuiradsorptionisotherm
x/m =CK
CK
2
1
1

9. Wherexisamountofsolutegetting
adsorbedonmgmofstationaryphase,Cis
concentrationofsoluteingaseousstateandallK
areconstants.nisinteger.
Ifstationaryphaseisliquidthesolutegets
dissolvedandHenry'slawisfollowed
x/m = KC

10. Boththephenomenonareselective.K
valuesaredifferentfordifferentsolutesonthe
samesorbentandhencedifferentamountofsolute
isgoingtostationaryphase.Anequilibriumis
establishedbetweenthesoluteonstationary
phaseandsoluteinmobilephase.

11. Asmobilephaseisconstantlyflowing,the
amountofsolutenotadsorbedissweptawaywith
mobilephaseandtomaintaintheequilibriumand
Kvalue,outofthesweptamountofsolute,some
willbeadsorbedonthenextpointagainto
maintainKvalue.Thisadsorptionanddesorption
keepsongoingsuccessivelyateverypointinthe
column.

12. NowcomponentshavingdifferentKvalueon
thatstationaryphase,willberetainedoncolumn
differentlyandhenceeachsolutewilltravelwith
differentspeed.EachsolutefollowsaGuassian
distributionandtravelswiththeshapeofapeak.
Asthelengthofcolumnisquitelarge,eachsolute
willcomeoutofthecolumnatdifferenttime.

13. Ifasuitabledetectoriskeptattheotherend
ofcolumn,thenassoonassoluteentersit,a
signalwillbeobtainedwhichisfedtorecorder.On
recorderthepeakforeachsoluteappearsat
differenttime,andthegraphshowingmultitudeof
peakscalledChromatogramisobtained.

14. 0X
0X
0X
0X
oX
Carrier
gas
Detector
Sample
(Binary mixture)
Sharp plug
Of sample
OX
OX
OX
OX
OOXX
OOXX
OOXX
OOXX
OO
OO
OO
OO
OO
OO
OO
OO
XX
XX
XX
XX
(i) (ii) (iii)
(iv)
XX
XX
XX
XX
Progress of separation

15. InstrumentusedinGC

16. MobilephaseusedinGC
Dependingonthemixturetobeseparated
anddetectorused,aninertgaslikeN
2,Argon,H
2
orHeisusedascarriergas.GenerallywithFID,
N
2orArgonisusedandwithTCDH
2orHeis
used.

17. Sampleintroductionsystem
Aliquidsampleinmicrolitrequantityis
introducedusinghypodermicsyringe,while
gaseoussampleisintroducedwithgastight
syringeorusinggassamplingvalve.

18. InGCseparationoccursingaseousstate
andhencetheliquidsampleintroducedhastobe
vaporisedinstantly.Injectionportisthereforekept
heatedatatemperatureabovethehighestboiling
pointofacomponentofamixture.

19. Column
ItisconsideredtobeheartofGC,where
separationoccurs.Columnsareof3types.
(1)Packedcolumns
(2)Wallcoatedopentubularcolumns(WCOT)
(3)Supportcoatedopentubularcolumns(SCOT)

20. Packedcolumnsareof3to5mlengthwith
internaldiameterof1.5to6mm.Herestationary
phasesolidorinertsolidonwhichliquidstationary
phaseiscoatedispackedasmicronsized
particles.
(1)Packedcolumns

21. WCOTiscalledacapillarycolumnwhichis
havinglengthof10-100m.withinternaldiameter
0.2to0.8mm.Hereliquidstationaryphaseis
directlycoatedonwall.Duetoextralength,any
complexmixturecanbeseparatedwithgood
resolution.Asthereisnopackinginside,the
carriergasdoesnotsufferanyresistanceand
separationisveryfast.Itsdrawbackisthat,it
cannotaccommodatemoresample.
(2)Wallcoatedopentubularcolumns

22. SCOThaslength10-25m.andinternal
diameter0.5mm.Hereliquidstationaryphaseis
coatedonmicronsizelayerofinertsolid.With
similaradvantagesofWCOT,moresamplecanbe
loadedonthiscolumn.
(3)Supportcoatedopentubularcolumns

23. GSChassolidstationaryphase.Solid
stationaryphaseusedareactivatedcarbon,silica
gel,aluminaormolecularsieves.GSChasvery
limitedapplications.Itisusedtoseparate
permanentgasesandlowmolecularweight
hydrocarbons.GSChasdrawbackthatduetonon
linearadsorptionisotherms,peaksshowtrailing.

24. Useofporouspolymerasstationaryphaseis
important.Divinylbenzeneandstyreneare
copolymerisedundercontrolledconditiontogive
porousbeadsofpolymer.Manytimesapolarliquid
monomerisincorporatedtogiveporouspolymer
withdefinitepolarity.Suchpolymersare
manufacturedandavailablewithtradenameof
“Porapaks”.Theyareavailablewithdifferent
polarityandtermedasPorapak-P,Q,R,S,NandT.

25. Someseparationsusingporouspolymers
areunique.(1)Separationofamixtureofreactive
gaseslikeCl
2,HCl,HCNetc.(2)Separationofa
mixtureofformaldehyde,waterandmethanol.
(3)Mixtureofethylene,ethane,acetylene.
Recentlypolymerlinedopentubularcolumns
(PLOT)whichis30mlongandID0.53mm.is
introduced.Itcanseparateamixtureofair,
methane,CO
2,ethylene,ethane.

26. GLCismoreusedthanGSCduetocertain
reasons.
(1)Itcanbeusedtoseparatehighmolecular
weightliquids.
(2)ItisfasterthanGSC.
(3)Thepeaksdonotshowtrailingandtheyare
symmetricalpeaks.
(4)Widechoiceofliquidstationaryphase.
GasLiquidChromatography(GLC)

27. Theliquidstationaryphaseiscoatedoninert
solid.Suchinertsolidparticlesareuniformly
shaped,sizedandhavesizelessthan10µm.
Mostwidelyusedinertsolidsare
diatomaceousearths.Theyareskeletal
remainsoftheunicellularalgaeknownas
diatoms.
(i)whitediatomaceousearths
(ii)pinkdiatomaceousearths.

28. Theydifferinmethodofpreparation.They
aremanufacturedbyJohn-ManvilleCorp.under
tradenameofChromosorb-W,Chromosorb-P.
Besidesthese,teflon,glassbeads,etc.areused
butnothavebecomepopular.
Liquidstationaryphaseusedshouldbenon
volatileatoperatingtemp.ofcolumn,shouldnot
beviscous,andshouldbesuitableforthe
componentstobeseparated.

29. Mostwidelyusedliquidstationaryphases
withtheirtemperaturelimitsandapplicationsare
givenbelow.
Theyareclassifiedas;
(1)Nonpolar
(2)Polar
(3)Intermediatepolaretc.

30. (a)Polarliquidstationaryphases
(1)Dimethylsulpholane
(2)Oxydipropionitrile
(3)Carbowax-M
(4)Versamid(Polyamideresin)
(b)NonPolar
(1)Hexadecane
(2)Squalene
(3)Methylsilicongum
(4)SE-30
(c)IntermediatePolar
(1)SE-52
(2)Didecylphthalate
(3)DC-550
(4)Benzylbiphenyl

31. Generallytoseparatepolarsolutes,polar
liquidsareusedandtoseparatenon-polarsolutes,
non-polarliquidsareusedasstationaryphase.
Liquidstationaryphaseisdissolvedin
volatilesolvents,mixedwithinertsolidsupportin
definiteproportionandthenvolatilesolvent
removedbyevaporation,whenliquidstationary
phasegetscoatedoninertsolidsupport.

32. Itisthenfilledinacolumntogetapacked
column.Generallyreadymadecolumnsare
purchasedfromskilledmanufacturer.
Columnsarekeptinovenwhichisheated
electrically.Itstemperatureiscontrolled
electronically.

33. Mostwidelyuseddetectorsare;
(1)Thermalconductivitydetector(TCD)
(2)Flameionisationdetector(FID)
(3)Thermionicdetector(TID)
(4)Electroncapturedetector(ECD)
(5)Flamephotometricdetector(FPD)and
(6)Sulphurchemiluminiscencedetector(SCD)
Thefirsttwodetectorsbeingverycommonwill
notbediscussed.Onlytheircomparisonisgiven.
Detectors

34. TCD FID
It responds to all compounds. It is
universal detector.
It responds only to organic compounds
and does not respond to NH3, H2O,
H2S, nitrogen oxides, sulphur oxides
etc.
It is less sensitive than FID. It is more sensitive than TCD.
Its sensitivity depends on flow rate of
carrier gas.
Its sensitivity does not depend upon
flow rate of carrier gas.
It is non destructive detector and hence
suitable with preparative GC.
It is destructive detector and can not be
used with preparative GC.
Here carrier gas used are He or H2. Here N2 or Ar used as carrier gas.

35. ItismodifiedFID.Itisselectivefor
phosphorousandnitrogencontainingcompounds.
ComparedtoFID,thisdetectoris500timesmore
sensitiveforPcompoundsand50timesmore
sensitivetoNcompounds.Itisusefulfordetection
andmeasurementofPcontainingpesticides.
ThermionicDetector(TID)

36. Hydrogen
inlet
Igniter
Chimney
Exhaust
Collector electrode
Polarizing electrode
Air or oxygen
inlet
Column effluent

37. ItissimilartoFID,butinadditionitcontainsan
electricallyheatedrubidiumsilicatebeadnear
collectorelectrode,whichismaintainedatabout
180Vwithrespecttocollectorelectrode.Thehot
gasesthenflowaroundthebead.Theheatedbead
formsaplasma(gaseousconductingmixture
containingions)havingtemperature600
0
to800
0
C.
Inthisplasma,N&Pcompoundsformlargeno.of
ions,togivelargecurrent,andhighsensitivity.

38. Thisisaselectivedetectorwhichisusedfor
phosphorousandsulphurcontainingcompounds.
ItsconstructiondiffersfromFID.LikeFIDit
doesnotcontainthecollectorelectrodearoundthe
flameanddoesnotmeasuretheamountof
ionisation.Insteaditmeasurestheradiation
emittedbytheflamebythesamplecomponents.
Forthisitcontainsaphotomultipliertubewith
suitablefilters.Itisshowninthediagram.
Flamephotometricdetector(FPD)

39. Whentheeluentispassedinlow
temperaturehydrogen-airflame,phosphorous
convertstoHPOspecieswhichemitsradiation
centeredabout510and526nm.
SulphursimultaneouslyisconvertedtoS
2
specieswhichemitsradiationcenteredat394nm.
Desiredwavelengthisisolatedbyusingafilter
betweentheflameandphotomultipliertube.

40. Itcanalsobeusedtodetecthalogens,
nitrogencompounds and organometallic
compoundscontainingmetalchromium,selenium
andgermanium.
For sulphurcompounds, sulphur
chemiluminescencedetector(Tobedescribed
later)providesgreaterworkingrangeandmore
sensitivitythanFPD.

41. H
2gas Column
effluent
Air
PM
tube
Filter

42. Itistherecentaddition,tothefamilyof
detectorsusedinGC.
Itisbaseduponthereactionbetweencertain
sulphurcompoundsandozone.Thereaction
producesluminescenceandtheintensityofthis
chemiluminescenceisproportionaltothe
concentrationofsulphur.Thedetectoris
particularlyusefulforthedeterminationof
pollutantssuchasmercaptans.
SulphurChemiluminiscenceDetector(SCD)

43. InSCD,theeluentismixedwithhydrogen
andairandisburnedasinFID.Theresulting
gasesarethenmixedwithozoneandtheintensity
ofresultingluminescenceismeasured.
Forsulphurcompounds,itoffersveryhigh
sensitivityandmorelinearworkingrange.

44. Temperatureofcolumnisveryimportantin
separation,asthepartitioncoefficientofthesolute
betweenthetwophasesmuchdependson
temperature.Ascolumntemperatureincreases,
thesamplespendsmoretimeinmobilephase.
Thisresultsindecreasingretentiontime.(Timefor
whichsoluteremainsinthecolumn).Oppositeto
thatatlowercolumntemperature,thesoluteis
retainedmoreonstationaryphaseandretention
timeincreases.
TemperatureProgramming

45. Ifacomplexmixtureistobeanalysedinwhich
solutecomponentshavealargedifferenceinboiling
point,thentheseparationatconstantcolumn
temperature(isothermalmode)cancreate
difficulties.Atlowcolumntemperature,thelower
boilingsolutesarewellresolvedandelutedin
reasonabletimebutthehigherboilingsolutestake
longtimetoeluteandfewmaynotcomeoutofthe
column.Todecreasetheretentiontimeofhigher
boilingsolutes,ifcolumntemperatureistakenhigh
thenlowerboilingsolutesareelutedsoquicklythat
theyarenotproperlyresolved.

46. Thisproblemcanbeovercomebyusing
temperatureprogramming.Inthistechniqueinitially
thecolumniskeptatlowtemperature,andthenits
temperatureisraisedinaprogrammedmannerat
desiredheatingratelike5
0
C/min,10
0
C/minetc.
Whenthecolumnisatlowtemperature,thelower
boilingsoluteselutedeffectively,whilethehigher
boilingsolutesremaincondensedonthecolumn.
Asthetemperatureisgraduallyraised,theystart
evaporatingfromcolumnandeluteintheorderof
theirboilingpoints.

47. Todotemperatureprogramming,initial
temperature,desiredheatingrateandfinal
temperaturearesetonelectroniccontrols.Many
gaschromatographshavefacilityforthis.

48. Thoughveryefficientseparationand
analyticaltechnique,GLChaslimitationthatit
cannotbeusedforseparationofnonvolatileliquids
andsolids.Liquidshavingmaximumboilingpoint
250
0
CcanbeseparatedwithGLC.Nonvolatile
liquidsorsolids,ifdesiredtobeseparatedbyGC,
theyhavetobeconvertedtotheirvolatile
derivatives.Thisiscalledderivatisation.
Derivatisation

49. Thereactionusedtopreparevolatile
derivativesofnon-volatilecompounds,mustbe
simple,fastandquantitativeevenatverylow
concentrationsencounteredwithGLC.Afew
reagentsusedtopreparevolatilederivativesfor
differentclassofcompoundshavebeenlisted
below.

50. No. Derivative Reagent Applications
1 Methyl esters Diazomethane, BF3 methanol Acids
2 Silyl ethers N, o-Bis (trimethylsilyl) acetamide,
Trimethylsilylimidazole
Alcohols, Acids,
Amides, amines.
3 Fluoroacetylaceronates
(Volatile complexes)
Trifluoroacetylacetone Metals
4 Fluoroacetates Trifluoroacetic anhydride Alcohols, Amines,
aminoacids.

51. Head Space Gas Chromatography
Duetoverystingqualitycontrolparameters
neededinfinishedproducts,tobeexported,it
becomesessentialtofindpercentageofvolatiles
infinishedproducts(solidorliquid)e.g.impurityof
solventorPCB,insolidorliquidhastobe
determined.ForthisHSGCisused.

52. Themeasuredquantityofproductistakenin
aninjectionvialandsealed.Itiskeptinaheated
portwherevolatilesareevaporatedandcollected
inheadspace.Asyringeneedleisinsertedin
rubbercapofvialandvapourscollectedfromhead
space,aresuckedinsyringe.Itisthenremoved
andgasesfromsyringeareintroducedininjection
portofGCandanalysed.Alloperationsare
performedbyarobot.

53. GC-interfaced with Mass Spectrometer
ThoughGCisveryusefulinquantitative
analysis,qualitativeanalysisusingGCisalmost
impossible.
Inresearch,whenaplantextractisanalysed
byGC,toidentifyeachorganiccompound
obtainedfromit,GCisinterfacedwithMS.

54. Eachcomponentseparatedisundergoing
fragmentationpatternandmassspectraobtained,
theunknowncanbeidentified.Hereagain
computeracquisitiondatamakesthethingsvery
simple.

55. Modern Elemental Analyser
Modernelementalanalysertodetermine
percentofC,HandNinorganiccompound,
simultaneously,principleofGCisused.
Atincruciblecontainingexactweighed
quantityoforganiccompound,isdroppedina
furnaceheatedat9500C.Aflowofoxygenis
introduced.C,H&NareconvertedtoCO
2,H
2O&
NOxrespectively.

56. Themixturepassedovercoppergauze
when,NOxareconvertedtoN
2.Thismixtureof
CO
2,H
2OandN
2isthenledtoachromatographic
columnfilledwithporapak,wheretheyare
separatedanddetectedbyTCD.Frompercentof
CO
2,H
2OandN
2,percentofC,HandNcanbe
determined.Ithasfollowingadvantages.
• Hardly2-5mgsubstanceneeded.
• Percentageofelementdeterminedwithin
5minutewithexcellentaccuracy.