GAS CHROMATOGRAPHY

1. Gas chromatography
It isa technique of separationinwhichflow of solvents(mobile phase) promotesthe separationof
substancesbydifferentialmigrationfromnarrow initial zone inaporoussorptive medium(stationary
phase)
It containsthree phases,
1)mobile phase
2)stationaryphase
2)mixture tobe separated
Polarity:
Means havingchargesintwodifferentsubstances/.
Havingpolarpositive negative charge.
Formationof twopolesi.e partial +ve& -ve.
- It is(temporary) stage (effect) whereas(ionicbond=permanent)
- Polaritycanalso be producedina substance (neutral molecule)by,Inductioneffect(dueto
presence of electronegative property)
Basesof separation:
1) (mixture tobe separated) separatedby,polarity.Theyremainpolarnotbecome ionic.
2) Mobile & stationaryphase notreact witheachother.
3) polaritydifference isnotbiggerb/cthe attractive force isnecessary.If polarityisgreatthanthey
no attractive force.(B/cRf value of eachcomponentisdifferentb/cpolarityisdifferent)
Rf value:
Ratioof distance traveledbysolute tothe distance traveledbysolvent.
[INFONOTE….. Q: chromatographyisphysical or chemical process?
Both.But 99% physical b/cthere isphysical phenomenanottypical reaction.itisphysical mixing.
Chemical:b/cstudyof compositionischemistry.so, itis1% chemical processb/cseparationof
componentsoccurhere and socompositionchange .
Absorption:micingof twodifferentsubstances.
Adsorption:mixingonsurface.
Sorptive:onsurface forshort time.]

2. MichealTsvetRussianbotanistgive chromatographrconcept.
GAS CHROMATOGRAPHY:
Mobile phase:
It isthe type of analytical techniqueinwhichmobile phase isgasorgases.
Most commonlyusedgasesare,H, N,He.(One ata time)
- mobile phase shouldbe inert& non-reactive.( chemical nature isinert)
Propertiesof mobilephase (MP)
1) results,withHis precise buthydrogencan create fire hazardsand can react withunsaturated
compounds.
2) Results,withN are not verymuchprecise.ButN gas isverycheap.
3) Results withHe isverymuch precise.ButHe gas is verymuch expensisve .
(somostlyusedgasis He inchromatography.)
Stationaryphase:
In G.C stationaryid“column”
Columnisinthe “coil form”.itisplacedinthe oven.The temperature of whichiscontrolled.
Inside the columnthere isalayerof “silica”ehichisweakpolar.
There are twotypesof colmn:
- Metal tubing(stainlesssteel mainly)
- Glasstubing
The columnis hollowfrominside.
Typesof GC:
There are 2 types,
1) GSC (Gas solidchromatography)
2) GLC (Gasliquidchromatography)
GSC:
In thisMP is gas & SP issolidsilica(inside the column)
GLC:
In GLC, MP is gas & SP isliquidlayer.Itpresentinside the columnbyoutsidethe silica
layer.
Most commonlyusedliquidlayerare,
i. PEG ( poly ethyleneglycol) polarinnature.
ii.polysiloxane (non-polarinnature)

3. comparison of GC withFractional distillation:
bothwork onbsasisof boilingpointtechnique.
GC use on small scale or labscale while fractional distillationusedonlarge &industrial scale.
Principal of GC:
The principle of GC ispolarity.
- We considerthe polarityof micture tobe separated.
- Chemical nature of mobile phase shouldbe inert&non-reactive.
Comparisonof GC eithTLC & HPLC:
- In GC
we use sample containingcompoundwhichare,
Thermostableupto300Ċ so,In GC there isuse of hightemperature.
- In TLC (thinlayerchromatography)
there isno use of concept of temperature ortemperature control.We use TLCwhenwe have
insufficientbudget.
The resultwithTLC isnot soprecise.
- In HPLC (highperformance liquidchromatography)
We usedcompoundwhichare heatsensitivei.e Enzymes&Hormones.
In thistechnique use of lowtemperature.
Resultismore precise &we use this whenwe have sufficientbudget.
Workingwithinstrumentation(workof GC) :
Diagram:
Working:
A knowncolume of liquidmixture whichistobe separatedisinjextedintothe headof the
column.
A carriergas or MP, carriesall the componentsof micture alongthe lengthof column.

4. Thismotionisinhibitedbythe adsorptionof componentsinthe column.
So,in thiswayall the componentsinthe columnare separatedaccordingtotheirretentiontime.
Detectordetectsall the componentsastheyreach to the detectorinthe form of peaks.
Retentiontime:
The time inbwfrom injectiontodetection.
Retentiontime isdifferentforeachcomponentdependontheirpolarity.
Selectionof carriergasor mobile phase:
There are 3 typesof gradesonbase of purity.
i.100% pure gas isconsideras7th
grade.It is alsocalledultrahighpure gas.
ii.99.999% pure gas isconsideras6th
grade.
iii.99.998% pure gas isconsideras 5th
grade.
Selectionof column:
We selectcolumnon basisof polarityof the silica& polarityof sample tobe separated.
- We alsoconsiderthe,thicknessof silica. Thisisalsocalledfilmthickness.
- We alsoconsidercolumnlength&columndiameter.
Columntemperature &temperature program:
InGC, columnisplacedinthe oven& the temperature of ovenisespeciallycontrolled.
If initial temperature&final temperatureof the ovenremainsame.The processiscalledisothermal
process. OR
if thisis a change in the initial &final temperatureof the oventhenthischange intemperature iscalled
temperature program.
Detectors:
Followingsdetectorsuse inGC,
i. TCD(thermal conductivitydetector)
ii. FID (flame ionizationdetector)

5. iii. CCD (catalyticcombustiondetector)
iv. DID (discharge ionizationdetector)
v. HID (heliumionizationdetector)
vi. PID (photoionizationdetector)
vii. ECD (electrocapture detector)
viii. AED (atomicemissiondetector)
GC Analysis:
Resultforminform of peak.We perform2 typesof analysisbyGC.
i) qualitative analysis(bypeak-height)
ii) quantitativeanalysis(bypeak-area)
Orderof polarity:
It isthe arrangementof solventinincreasingorderof polarityordecreasingorderof polarity.Tisis
calledorderof polarity.
GC applications: