Gas Chromatography

Gas chromatography head points:
Invention of Chromatography
original chromatography Experiment
Common types of chromatography
Paper and Thin layer chromatography
How does chromatography work?
Theoretical Plate
gas chromatography
schematic of GC
carrier gas-supply
Injection port
sample Injection system
split/spitless Injection
sample valves
GC columns
open tubular columns
Temperature Control
Solid Support Materials
Particle size of Supports
The stationary Phase
Detection systems
Characteristics of the Ideal Detector
Flame Ionization Detectors
Thermal Conductivity Detector
Electron-capture Detectors

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Preeti Choudhary

1. Gas Chromatography
Presented by: Preeti Choudhary
MSc (Applied Physics)
[email protected]

2. Invention of Chromatography
Mikhail Tswett
Russian Botanist
(1872-1919)
MikhailTswettinvented
chromatographyin1901
duringhisresearchon
plantpigments.
Heusedthetechniqueto
separatevariousplant
pigmentssuch as
chlorophylls,xanthophylls
andcarotenoids.

3. Original Chromatography Experiment
Later
Start:A glass
column is filled
with powdered
limestone
(CaCO
3).
End:A series of
colored bands is
seen to form,
corresponding to
the different
pigments in the
original plant
extract. These
bands were later
determined to be
chlorophylls,
xanthophylls and
carotenoids.
An EtOH extract
of leaf pigments
is applied to the
top of the column.
EtOH is used to
flush the pigments
down the column.

4. Chromatography:(Greek = chroma “color”and
graphein “writing”)Tswett named this new technique
chromatography based on the fact that it separated the
components of a solution by color.
Common Types of Chromatography
Tswett’s technique is based on Liquid Chromatography.
There are now several common chromatographic
methods. These include:
Paper Chromatography
Thin Layer Chromatography (TLC)
Liquid Chromatography (LC)
High Pressure Liquid Chromatography (HPLC)
Ion Chromatography
Gas Chromatography (GC)

5. Paper and Thin Layer Chromatography
Later
The solvent moves up paper by capillary action,
carrying mixture components at different rates.
solvent
solvent
front

6. How Does Chromatography Work?
In all chromatographic separations, the sample is transported
in a mobile phase. The mobile phase can be a gas, a liquid,
or a supercritical fluid.
The mobile phase is then forced through a stationary phase
held in a columnor on a solid surface. The stationary phase
needs to be something that does not reactwith the mobile
phase or the sample.
Thesamplethen has the opportunity to interactwith the
stationary phase as it moves past it. Samples that interact
greatly, then appear to move more slowly. Samples that
interact weakly, then appear to move more quickly. Because
of this difference in rates, the samples can then be separated
into their components.

7. Chromatographyis based on a physical equilibrium
that results when a solute is transferred between the
mobile and a stationary phase.
A
A
A
A
A
A
A
A
A
A
A
A
K= distribution
coefficient or
partition ratio
K =
C
S
C
M
Where C
Sis the molar
concentration of the
solute in the stationary
phase and C
Mis the
molar concentration in
the mobile phase.
Cross Section of Equilibrium in a column.
“A” are adsorbed to the stationary phase.
“A” are traveling in the mobile phase.

8. Flow
As a material travels through the column, it assumes a
Gaussian concentration profile as it distributes between the
stationary packing phase and the flowing mobile gas or
liquid carrier phase.
In a chromatography column, flowing gas or liquid
continuously replaces saturated mobile phase and results
in movement of A through the column.
Column is packed
with particulate
stationary phase.

9. Flow
Flow
Flow
Flow
In a mixture, each component has a different distribution coefficient,
and thus spends a different amount of time absorbed on the solid
packing phase vs being carried along with the flowing gas
More volatile materials are carried through the column more rapidly
than less volatile materials, which results in a separation.

10. Note: The first two components were not completely separated.
Peaks in general tend to become shorter and wider with time.
If a detector is used to determine when the components elute
from the column, a series of Gaussian peaks are obtained,
one for each component in the mixture that was separated
by the column.

11. TheoreticalplateisatermcoinedbyMartin&
Synge.Itisbasedonastudyinwhichtheyimaginedthat
chromatographiccolumnswereanalogoustodistillation
columnsandmadeupornumerousdiscretebutconnected
narrowlayersorplates.Movementofthesolutedownthe
columnthencouldbetreatedasastepwisetransfer.
Theoreticalplates(N)measurehowefficientlya
columncanseparateamixtureintoitscomponents.This
efficiencyisbasedontheretentiontimeofthe
componentsandthewidthofthepeaks.
The Theoretical Plate

12. w
b
t
RN = 16(
t
R
w
b
)
2
N = Number of theoretical plates(a measure of efficiency)
t
Ris the retention time; it is measured from the injection peak
(or zero) to the intersection of the tangents.
w
bis the width of the base of the triangle; it is measured
at the intersection of the tangents with the baseline.

13. When the retention time, t
R, is held constant, the column that produces
peaks with narrower bases, w
b, will be more efficient–have a greater
N value.
Likewise a column that produces wider peakswill be less efficient–
have a smaller N value.
This is because a smaller denominator, w
b, will yield a larger overall
number and a larger denominator will yield a smaller number.
Larger N Smaller N
t
R
t
R
w
b w
bN = 16(
t
R
w
b
)
2

14. Gas Chromatography
Gaschromatography(GC),isachromatographictechnique
thatcanbeusedtoseparatevolatileorganiccompounds.
Ingaschromatographythesampleisvaporizedand
injectedontotheheadofachromatographiccolumn.
Elutionisbroughtaboutbytheflowofaninertgaseous
mobilephase.
Themobilephasedoesnotinteractwithmoleculeofthe
analyte.Itsonlyfunctionistotransporttheanalyte
throughthecolumn,hencecalledcarriergas.
Gas-liquidchromatographyisbaseduponthepartitionof
theanalytebetweenagaseousmobilephaseandaliquid
phaseimmobilizedonthesurfaceofaninertsolid.

16. Gas Chromatography (GC)
It consists of
•a flowing mobile phase-carrier gas
•pressure regulators and gauges
•a flow meter
•an injection port
•aseparation column (the stationary phase)
•an oven
•a detector
•a recorder
•a display device

17. Schematic of GC

18. Carrier Gas-Supply
Mobile phases are generally inert gases such as helium, argon, hydrogen or
nitrogen.
Associated with the gas supply are pressureregulators, gauges, and flow
meters.
Inaddition,thecarriergassystemoftencontainsamolecularsieveto
removewaterorotherimpurities.

19. •The injection portconsists of a rubber
septum through which a syringe needle is
inserted to inject the sample.
•Theinjection port is maintained at a higher
temperature than the boiling point of the least
volatilecomponent in the sample mixture.
Injection Port

20. Sample Injection System
Column efficiency requires that
the sample be of suitable size
and be introduced as a “plug”
of vapor; slow injection of
oversized samples causes band
spreading and poor resolution.

21. Themostcommonmethodofsample
injectioninvolvestheuseof
microsyringetoinjectaliquidor
gaseoussamplethroughaself-sealing,
silicone-rubberdiaphragmorseptum
intoaflashvaporizerportlocatedatthe
headofthecolumn

22. Split/splitless injection
Theamountofgasthatgoesout
thesplitventcontrolstheamount
ofsamplethatentersthecolumn.
Ifthesplitventisclosed,viaa
computer-controlledsplitvalve,
thenallofthesampleintroduced
intotheinjectorvaporizesand
goesintothecolumn.
Ifthesplitventisopenthen
mostofthevaporizedsampleis
thrownawaytowasteviathesplit
ventandonlyasmallportionof
thesampleisintroducedto
thecolumn.

23. Sample valves
Forquantitativework,morereproduciblesamplesizesforbothliquidsand
gasesareobtainedbymeansofarotarysamplevalve.
Errorsduetosamplesizecanbereducedto0.5%to2%relative.
Thesamplingloopisfilledbyinjectionofanexcessofsample.
Rotationofthevalveby45degthenintroducesthereproduciblevolume
ACBintothemobilephase.

24. Sample valves

25. Column Configurations
Twogeneraltypesofcolumnsareencounteredingaschromatography,
packedandopentubularorcapillary.
Chromatographiccolumnsvaryinlengthfromlessthan2mto50mor
more.Theyareconstructedofstainlesssteel,glass,fusedsilica,orTeflon.
Inordertofitintoanovenforthermostating,theyareusuallyformedas
coilshavingdiametersof10to30cm.

26. GC Columns
Capillary columnsPacked columns
•Typically a glass,stainless
steel,copper or aluminumcoil.
•1-5mtotal length and 5 mm
inner diameter.
•Filled with the st. ph. or a
packing coated with the st.ph.
•Thin fused-silica,metal or glass
•Typically 10-100 m in length and
250mm inner diameter.
•St. ph. coated on the inner surface.
•Provide much higher separation
efficiency
•But more easily overloaded by too
much sample.

27. Open tubular Columns
Opentubularorcapillarycolumnsareoftwobasictypes:
wall-coatedopentubular(WCOT)andsupport-coatedopen
tubular(SCOT).
Wall-coatedcolumnsaresimplycapillarytubescoatedwitha
thinlayerofthestationaryphase.
Insupport-coatedopentubularcolumns,theinnersurfaceof
thecapillaryislinedwithathinfilm(~30mm)ofasupport
material,suchasdiatomaceousearth.Thistypeofcolumn
holdsseveraltimesasmuchstationaryphaseasdoesawall-
coatedcolumnandthushasagreatersamplecapacity.

29. Since the partitioning behavior is dependent
on temperature, the column is usually
contained in a thermostat-controlled oven.
Separating componentswith a wide range of
boiling points is accomplished by starting at a
low oven temperature andincreasing the
temperature over time to elute the high-boiling
point components. This is called temperature
programming.
Temperature control

30. Temperature Programming
Temperature programming:
Temperature is raised during the
separation which increases
solute vapor pressure and
decreaseretention time, thus
improving column efficiency
Temperature gradient
improves resolution while
also decreasing retention
time

31. Open Tubular Columns-Increasing Resolution
Decrease tube diameter
Increase resolution
Increase Column Length
Increase resolution

32. Solid Support Materials
The most widely used support material is
prepared from naturally occurring
diatomaceous earth, which is made up
of the skeletonsof thousands of species
of single-celled plants (diatoms) that
inhabited ancient lakes and seas.
Such plants received their nutrients and
disposed of their wastes via molecular
diffusion through their pores.
As a consequence, their remains are well-
suited as support materials because gas
chromatography is also based upon the
same kind of molecular diffusion.

33. Particle Size of Supports
Theefficiencyofagas-chromatographiccolumnincreasesrapidlywithdecreasing
particlediameterofthepacking.
Thepressuredifferencerequiredtomaintainagivenflowrateofcarriergas,
however,variesinverselyasthesquareoftheparticlediameter.

35. The Stationary Phase
Desirablepropertiesfortheimmobilizedliquid
phaseinagas-liquidchromatographiccolumn
include:
lowvolatility(ideally,theboilingpointofthe
liquidshouldbe100
o
Clowerthanthemaximum
operatingtemperatureforthecolumn)
thermalstability
chemicalinertness
solventcharacteristicssuchthatk`and
valuesforthesolutestoberesolvedfallwithina
suitablerange.
Theretentiontimeforasoluteonacolumn
dependsuponitsdistributionconstantwhichin
turnisrelatedtothechemicalnatureofthe
stationaryphase.

36. The Stationary Phase
Tohaveareasonableresidencetimeinthecolumn,a
speciesmustshowsomedegreeofcompatibility
(solubility)withthestationaryphase.Here,the
principleof“likedissolveslike”applies,where“like”
referstothepolaritiesofthesoluteandtheimmobilized
liquid.
Polarstationaryphasescontainfunctionalgroupssuchas
–CN,--COand–OH.
Polyesterphasesarehighlypolar.Polarsolutesinclude
alcohols,acids,andamines;
Hydrocarbon-typestationaryphaseanddialkylsiloxanes
arenonpolar
Solutesofmediumpolarityincludeethers,ketones,and
aldehydes.

38. Film Thickness
Commercialcolumnsareavailablehavingstationary
phasesthatvaryinthicknessfrom0.1to5mm.Film
thicknessprimarilyaffectstheretentivecharacterand
thecapacityofacolumn.
Thickfilmsareusedwithhighlyvolatileanalytesbecause
suchfilmsretainsolutesforalongertime,thusproviding
agreatertimeforseparationtotakeplace.
Thinfilmsareusefulforseparatingspeciesoflow
volatilityinareasonablelengthoftime.
Formostapplicationswith0.26-or0.32-mmcolumns,a
filmthicknessof0.26mmisrecommended.With
megaborecolumns,1-to1.5mmfilmsareoftenused.

39. Detection Systems
CharacteristicsoftheIdealDetector:Theidealdetectorforgas
chromatographyhasthefollowingcharacteristics:
1.Adequatesensitivity
2.Goodstabilityandreproducibility.
3. A linear response to solutes that extends over several orders of
magnitude.
4. A temperature range from room temperature to at least 400
o
C.

40. Characteristics of the Ideal Detector
5.Ashortresponsetimethatisindependentofflowrate.
6.Highreliabilityandeaseofuse.
7.Similarityinresponsetowardallsolutesorahighlyselectiveresponse
towardoneormoreclassesofsolutes.
8.Nondestructiveofsample.

41. GC Detectors
After the components of a mixture are separated using gas
chromatography, they must be detected as they exit the GC
column.
Thermal-conductivity(TCD) and flame ionization (FID)
detectors are the two most common detectors on
commercial GCs.
The others are
1.Atomic-emmision detector (AED)
2.Chemiluminescence detector
3.Electron-capture detector (ECD)
4.Flame-photometric detector (FPD)
5.Mass spectrometer (MS)
6.Photoionization detector (PID)

43. Flame Ionization Detectors (FID)
Theflameionizationdetectoristhemost
widelyusedandgenerallyapplicabledetector
forgaschromatography.
Theeffluentfromthecolumnismixedwith
hydrogenandairandthenignitedelectrically.
Mostorganiccompounds,whenpyrolyzedatthe
temperatureofahydrogen/airflame,produce
ionsandelectronsthatcanconductelectricity
throughtheflame.Thiscurrentisamplifiedand
recorded.

44. Flame Ionization Detectors (FID)
Apotentialofafewhundredvoltsisappliedon
collectorelectrode.
Theresultingcurrent(~10
-12
A)isthen
measured.
Theflameionizationdetectorexhibitsahigh
sensitivitylargelinearresponserange(~10
7
),
andlownoise.Itisruggedandconvenientto
use.
Adisadvantageoftheflameionizationdetector
isthatitisdestructiveofsample.
Little or no response to (use a Thermal
Conductivity Detector for these gases)
CO, CO
2, CS
2, O
2, H
2O, NH
3, inert gasses

45. Flame Ionization Detector
Hydrogen
Air
Capillary tube (column)
Platinum jet
Collector
Sintered disk
Teflon insulating ring
Flame
Gas outlet
Coaxial cable to
Analog to Digital
converter
Ions
Why do we need
hydrogen?

46. Thermal Conductivity Detectors(TCD)
Averyearlydetectorforgaschromatography,and
onethatstillfindswideapplication,isbasedupon
changesinthethermalconductivityofthegas
streambroughtaboutbythepresenceofanalyte
molecules.
ThesensingelementofTCDorkatharometeris
anelectricallyheatedelementwhose
temperatureatconstantelectricalpowerdepends
uponthethermalconductivityofthesurrounding
gas.
Theheatedelementmaybeafineplatinum,gold,
ortungstenwireorasemiconductingthermistor.

47. Thermal Conductivity Detector
Measures amount of compound leaving
column by its ability to remove heat.
Helium and hydrogen have high thermal
conductivity , so the presence of any
compound will lowerthe thermal
conductivity increasing temperature of
filament.
As heat is removed from filament, the
resistance (R) of filament changes, causing
a change in an electrical signal that can be
measured
Responds to all compounds (universal)
Signal changes in response to flow rate of
mobile phase and any impurities present

48. Thermal Conductivity Detector(TCD)
Theadvantageofthethermalconductivitydetectorisitssimplicity,its
largelineardynamicrange(~10
5
),itsgeneralresponsetobothorganicand
inorganicspecies,anditsnondestructivecharacter,whichpermits
collectionofsolutesafterdetection.
Alimitationofthekatharometerisitsrelativelylowsensitivity.
Otherdetectorsexceedthissensitivitybyfactorsaslargeas10
4
to10
7
.

49. Electron-Capture Detector (ECD)
Theelectron-capturedetectorhasbecomeoneofthe
mostwidelyuseddetectorsforenvironmentalsamples
becausethisdetectorselectivitydetectshalogen
containingcompounds,suchaspesticidesand
polychlorinatedbiphenyls.
Theeffluentfromthecolumnispassedoveraemitter,
usuallynickel-63.
Anelectronfromtheemittercausesionizationofthe
carriergasandtheproductionofaburstofelectrons.
Intheabsenceoforganicspecies,aconstantstanding
currentbetweenapairofelectrodesresultsfromthis
ionizationprocess.
However,inthepresenceoforganicmoleculesthattend
tocaptureelectrons,thecurrentdecreasesmarkedly.

51. Electron-Capture Detectors(ECD)
Theelectron-capturedetectorisselectiveinitsresponsebeinghighly
sensitivetomoleculescontainingelectronegativefunctionalgroupssuch
ashalogens,peroxides,quinones,andnitrogroups.
Itisinsensitivetofunctionalgroupssuchasamines,alcohols,and
hydrocarbons.
Animportantapplicationoftheelectron-capturedetectorhasbeenfor
thedetectionanddeterminationofchlorinatedinsecticides.

53. Thank-You
