Mass Spectrometry

Covers an illustrative discussion on Mass Spectrometry

1. Mass Spectrometry
ER. FARUK BIN POYEN, Asst. Professor
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
[email protected]

2. Contents
Introduction
BasicPrinciple
BasicComponents
WorkingPrinciple
Isotopes
HighResolutionMassSpectrometry
MassAnalyzers
IonDetectors
MassSpectrometryIonizationMethods
InductivelyCoupledPlasma
TypesofMassSpectrometers
ApplicationsofMassSpectrometers
MeritsandDemeritsofMS
HypenatedTechniques
2

3. Introduction:
Massspectrometryisananalyticalprocedureemployedtoquantifyknown
materials,toidentifyunknowncompoundswithinasampleandtoexposethe
structureandchemicalpropertiesofvariousmolecules.
Thecompleteprocessencompassestheconversionofthesampleintogaseous
ions,withorwithoutfragmentation,whichlaterarecategorizedbytheirmass
tochargeratios(m/z)andrelativeabundances.
ItsUSPoverotherspectroscopicmethodsisitssuperiorsensitivity.
TheheartoftheMSistheionsource.
Principally,amassspectrometerexecutesthreefunctions:
1.Createspositiveionsfromaneutralsample.
2.Separatestheionsaccordingtotheirmass/chargeratio..
3.Measurestherelativeabundancesofionsandtheirrelativemasses;the
informationbeingrepresentedasamassspectrum.
3

4. Basic Principle:
Amassspectrometerproducesmultipleionsfromthesampleunder
examination;itthensplitsthemaccordingtotheirspecificmass-to-
chargeratio(m/z)andthenrecordstherelativeprofusionofeachion
type.
Theanalysisofcompoundsstartswithgasphaseionsformationofthe
compound,generallybyelectronionizationwheremolecularions
undergofragmentation.
Theseparationofionsaremadebasedontheirmass-to-chargeratioand
aredetectedinproportiontotheirabundance.Hencemassspectrumfor
amoleculeisobtained.
Theresultisdisplayedonaplotofionabundanceversusmass-to-charge
ratioandallrelatedinformationregardingthenatureandstructureofthe
moleculeisderived.
4

5. Basic Components:
Theinstrumentcomprisesthreemajorcomponents:
1.IonSource:Fortheproductionofgaseousionsfromthesubstance
understudy.
2.Analyzer:Forresolvingtheionsintotheircharacteristicsmass
componentsbasedontheirmass-to-chargeratio.
3.DetectorSystem:Fordetectingtheionsandcopyingtherelative
abundanceofeachoftheresolvedionicspecies.
Fortheadmissionoftestsample,asampleintroductionsystemis
alsorequiredmaintainingahighvacuum(~10
-6
to10
-8
mmof
mercury)andacomputercontrols,acquiresandmanipulatesdataand
comparesspectratoreferencedata.
5

6. Basic Components:
Accelerationandcollimationregion:Theionsarethenacceleratedand
collimated(madeintoparallelbeams)intoafinebeambyanintenseelectric
field;theoverallproductisaveryfinebeamofpositiveionstravellingwitha
uniformhighvelocity.
Deflectionchamber:Thebeamofionsthenpassesintothedeflection
chamber.
Herethepolesofanelectromagnetareplacedastridethebenttube.
Thisproducesahighintensity,variablemagneticfieldwithinthetube,normal
toionbeam.
Thefieldcausesachangeinthedirectionofmovementofthesepositiveions.
Themagnitudeofthisdeflectiondependsentirelyonthemass/chargeratioof
theion.Thustheionbeamissplitupintoaseriesofseparatebeams,eachof
whichhasparticlesofonespecificmass/chargeratio.
6

7. Basic Components:
AMSundergoesthefollowingsteps:
1.Ionsproducedfromthesampleintheionizationsource.
2.Theseionsareseparatedaccordingtotheirmass-to-chargeratiointhe
massanalyzer.
3.Selectedionsarefragmentedandanalyzedinasecondanalyzer.
4.Ionsemergingfromthelastanalyzeraredetectedandtheirabundance
withthedetectorismeasuredthatconvertstheionsintoelectrical
signals.
5.Signalsareprocessed,thosearetransmittedtothecomputerand
instrumentiscontrolledusingfeedback.
7

8. Basic Components: 8

9. Working Principle: 9
Ionsbeinghighlyreactiveandshort-lived,formationandmanipulationofionsneedto
beconductedinavacuum.
Thepressureunderwhichionsmaybehandledisroughly10
-5
to10
-8
torr.
Inonecommonmethod,ionizationisachievedbyahighenergyelectronbeam
bombardmentandionseparationisattainedbyacceleratingandfocusingtheionsina
beam,bentbyanexternalmagneticfield.
Theionsarethendetectedelectronicallyandtheresultisstoredand
analyzed.

10. Working Principle: 10
Amassspectrometeroperatinginthisfashionisschemedinthediagram.
Moleculesofthesample(blackdots)arebombardedbyelectrons(lightbluelines)
dispensingfromaheatedfilament.
ThisistermedasanEI(electron-impact)source.Gasesandvolatileliquidsamplesare
allowedtoescapeintotheionsourcefromareservoir(asshown).Non-volatilesolids
andliquidsareintroduceddirectly.

11. Working Principle: 11

12. Working Principle:
Cationsformedbytheelectronbombardment(reddots)arepushedawaybya
chargedrepellingplate(anionsareattractedtoit)andacceleratedtowardother
electrodes,havingslitsthroughwhichtheionspassasabeam.Someofthese
ionsfragmentintosmallercationsandneutralfragments.
Aperpendicularmagneticfieldbendstheionbeaminanarcwhoseradiusis
inverselyproportionaltothemassofeachion.
Lighterionssuffermoredeflectionthanheavierions.
Byalteringthestrengthofthemagneticfield,ionsofdifferentmasscanbe
focusedprogressivelyonadetectorfixedatthecurvedtubeend(alsoundera
highvacuum).
12

13. Working Principle:
Highenergyelectroncollidingwithamolecule,itoftenionizesitbycracking
awayoneofthemolecularelectrons(eitherbondingornon-bonding).
Thisleavesbehindamolecularion(coloredredinthefollowingdiagram).
Remainingenergyfromthecollisionmaycausethemoleculariontofragment
intoneutralpieces(coloredgreen)andsmallerfragments(coloredpinkand
orange).
Themolecularionisaradicalcation,butthefragmentionsmayeitherbe
radicalcations(pink)orcarbocations(orange),dependingonthenatureofthe
neutralfragment.
13

14. Isotopes:
Sinceamassspectrometerseparatesanddetectsionsofslightlydifferent
masses,iteasilydistinguishesdifferentisotopesofagivenelement.
Thisismanifestedmostdramaticallyforcompoundscontainingbromine
andchlorine,asillustratedbythefollowingexamples.
Sincemoleculesofbrominehaveonlytwoatoms,thespectrumonthe
leftwillcomeasasurpriseifasingleatomicmassof80amuisassumed
forBr.
Thefivepeaksinthisspectrumdemonstrateclearlythatnaturalbromine
consistsofanearly50:50mixtureofisotopeshavingatomicmassesof
79and81amurespectively.
Thus,thebrominemoleculemaybecomposedoftwo
79
Bratoms(mass
158amu),two
81
Bratoms(mass162amu)orthemoreprobable
combinationof
79
Br-
81
Br(mass160amu).
14

15. Isotopes:
FragmentationofBr
2toabrominecationthengivesrisetoequalsized
ionpeaksat79and81amu.
Thecenterandrighthandspectrashowthatchlorineisalsocomposed
oftwoisotopes,themoreabundanthavingamassof35amuandthe
minorisotopeamass37amu.
Thepreciseisotopiccompositionofchlorineandbromineis:
Chlorine:75.77%
35
Cl and 24.23%
37
Cl
Bromine:50.50%
79
Br and 49.50%
81
Br
15

16. High Resolution Mass Spectrometry:
ByproducingMScapableofdeterminingm/zvaluesaccuratelytofour
decimalplaces,distinguishingbetweenformulashavingsamenominal
massishighlypossible.
Thetablebelowshowssuchascenarioandadouble-focusinghigh-
resolutionmassspectrometercaneasilydistinguishionshavingthese
compositions.
Massspectrometrythereforenowcanprovidespecificmolecularmass
valueandalsothemolecularformulaofanunknowncompound.
16
Formula C
6H
12 C
5H
8O C
4H
8N
2
Mass 84.0939 84.0575 84.0688

17. Mass Analyzers:
Analyzer System Highlights
Quadrupole Unit mass resolution, fast scan, low cost
Sector (Magnetic and/or
Electrostatic)
High resolution, exact mass
Time-of-Flight (TOF)
Theoretically, no limitation for m/z
maximum, high throughput
Ion Cyclotron Resonance (ICR)
Very high resolution, exact mass,
perform ion chemistry
17

18. Mass Analyzers:
FollowingissomegeneralinformationwhichwillaidEI(ElectronImpact)massspectra
interpretation:
Molecularion(M
+
):Ifthemolecularionappears,itwillbethehighestmassinanEIspectrum
(exceptforisotopepeaksdiscussedbelow).Thispeakwillrepresentthemolecularweightofthe
compound.Itsappearancedependsonthestabilityofthecompound.Doublebonds,cyclic
structuresandaromaticringsstabilizethemolecularionandincreasetheprobabilityofits
appearance.
ReferenceSpectra:Massspectralpatternsarereproducible.Themassspectraofmany
compoundshavebeenpublishedandmaybeusedtoidentifyunknowns.Instrumentcomputers
generallycontainspectrallibrarieswhichcanbesearchedformatches.
Fragmentation:Generalrulesoffragmentationexistandarehelpfultopredictorinterpretthe
fragmentationpatternproducedbyacompound.Functionalgroupsandoverallstructure
determinehowsomeportionsofmoleculeswillresistfragmenting,whileotherportionswill
fragmenteasily.Adetaileddiscussionofthoserulesisbeyondthescopeofthisintroductionand
furtherinformationmaybefoundinmassspectrometryreferencebooks.
Isotopes:Isotopesoccurincompoundsanalyzedbymassspectrometryinthesameabundances
thattheyoccurinnature.
18

19. Ion Detectors:
Channeltron
Achanneltronisahorn-shapedcontinuousdynodestructurethatiscoatedonthe
insidewithanelectronemissivematerial.Anionstrikingthechanneltroncreates
secondaryelectronsthathaveanavalancheeffecttocreatemoresecondary
electronsandfinallyacurrentpulse.
DalyDetector
ADalydetectorconsistsofametalknobthatemitssecondaryelectronswhen
struckbyanion.Thesecondaryelectronsareacceleratedontoascintillatorthat
produceslightthatisthendetectedbyaphotomultipliertube.
ElectronMultiplierTube(EMT)
Electronmultipliertubesaresimilarindesigntophotomultipliertubes.They
consistofaseriesofbiaseddynodesthatejectsecondaryelectronswhentheyare
struckbyanion.Theythereforemultiplytheioncurrentandcanbeusedin
analogordigitalmode.
19

20. Ion Detectors:
Faradaycup
AFaradaycupisametalcupthatisplacedinthepathoftheionbeam.Itis
attachedtoanelectrometer,whichmeasurestheion-beamcurrent.Sincea
Faradaycupcanonlybeusedinananalogmode,itislesssensitivethan
otherdetectorsthatarecapableofoperatinginpulse-countingmode.
Microchannelplate
Amicrochannelplate(MCP)consistsofanarrayofglasscapillaries(10-25
mminnerdiameter)thatarecoatedontheinsidewithaelectron-emissive
material.Thecapillariesarebiasedatahighvoltageandlikethe
channeltron,anionthatstrikestheinsidewallofoneofthecapillaries
createsanavalancheofsecondaryelectrons.Thiscascadingeffectcreatesa
gainof10
3
to10
4
andproducesacurrentpulseattheoutput.
20

21. Sample Introduction/Ionization Method:
Ionization
method
Typical
Analytes
Sample
Introduction
Mass
Range
Method
Highlights
ElectronImpact (EI)
Relatively
small
volatile
GC or
liquid/solid
probe
to
1,000
Daltons
Hard method
versatile
provides
structure info
Chemical Ionization
(CI)
Relatively
small
volatile
GC or
liquid/solid
probe
to
1,000
Daltons
Soft method
molecular ion
peak [M+H]
+
Electrospray (ESI)
Peptides
Proteins
nonvolatile
Liquid
Chromatography
or syringe
to
200,000
Daltons
Soft method
ions often
multiply
charged
Fast Atom
Bombardment (FAB)
Carbohydrates
Organometallics
Peptides
nonvolatile
Sample mixed
in viscous
matrix
to
6,000
Daltons
Soft method
but harder
than ESI or
MALDI
Matrix Assisted Laser
Desorption
(MALDI)
Peptides
Proteins
Nucleotides
Sample mixed
in solid
matrix
to
500,000
Daltons
Soft method
very high
mass
21

22. Mass Spectrometry Ionization Methods:
A) Chemical ionization (CI)
CIusesareagentiontoreactwiththeanalytemoleculestoformionsbyeitheraprotonor
hydridetransfer:
MH + C
2H
5
+
--> MH
2
+
+ C
2H
4
MH + C
2H
5
+
--> M
+
+ C
2H
6
Thereagentionsareproducedbyintroducingalargeexcessofmethane(relativetothe
analyte)intoanelectronimpact(EI)ionsource.
ElectroncollisionsproduceCH
4
+
andCH
3
+
whichfurtherreactwithmethanetoform
CH
5
+
andC
2H
5
+
:
CH
4
+
+ CH
4--> CH
5
+
+ CH
3
CH
3
+
+ CH
4--> C
2H
5
+
+ H
2
22

23. Mass Spectrometry Ionization Methods:
B)Plasmaandglowdischarge
Aplasmaisahot,partially-ionizedgasthateffectivelyexcitesandionizesatoms.
Themostcommonplasmasourceisaninductively-coupledplasma(detailsareina
separateICPdocument).
Aglowdischargeisalow-pressureplasmamaintainedbetweentwoelectrodes.Itis
particularlyeffectiveatsputteringandionizingmaterialfromsolidsurfaces.
C)Electronimpact(EI)
AnEIsourceusesanelectronbeam,usuallygeneratedfromatungstenfilament,toionize
gas-phaseatomsormolecules.
Anelectronfromthebeamknocksanelectronoffofanalyteatomsormoleculestocreate
ions.
23

24. Mass Spectrometry Ionization Methods: 24

25. Mass Spectrometry Ionization Methods:
D)ElectrosprayIonization(ESI)
TheESIsourceconsistsofaveryfineneedleandaseriesofskimmers.
Asamplesolutionissprayedintothesourcechambertoformdroplets.
Thedropletscarrychargeattheexitandthecapillaryandasthesolventevaporates,the
dropletsdisappearleavinghighlychargedanalytemolecules.
ESIisparticularlyusefulforlargebiologicalmoleculesthataredifficulttovaporizeor
ionize.
25

26. Mass Spectrometry Ionization Methods:
E) Fast-atom Bombardment (FAB)
InFABahigh-energybeamofnaturalatoms,typicallyXeorAr,strikesasolidor
low-vapor-pressureliquidsamplecausingdesorptionandionization.
Itisusedforlargebiologicalmoleculesthataredifficulttogetintothegasphase.
Thesampleisusuallydispersedinamatrixsuchasglycerol.
FABcauseslittlefragmentationandusuallygivesalargemolecularionpeak,
makingitusefulformolecularweightdetermination.
Theatomicbeamisproducedbyacceleratingionsfromanionsourcethougha
charge-exchangecell.
Theionspickupanelectronincollisionswithnaturalatomstoformabeamof
high-energyatoms.
26

27. Mass Spectrometry Ionization Methods:
F)FieldIonization
Moleculescanloseanelectronwhenplacedinaveryhighelectricfield.
Highfieldscanbecreatedinanionsourcebyapplyingahighvoltage
betweenacathodeandananodecalledafieldemitter.
Afieldemitterconsistsofawirecoveredwithmicroscopiccarbon
dendrites,whichgreatlyamplifytheeffectivefieldatthecarbonpoints.
G)LaserIonization(LIMS)
Alaserpulseablatesmaterialfromthesurfaceofasampleandcreatesa
microplasmathationizessomeofthesampleconstituents.
Thelaserpulseaccomplishesbothvaporizationandionizationofthe
sample.
27

28. Mass Spectrometry Ionization Methods:
H)Matrix-AssistedLaserDesorptionIonization(MALDI)
MALDIisaLIMSmethodofvaporizingandionizinglargebiologicalmolecules
suchasproteinsorDNAfragments.
Thebiologicalmoleculesaredispersedinasolidmatrixsuchasnicotinicacidor
dihydroxybenzoicacid.
AUVlaserpulseablatesthematrixwhichcarriessomeofthelargemolecules
intothegasphaseinanionizedformsotheycanbeextractedintoamass
spectrometer.
I)ResonanceIonization(RIMS)
Oneormorelaserbeamsaretunedinresonancetotransistionsofagas-phase
atomormoleculetopromoteitinastepwisefashionaboveitsionizationpotential
tocreateanion.
Solidsamplesmustbevaporizedbyheating,sputtering,orlaserablation.
28

29. Mass Spectrometry Ionization Methods: 29

30. Mass Spectrometry Ionization Methods:
J)Sparksource
Asparksourceionizesanalytesinsolidsamplesbypulsinganelectric
currentacrosstwoelectrodes.
Ifthesampleisametalitcanserveasoneoftheelectrodes,otherwiseit
canbemixedwithgraphiteandplacedinacup-shapedelectrode.
K)Thermalionization(TIMS)
Thermalionizationisusedforelementalorrefractorymaterials.
Asampleisdepositedonametalribbon,suchasPtorRe,andanelectric
currentheatsthemetaltoahightemperature.
Theribbonisoftencoatedwithgraphitetoprovideareducingeffect.
30

31. Mass Spectrometry Ionization Methods:
L)Plasma-desorptionionization(PD)
Decayof
252
Cfproducestwofissionfragmentsthattravelinopposite
directions.
Onefragmentstrikesthesampleknockingout1-10analyteions.
Theotherfragmentstrikesadetectorandtriggersthestartofdata
acquisition.
Thisionizationmethodisespeciallyusefulforlargebiologicalmolecules.
31

32. Mass Spectrometry Ionization Methods:
M)Secondaryionization(SIMS)
Aprimaryionbeam;suchas
3
He
+
,
16
O
+
,or
40
Ar
+
;isacceleratedand
focusedontothesurfaceofasampleandsputtersmaterialintothegas
phase.
Approximately1%ofthesputteredmaterialcomesoffasions,whichcan
thenbeanalyzedbyamassspectrometer.
SIMShastheadvantagethatmaterialcanbecontinuallysputteredfroma
surfacetodetermineanalyteconcentrationsasafunctionofdistancefrom
theoriginalsurface(depthprofiling).
32

33. InductivelyCoupledPlasma:
Inductivelycoupledplasma(ICP)isaveryhightemperature(7000-
8000K)excitationsourcethatefficientlydissolves,vaporizes,excites
andionizesatoms.
Molecularinterferencesaregreatlyreducedwiththisexcitationsource
butarenoteliminatedcompletely.
ICPsourcesareusedtoexciteatomsforatomic-emission
spectroscopyandtoionizeatomsformassspectrometry.
33

34. Inductively Coupled Plasma:
Thesampleisnebulizedandentrainedintheflowofplasmasupport
gas,whichistypicallyAr.
Theplasmatorchconsistsofconcentricquartztubes,withtheinnertube
containingthesampleaerosolandArsupportgasandtheoutertube
containinganArgasflowtocoolthetubes(seeschematic).
Aradiofrequency(RF)generator(typically1-5kW@27MHzor41
MHz)producesanoscillatingcurrentinaninductioncoilthatwraps
aroundthetubes.
34

35. Inductively Coupled Plasma:
Theinductioncoilcreatesanoscillatingmagneticfield,whichproduces
anoscillatingmagneticfield.
Themagneticfieldinturnsetsupanoscillatingcurrentintheionsand
electronsofthesupportgas.
Theseionsandelectronstransferenergytootheratomsinthesupport
gasbycollisionstocreateaveryhightemperatureplasma.
35

36. Types of Mass Spectrometers:
MagneticDeflectionMassSpectrometer
TimeOfFlightMassSpectrometer
RadiofrequencyMassSpectrometer
QuadrupoleMassSpectrometer
FourierTransformIonCyclotronResonanceMassSpectrometer
36

37. Magnetic Deflection Mass Spectrometer
Theionopticsintheion-sourcechamberofamassspectrometerextractandaccelerate
ionstoakineticenergygivenby:K.E.=0.5mv
2
=eVwheremisthemassoftheion,
visit'svelocity,eisthechargeoftheionandVistheappliedvoltageoftheionoptics.
Itisknownthatwhenaccelerationisappliedperpendiculartothedirectionofion
motion,theobject’svelocityremainsconstantbuttheobjecttravelsinacircularpath.
Theionsentertheflighttubebetweenthepolesofamagnetandaredeflectedbythe
magneticfield,H.Onlyionsofmass-to-chargeratiothathaveequalcentrifugaland
centripetalforcespassthroughtheflighttube:
mv
2
/ r = H* e * v
centrifugalforces=centripetalforces.
Whereristheradiusofcurvatureoftheionpath:
r= mv / eH
Thisequationshowsthatthem/eoftheionsthatreachthedetectorcanbevariedby
changingeitherHorV.
37

38. Magnetic Deflection Mass Spectrometer
SingleFocusinganalyzers:Acircularbeampathof180,90or60
degreescanbeused.Thevariousforcesinfluencingtheparticleseparate
ionswithdifferentmass-to-chargeratios.
DoubleFocusinganalyzers:Anelectrostaticanalyzerisaddedinthis
typeofinstrumenttoseparateparticleswithdifferenceinkinetic
energies.
Theheatedtungstenfilamentproducesanelectronbeam,whichpasses
betweentheplates.Adifferenceinelectricpotentialbetweentheplates
pullsionsoutofthebeam,sothattheypassthroughtheslit.
38

39. Magnetic Deflection Mass Spectrometer 39

40. Time Of Flight Mass Spectrometer
Atime-of-flightmassspectrometerusesthedifferencesintransittime
throughadriftregiontoseparateionsofdifferentmasses.
Itoperatesinapulsedmodesoionsmustbeproducedorextractedin
pulses.
Anelectricfieldacceleratesallionsintoafield-freedriftregionwitha
kineticenergyofqV,whereqistheionchargeandVistheapplied
voltage.
Sincetheionkineticenergyis0.5*mv
2
,lighterionshaveahigher
velocitythanheavierionsandreachthedetectorattheendofthedrift
regionsooner.
Ionsareformedbypulseionizationmethod.Theionstraversean
evacuatedtubecalledthedrifttubetoreachthedetector.
40

41. TimeOfFlightMassSpectrometer
Theory:
K.E. =
1
/
2mv
2
= qV
V = (
2qV
/
m)
1/2
Thetransittime(t)throughthedrifttubeisL/VwhereListhelengthofthe
drifttube.t=L/(
2V
/
m/q)
1/2
m/e = 2V/L
2
* t
2
TheessentialpartsofaToFinstrumentconsistof
1.Anelectrongunfortheproductionofions
2.Agridsystemforacceleratingionstouniformvelocitiesinapulsedmode
3.Anevacuatedtube,calledthedrifttube
4.Aniondetector
41

42. Time Of Flight Mass Spectrometer 42

43. Time Of Flight Mass Spectrometer
Thisschematicshowsablationofionsfromasolidsamplewithapulsed
laser.
Thereflectronisaseriesofringsorgridsthatactasanionmirror.
TheionsleavingtheionsourceofaToFMShaveneitherexactlythe
samestartingtimenorexactlythesamekineticenergies.
Thismirrorcompensatesforthespreadinkineticenergiesoftheionsas
theyenterthedriftregionandimprovestheresolutionoftheinstrument.
Theoutputofaniondetectorisdisplayedonanoscilloscopeasa
functionoftimetoproducethemassspectrum.
ThemainadvantagesofaToFincludeitsspeedandabilitytorecord
entiremassspectrumatonetime.
Thedisadvantageisitspoorresolution.
43

44. Radiofrequency Mass Spectrometer
Thearrangementinthisinstrumentissuchthatthechargedparticles
emergingfromtheionsourceareallacceleratedtothesameenergyin
anelectrostaticfieldandthentheypassthroughasystemofradio
frequencyelectrodes.
Theenergyacquiredbytheionsinthisprocessisafunctionoftheirof
theirspecificm/eratio.
Apotentialenergyselectorisplacedbeforethedetector,whichbalances
outtheenergyoftheionbeamandthemassspectrumisrecordedby
detectionoftheionswiththehighestenergiesasthefrequencyofthe
alternatingRFvoltageisvaried.
44

45. Quadrupole Mass Spectrometer
Aquadrupole(simpleinconstructionandlightweight)massfilterconsistsof
fourparallelmetalrodsarrangedasinthefigurebelow.
Twooppositerodshaveanappliedpotentialof(U+Vcos(wt))andtheother
tworodshaveapotentialof-(U+Vcos(wt)),whereUisadcvoltageandVcos
(wt)isanacvoltage.
Theappliedvoltagesaffectthetrajectoryofionstravelingdowntheflightpath
centeredbetweenthefourrods.
Forgivendcandacvoltages,onlyionsofacertainmass-to-chargeratiopass
throughthequadrupolefilterandallotherionsarethrownoutoftheiroriginal
path.
Amassspectrumisobtainedbymonitoringtheionspassingthroughthe
quadrupolefilterasthevoltagesontherodsarevaried.
Therearetwomethods:varyingwandholdingUandVconstant,orvaryingU
andV(U/V)fixedforaconstantw.
45

46. Quadrupole Mass Spectrometer
Quadrupolemassspectrometersconsistofanionsource,ionopticsto
accelerateandfocustheionsthroughanapertureintothequadrupolefilter,the
quadrupolefilteritselfwithcontrolvoltagesupplies,anexitaperture,anion
detector,detectionelectronics,andahigh-vacuumsystem.
Byproperselectionofpotentialsandfrequency,aninofthedesiredmasscan
bemadetopassthroughthesystemwhileunwantedmasseswillbecollected
ononeoftheelectrodes.
ByincreasingtheratioU/V,thestableintervalwhichcorrespondstoastable
massintervalisreduced,sothatonlyionsofonemasscanpassthequadrupole
filter.
Whenthevalueofqiskeptconstant,thevalueofm/eisproportionaltothe
valueofV.
m/e=k.V/R
o
2
f
2
.R=radiusoftheelectricfield.Kisaconstant.
46

47. Quadrupole Mass Spectrometer 47

48. Fourier Transform Ion Cyclotron Resonance
Mass Spectrometer
Fouriertransformioncyclotronresonancemassspectrometryisatype
ofmassanalyzer(ormassspectrometer)fordeterminingthemass-to-
chargeratio(m/z)ofionsbasedonthecyclotronfrequencyoftheionsin
afixedmagneticfield.
TheionsaretrappedinaPenningtrap(amagneticfieldwithelectric
trappingplates)wheretheyareexcited(attheirresonantcyclotron
frequencies)toalargercyclotronradiusbyanoscillatingelectricfield
orthogonaltothemagneticfield.
Aftertheexcitationfieldisremoved,theionsarerotatingattheir
cyclotronfrequencyinphase(asa"packet"ofions).
48

49. Fourier Transform Ion Cyclotron Resonance
Mass Spectrometer
Theseionsinduceacharge(detectedasanimagecurrent)onapairof
electrodesasthepacketsofionspassclosetothem.
Theresultingsignaliscalledafreeinductiondecay(FID),transientor
interferogramthatconsistsofasuperpositionofsinewaves.
TheusefulsignalisextractedfromthisdatabyperformingaFourier
transformtogiveamassspectrum
Inthesimplestform(idealized)therelationshipbetweenthecyclotron
frequencyandthemasstochargeratioisgivenby:
wheref=cyclotronfrequency,q=ioncharge,B=magneticfield
strengthandm=ionmass.
49

50. Fourier Transform Ion Cyclotron Resonance
Mass Spectrometer
50

51. Applications of Mass Spectrometers:
Massspectrometersaresensitivedetectorsofisotopesbasedontheir
masses.
Theyareusedincarbondatingandotherradioactivedatingprocesses.
Thecombinationofamassspectrometerandagaschromatograph
makesapowerfultoolforthedetectionoftracequantitiesof
contaminantsortoxins.
Anumberofsatellitesandspacecrafthavemassspectrometersforthe
identificationofthesmallnumbersofparticlesinterceptedinspace.
Forexample,theSOHOsatelliteusesamassspectrometertoanalyze
thesolarwind.
Massspectrometersareusedfortheanalysisofresidualgasesinhigh
vacuumsystems.
51

52. Merits of MS
1.Increasedsensitivityovermostotheranalyticaltechniquesbecausethe
analyzer,asamass-chargefilter,reducesbackgroundinterference
2.Excellentspecificityfromcharacteristicfragmentationpatternsto
identifyunknownsorconfirmthepresenceofsuspectedcompounds.
3.Informationaboutmolecularweight.
4.Informationabouttheisotopicabundanceofelements.
5.Temporallyresolvedchemicaldata.
52

53. DemeritsofMS
1.Oftenfailstodistinguishbetweenopticalandgeometricalisomers
2.Thepositionsofsubstituentino-,m-andp-positionsinanaromatic
ring.
3.Also,itsscopeislimitedinidentifyinghydrocarbonsthatproduce
similarfragmentedions.
53

54. MS –GC Hyphenated Techniques
ItwasintroducedbyHirschfeld
Itreferstoanon-linecombinationofachromatographicseparation
techniquewithasensitiveandelement-specificSpectroscopicdetector.
54

55. WorkingofGC-MS
VaporizedSampleintroducedintoGCinlet.
sweptontothecolumnbyHecarriergas&separatedoncolumn.
SamplecomponentselutedfromcolumnmovedtotheMS(He
removed).
ThecomputerdrivestheMS,recordsthedata.
Identificationbasedonit'smassspectrum.
Alargelibraryofknownmassspectraisstoredonthecomputerandcan
besearchedforidentification.
55

56. GC –MS Block Diagram 56

57. Schematic of a GC-MS Instrument 57

58. Applications of GC -MS
Petrochemicalandhydrocarbonsanalysis
Geochemicalresearch
Forensic(arson,explosives,drugs,unknowns)
Environmentalanalysis
Pesticideanalysis,foodsafetyandquality
Pharmaceuticalanddruganalysis
Clinicaltoxicology
Foodandfragrance
58

59. Limitations of GC –MS
Onlycompoundswithvaporpressuresexceedingabout10
–10
torrcanbe
analyzedbygaschromatography-massspectrometry(GC-MS).
Determiningpositionalsubstitutiononaromaticringsisoftendifficult.
Certainisomericcompoundscannotbedistinguishedbymass
spectrometry(forexample,naphthaleneversusazulene),buttheycan
oftenbeseparatedchromatographically.
59

60. References
AnalyticalInstrumentation,BelaG.Liptak
PrinciplesofInstrumentalAnalysis6E,Skoog,HollerandCrouch
HandbookofAnalyticalInstruments,2
nd
Edition,RSKhandpur
GasChromatographyMassSpectrometry(GC-MS)-AyeshaAbdulGhafoor.
McLafferty,F.W.Hertel,R.H.andVillwock,R.D.(1974),"Probabilitybased
matchingofmassspectra.Rapididentificationofspecificcompoundsin
mixtures".OrganicMassSpectrometry9(7):690–702
Amirav,A.Gordin,A.Poliak,M.Alon,T.andFialkov,A.B.(2008),"Gas
ChromatographyMassSpectrometrywithSupersonicMolecularBeams".
JournalofMassSpectrometry43:141–163.
R.A.HitesandK.Biemann(1968),AnalyticalChemistry,40,1217–21.
McMaster,C.McMaster,MarvinC.(1998).GC/MS:apracticaluser'sguide.
NewYork:Wiley.
60