Mass spectrometry

1. 1
MassSpectrometry
By: Bijaya Kumar Uprety

2. 2
•Massspectrometryisatechniqueusedformeasuringthe
molecularweightanddeterminingthemolecularformulaofan
organiccompound.
Note:Atomscanbedeflectedbymagneticfields-providedtheatomisfirst
turnedintoanion.Electricallychargedparticlesareaffectedbyamagneticfield
althoughelectricallyneutralonesaren't.Thisiswhyinmassspectrometer
atomsareionized.
•Inamassspectrometer,amoleculeisvaporizedundervacuum
andthenionizedbybombardmentwithabeamofhigh-energy
electronscausingthelossofanelectron.
•Theenergyoftheelectronsis~1600kcal(or70eV).
•Sinceittakes~100kcalofenergytocleaveatypicalsbond,
1600kcalisanenormousamountofenergytocomeintocontact
withamolecule.
Introductionandprinciple:

3. 3
•Whentheelectronbeamionizesthemolecule,the
speciesthatisformediscalledaradicalcation,and
symbolizedasM
+•
.
•TheradicalcationM
+•
iscalledthemolecularionor
parentionandthemassofM
+•
representsthe
molecularweightofM.
•BecauseMisunstable,someionsdecomposeto
formfragmentsofradicalsandcationsthathavea
lowermolecularweightthanM
+•
.
•Theionsarethenacceleratedthroughapotentialof
about10,000voltssothattheyallhavethesame
kineticenergy.

4. •Theionsarethendeflectedbyamagneticfield
accordingtotheirmasses.Thelightertheyare,the
moretheyaredeflected.
•Theamountofdeflectionalsodependsonthe
numberofpositivechargesontheion-inother
words,onhowmanyelectronswereknockedoffin
thefirststage.Themoretheionischarged,themore
itgetsdeflected.
•Thebeamofionspassingthroughthemachineis
detectedelectrically.
4

5. 5
Figure 13.1 Schematic of a mass spectrometer
MassSpectrometry
Introduction:

6. Schematic diagram of mass spectrometer.
6

7. Instrumentation
The essential components of a mass spectrometer are:
• Inlet device
• Ionisationchamber or Ion Source
• Analyser
• Detector
• Processing and output devices
The arrangement of these components of the mass
spectrometer is schematically represented in Fig. 13.3.
7

8. 8
Fig. Components of mass spectrometer

9. •Theinletdeviceloadsthesampleintotheionisationchamber
wheretheanalyteisionisedbyasuitablemethodandthe
molecularionand/orthefragmentionsobtainedby
fragmentationofthemolecularionaredirectedtowardsthe
analyser.
•Intheanalysertheseionsobtainedbythefragmentationofthe
molecularionaresortedoutonthebasisoftheirm/zvalueby
usingoneofthemanyavailabletechniquesandaresenttothe
detector(transducer).
•Inthedetectortheionfluxgeneratesanelectricalcurrent
proportionaltothenumberofionsreachingit.
•Theprocessingunitrecordsthemagnitudeoftheseelectrical
signalsasafunctionofm/zandgivesanoutputintheformof
amassspectrum.
9

10. 1. Inlet devices
•Thepurposeoftheinletdeviceistoloadthesampleintotheionisation
chamber.Thedeviceuseddependsonthenatureofthesample.
•Thesolidsamplesareplacedonthetipofarodcalleddirectinsertionprobe
whichisinsertedintotheevacuatedchamberhavingavacuum-tightseal.This
isthenheatedtoevaporateorsublimethesampletogetthemoleculesinthegas
phase.
•Thegasesandheatvolatileliquids,ontheotherhandaregenerally
introducedthroughspeciallydesigneddeviceswithcontrolledflow.Theliquid
samplesarealsosuitablyevaporated.
•Oncethesampleisevaporatedthegaseousmoleculesarethenionisedbya
suitabletechnique;thisusuallyisaccompaniedbyfragmentationalso.
•Whentheanalyteisthermallylabilei.e.,itcandecomposeuponheating,then
weneedtouseothermethodslikedesorptionordesolvationmethodstobring
theanalyteintothevapourphase.
10

11. Ionization Chamber or Ion Source
•Sincemassspectrometerworksbysortingoutthechargedparticlesbyusing
magneticand/orelectricfields.Therefore,acompound/moleculemustbe
chargedorionizedtobeanalyzedbyamassspectrometer.
•Intheionizationchamber(alsocalledionsource)themoleculeisionizedby
usingoneofthemanymethodsavailableforthepurpose.Theionsourcesfall
intotwocategoriesasfollows:
•Gasphasesources
•Desorptionsources
Gasphasesources:Inthesesourcesthesampleisfirstvaporizedandthen
ionized.Theseareusedforlowmolecularweight(<1000Da)sampleswhichare
thermallystablei.e.,donotdecomposeonheatingandhavelowboilingpoints
(<500
o
C).Therearethreemethodsthatbelongtothiscategory.Theseare:
11

12. i)Electronionisation(EI)
ii)Chemicalionisation(CI)
iii)Fieldionisation(FI)
•Electronionisation:Thisistheoldestandprobablythebest-
characterizedofalltheionisationmethods.Inthismethod,ahigh
energybeamofelectronspassesthroughthesampleinthegas-
phase.Theseelectronsgenerallyhaveenergyof10-150eV.The
electronsoncollidingwiththeneutralanalytemoleculeknockoff
anelectronfromitandgenerateapositivelychargedion.This
processproduceseitheramolecularionoroneofitsfragments.
Thismethodisgoodforvolatilecompoundsbutthemolecularion
peakiseitherweakorabsent.
12

13. 13

14. Chemicalionisation:Thechemicalionisationmethoduses
ion/moleculereactionstoproduceionsfromtheanalyte.Forthis
purposeareagentgassuchasmethane,iso-butane,orammoniais
passedintotheionisationchamberwhereitgetsionisedby
electronionisation.Forexample,methanegasgivesmainlyCH
4
+
andCH
3
+
ionsasfollows
•Thesereagentgasionsthenreactwiththeneutralmoleculesofthe
reagentgasasfollow
14

15. •Theproductsoftheseion-moleculereactionsreactwiththeanalytemolecules
(M)toproduceanalyteions.Thereactionswiththeaboveionscanbeshown
as
M+CH
5
+
MH
+
+CH
4
M+C
2H
5
+
MH
+
+C
2H
4
•
Thesegiveanionat[M+1].FortheanalyteMofRHtype,wemayhave
reactionthatcanberepresentedas
•
Inthiscasetheionswouldbeobtainedat[M-1].Thus,themassspectrum
resultingfromchemicalionisationmethodgenerallycontainswelldefined
[M+1]
+
and[M-1]
+
ionpeaks.Furthersincethe(M+1)
+
ionsdonot
undergosignificantfragmentation,thespectraaresimplerascomparedto
theonesobtainedinEImethod.
15

16. •Desorptionsources:Inthesesourcesthesolidorliquidsampleisdirectly
convertedintothegaseousions.Theseareusedforhighmolecularweight
(>1000Da)sampleswhicharethermallyunstable(i.e.,decomposeon
heating)andarenonvolatile.Anumberofsourcesbelongingtothiscategory
areavailablebutweshalltakeupjustoneofthem,namelythefastatom
bombardment(FAB)method.
16

17. 17

18. Analyser
•Therearefourgeneraltypesofmassanalyzersthatcanbe
usedfortheseparationofionsinamassspectrometry.
1.QuadrupoleMassAnalyzer
•AQuadrupoleisamassanalyzerthatusesanelectricfield
toseparateions.TheQuadrupoleconsistsof4parallel
rods/poles,whereadjacentrodshaveoppositevoltage
polarityappliedtothem.Thevoltageappliedtoeachrodis
thesummationofaconstantDCvoltage(U)andavarying
radiofrequency(V
rfcos(wt)),wherew=angularfrequency
oftheradiofrequencyfield.Theelectricforceontheions
causestheionstooscillate/orbitintheareabetweenthe4
rods,wheretheradiusoftheorbitisheldconstant.
18

19. •Theionmovesinaverycomplexmotionthatisdirectly
proportionaltothemassoftheion,voltageonthequadrupole,and
theradiofrequency.Theionswillremainorbitinginthearea
betweenthepoleswithnotranslationalongthelengthofthepoles
unlesstheionshaveaconstantvelocitythatiscreatedastheions
enterthequadrupole.Beforeenteringtheanalyzer,theionstravel
throughapotentialofacertainvoltage,usuallycreatedbyring
electrode,inordertogivetheionsaconstantvelocitysotheycan
transversealongthecenterofthequadrupole.
•Whileinthequadrupole,thetrajectoriesoftheionschange
slightlybasedontheirmasses.Ionsofspecificmasshaveacertain
frequencybywhichtheyoscillate.Thegreaterthemass,the
greaterthefrequency.Acertainlimitisassociatedwitheach
quadrupoleanditselectsionswhicharewithinthedesirable
frequencyrange.
19

20. 20

21. 2. TOF (Time of Flight) Mass Analyzer
•TOF Analyzers separate ions by time without the use of an
electric or magnetic field. In a crude sense, TOF is similar to
chromatography, except there is no stationary/ mobile phase,
instead the separation is based on the kinetic energy and velocity
of the ions.
•Ions of the same charges have equal kinetic energies; kinetic
energy of the ion in the flight tube is equal to the kinetic energy of
the ion as it leaves the ion source:
KE = mv
2
/2 = zV
The time of flight, or time it takes for the ion to travel the length
of the flight tube is:
T
f= L(length of tube)/v(velocity of ion)
21

22. •Substitutingtheequationforkineticenergyinequationfortimeofflight:
T
f=L(m/z)
1/2
(1/2V)
1/2
=(Constant)*(m/z)
1/2
Duringtheanalysis,L,lengthoftube,V,Voltagefromtheionsource,areall
heldconstant,whichcanbeusedtosaythattimeofflightisdirectly
proportionaltotherootofthemasstochargeratio.
•Unfortunately,athighermasses,resolutionisdifficultbecauseflighttimeis
longer.Alsoathighmasses,notalloftheionsofthesamem/zvaluesreach
theiridealTOFvelocities.Tofixthisproblem,oftenareflectronisaddedtothe
analyzer.Thereflectronconsistsofaseriesofringelectrodesofveryhigh
voltageplacedattheendoftheflighttube.Whenaniontravelsintothe
reflectron,itisreflectedintheoppositedirectionduetothehighvoltage.
•Thereflectronincreasesresolutionbynarrowingthebroadbandrangeofflight
timesforasinglem/zvalue.Fasterionstravelfurtherintothereflectrons,and
slowerionstravellessintothereflector.Thiswaybothslowandfastions,of
thesamem/zvalue,reachthedetectoratthesametimeratherthenatdifferent
times,narrowingthebandwidthfortheoutputsignal.
22

23. •http://chemwiki.ucdavis.edu/Analytical_Chemistry/Instrumental_Analysis/Ma
ss_Spectrometry/Mass_Analyzers_(Mass_Spectrometry)
23

24. 3. Sector: Magnetic Sector Mass Analyzer
•Similartotimeofflightanalyzermentionedearlier,inmagneticsector
analyzersionsareacceleratedthroughaflighttube,wheretheionsare
separatedbychargetomassratios.Thedifferencebetweenmagnetic
sectorandTOFisthatamagneticfieldisusedtoseparatetheions.As
movingchargesenteramagneticfield,thechargeisdeflectedtoa
circularmotionofauniqueradiusinadirectionperpendiculartothe
appliedmagneticfield.Ionsinthemagneticfieldexperiencetwoequal
forces;forceduetothemagneticfieldandcentripetalforce.
F
B= zvB=F
c= mv
2
/r
The above equation can then be rearranged to give:
v = Bzr/m
If this equation is substituted into the kinetic energy
equation:
KE= zV=mv
2/2
m/z=B
2
r
2
/2V
24

25. •Basicallytheionsofacertainm/zvaluewillhaveauniquepathradius
whichcanbedeterminedifbothmagneticfieldmagnitudeB,and
voltagedifferenceVforregionofaccelerationareheldconstant.when
similarionspassthroughthemagneticfield,theyallwillbedeflected
tothesamedegreeandwillallfollowthesametrajectorypath.Those
ionswhicharenotselectedbyVandBvalues,willcollidewitheither
sideoftheflighttubewallorwillnotpassthroughtheslittothe
detector.Magneticsectoranalyzersareusedformassfocusing,they
focusangulardispersions.
25

26. 4.Sector: Electrostatic Sector Mass Analyzer
•Is similar to time of flight analyzer in that it separates the ions while in flight,
but it separates using an electric field. Electrostatic sector analyzer consists of 2
curved plates of equal and opposite potential. As the ion travels through the
electric field, it is deflected and the force on the ion due to the electric field is
equal to the centripetal force on the ion. Here the ions of the same kinetic energy
are focused, and ions of different kinetic energies are dispersed.
KE = zV =mv
2
/2
F
E= zE= F
c=mv
2
/R
Electrostatic sector analyzers are energy focusers, where an ion beam is focused
for energy.
Electrostatic and magnetic sector analyzers when employed individually are single
focusing instruments. However when both techniques are used together, it is called
a double focusing instrument., because in this instrument both the energies and the
angular dispersions are focused.
26

27. Detector or Ion Collector
•Inthemassspectrometerstheionsafterpassing
throughtheanalyseraregenerallydetectedbya
suitableelectronmultiplier.Theelectron
multipliersarecapableofprovidingquick
responsetimesandhighcurrentgains.The
electricalsignalsoobtainedcanbeprocessed,
storedorsuitablydisplayed.
27

28. Processing and Output Devices
•Atypicalmassspectrumcontainsalargeamountofstructuraldataintermsof
them/zvaluesandtherelativeintensitiesofallthefragmentsobtainedfrom
themolecule.
•Further,forthedatatobedependableandusefulanumberofinstrumental
parametersneedtobemonitoredandcontrolled.Thismeansalargeamount
ofdataanditsmanipulation.Itisachievedwiththehelpofmicroprocessors
andmicrocomputersthatareanintegralpartofallmassspectrometers.The
massspectrometerdatasystemsalsoincludesoftwaresforquantification,
interpretation,andidentificationofthemoleculesusingon-linespectral
libraries.
•Intheprocessingunitstheion-currentsignalsobtainedfromthedetectoris
digitalisedandextensivelyprocessedbeforebeingdisplayedintermsofa
massspectrum.Thespectrumdisplaysthem/zvaluesofallthefragments
andtheirintensitiesrelativetothatofthemostintensepeakcalledbase
peak.Sometimes,thedataisalsodisplayedintheformofatablewherein
them/zvaluesarelistedinanincreasingorderandthecorresponding
relativepeakintensitiesaregiveninnumbers.
28

29. 29
•Themassspectrometeranalyzesthemassesof
cations.
•Amassspectrumisaplotoftheamountofeach
cation(itsrelativeabundance)versusitsmassto
chargeratio(m/z,wheremismass,andzischarge).
•Sinceinmostofcases,zisalmost+1(sinceitis
muchmoredifficulttoremovefurtherelectronsfrom
analreadypositiveionorinsimpletermsinmostof
thecasesonlyoneelectronislostduringthe
bombardmentso‘z’isequivalentto+1),m/zactually
measuresthemass(m)oftheindividualions.
Mass spectrogram

30. 30
•Thetallestpeakinthemassspectrumiscalledthe
basepeak.
•ThebasepeakmayalsobetheMpeak,although
thismaynotalwaysbethecase.
•Isotopes:
•ThoughmostCatomshaveanatomicmassof
12,1.1%haveamassof13.Thus,
13
CH
4is
responsibleforthepeakatm/z=87inhexane.
ThisiscalledtheM+1peak.
•SomeisotopesshowM+2peaks.

31. 31
Characteristics of mass spectrum

32. 32

33. 33

34. 34
Figure 13.2 Mass spectrum of hexane (CH
3CH
2CH
2CH
2CH
2CH
3), C
6H
14.
MassSpectrometry
The molecular ion for hexane is at m/z= 86.
The base peak occurs a m/z= 57.
A small M + 1 peaks occurs at m/z= 87.

35. 35
ABranchedalkane:
MassSpectrometry
•Note that alkyl substituted benzenes generate a peak at
91 m/zdue to the tropylium ion.

36. 36
AnAlcohol:
MassSpectrometry
•Alcohols are easily dehydrated in the injection chamber
under full vacuum. As a result the molecular ion does
not appear and one observes an M-18 peak.

37. 37
McLaffertyRearrangement:
MassSpectrometry
•This fragmentation pattern is typically seen in
carbonyl compounds that have a γhydrogen.C H
O
CH
2
CH
2
CH
2
H
C H
O
CH
2
CH
2
CH
2
H
+
+
+ CH
3CH
2CH
2-CHO CH
3CH
2CH
2 + H - C = O
++
CarbonylCleavage:

38. 38
AlkylHalidesandtheM+2Peak:
•Most elements have one major isotope.
•Chlorine has two common isotopes:
35
Cl and
37
Cl,
which occur naturally in a 3:1 ratio.
Thus, there are two peaks in a 3:1 ratio for the
molecular ion of an alkyl chloride.
The larger peak, the M peak, corresponds to the
compound containing the
35
Cl. The smaller peak, (M + 2
peak), corresponds to the compound containing
37
Cl.
Thus, when the molecular ion consists of two peaks (M
and M + 2) in a 3:1 ratio, a Cl atom is present.
•Br has two isotopes:
79
Br and
81
Br, in a ratio of ~1:1.
Thus, when the molecular ion consists of two peaks
(M and M + 2) in a 1:1 ratio, a Br atom is present.
MassSpectrometry

39. 39
Figure 13.3 Mass spectrum of 2-chloropropane [(CH
3)
2CHCI]
MassSpectrometry
AlkylHalidesandtheM+2Peak:

40. 40
Figure 13.4 Mass spectrum of 2-bromopropane [(CH
3)
2CHBr]
MassSpectrometry
AlkylHalidesandtheM+2Peak:

41. 41
TheNitrogenRule:
•Hydrocarbons like methane (CH
4) and hexane
(C
6H
14), as well as compounds that contain only C,
H, and O atoms, always have a molecular ion with
an even mass.
•An odd molecular ion indicates that a compound
has an odd number of nitrogen atoms.
•The effect of N atoms on the mass of the
molecular ion in a mass spectrum is called the
nitrogen rule: A compound that contains an odd
number of N atoms gives an odd molecular ion. A
compound that contains an even number of N
atoms (including zero) gives an even molecular
ion.
MassSpectrometry

42. 42
HighResolutionMassSpectrometers:
•Low resolutionmass spectrometers report m/z
values to the nearest whole number. Thus, the
mass of a given molecular ion can correspond to
many different molecular formulas.
•High resolutionmass spectrometers measure
m/zratios to four (or more) decimal places.
MassSpectrometry

43. 43
HighResolutionMassSpectrometers:
This is valuable because except for
12
C whose
mass is defined as 12.0000, the masses of all
other nuclei are very close—but not exactly—
whole numbers.
Table 13.1 lists the exact mass values for a few
common nuclei. Using these values it is
possible to determine the single molecular
formula that gives rise to a molecular ion.
MassSpectrometry

44. 44
•Consider a compound having a molecular ion at m/z= 60
using a low-resolution mass spectrometer. The molecule
could have any one of the following molecular formulas.
MassSpectrometry
HighResolutionMassSpectrometers: