Mass 2021 2 (Electron impact)

Mass (Electron impact)

1. Lecture-2(Electron impact)
Ph.D.AhmedMetwaly
Mass spectrometry
ORCID account
Email: [email protected]
1
•Associate Professor of Pharmacognosy , faculty of Pharmacy, Al-Azhar University
•Senior research fellow, Liaoning University of Traditional Chinese Medicine, China (20118-2019)
•Visiting scholar, School of Pharmacy, University of Mississippi, USA (2012-2014)

2. Objectives
❑Methods of ionization
❑Electron impact
Nitrogen rule
Mechanisms of fragmentation
Sigma bond cleavage [σ]
Charge site initiation
Radical site initiation [α-cleavage]
Application of MS to organic compounds

3. Methods of sample ionization (Ionization
techniques):
Hard ionization technique:
a lot of energy involved.
extensive fragmentation of analyte molecules.
Electron Impact (EI) most common, uses an electron beam (70 eV electrons) to
ionize samples.
Soft ionization techniques:
Less energy is used to ionize the analyte molecule.
Less fragmentation occurs (softer on the molecule).

4. Electron Impact (EI):
Thismethodisusedforionizationoflowmolecularweightcompounds.
Inanelectronimpactmassspectrometer,ahighenergybeamofelectronsisusedto
displace(dislodge)anelectronfromtheorganicmoleculeinthegasphasetoforma
radicalcationknownasthemolecularionorparention.
Themolecularionfragmentedtogiveotherpositivelychargedsmallerions(fragment
ions)andaradical.M + e
-
M + 2 e
-
m
1
+
+ m
2
Ionization Fragmentation

7. Electron Impact (EI):
Advantages of EI:
Sensitive.
Its ability to produces a considerable number of fragment ions which help in
identification of the compound.
Fragmentation provides “fingerprint”.
Disadvantages of EI:
Molecular ion peak often eliminated.
Sample volatilization necessary(not suitable for thermo labile compounds).

8. Terminology:
Molecularion(M
+
):Theionobtainedbythelossofanelectronfromthe
moleculeandrepresentthemolecularweightofthecompound.
Basepeak:ThemostintensepeakintheMS,itisassignedavalueof100%
intensityandtheintensitiesofotherpeaksarereportedaspercentagesofthe
basepeak.
Radicalcation:Thepositivechargedspecieswithanoddnumberofelectrons.
Fragmentions:Thelightercationsformedbythedecompositionofthemolecular
ion.Theseoftencorrespondtostablecarbcations.

10. The Nitrogen Rule
-CompoundcontainingC,H,O,andanevennumberofnitrogenatoms(or
nonitrogen)willhaveanevenmolecularweight.
-CompoundcontainingC,H,O,andanoddnumberofnitrogenatomswill
haveanoddmolecularweight.Why???

11. Problem
Whichofthefollowingionswillappearatanevenmass
number?
NH
3
C
4H
9N
C
5H
6N
2
C
2H
5NH
2

12. Mechanisms of fragmentation
Sigma bond cleavage [σ]
Thehighermolecularweightcarbocationabletostabilizethe
positivecharge.-e
-
C
H
3C
H
3C
H
3C
CCH
2CH
3
H
3C
H
3C
H
3C
C
H
3C
H
3C
H
3C
+
+
CH
2CH
3

m/z = 86
m/z = 57
CH
2CH
3 C
2H
5S C
2H
5
C
2H
5S C
2H
5

C
2H
5S
+
+C
2H
5
-e
-

13. Let's have another look at the mass spectrum for pentane

14. Inthestickdiagramshowingthemassspectrumofpentane,
thelineproducedatm/z=72isduetothemolecularion.
Whatcausesthelineatm/z=57?(HowaboutC4H9+)
C4H9+wouldbe[CH3CH2CH2CH2]+,andthiswouldbe
producedbythefollowingfragmentation:
Themethylradicalproducedwillsimplygetlostinthe
machine.

15. Thetallestlineinthestickdiagramatm/z=43iscalledthe
basepeakcanbeworkedoutsimilarly.
Ifyouplayaroundwiththenumbers,youwillfindthatthis
correspondstoabreakproducinga3-carbonion:
Thelineatm/z=29istypicalofanethylion,[CH3CH2]+:

16. Charge site (inductive cleavage) [ί]
Chargesiteinitiation[ί]throughinductiveeffectC
2H
5O C
2H
5
i
C
2H
5
+
+C
2H
5OC
2H
5O C
2H
5
-e
-

17. Radical site initiation [α-cleavage] CH
3-CH
2OCH
2CH
3
-e
-

H
2C=OC
2H
5
+
+CH
3
CH
3-CH
2OCH
2CH
3

18. Retro Diels Alder fragmentation [RDA]
RDAfragmentationmaybesimplifiedas:-e
-
+ =
m/z = 55

 
 +
RDA

19. Hydrogen rearrangement; rH
[McLaffertyrearrangement]
Forunsaturatedcompounds.
Six-memberedtransitionstate.
Thismethodusedforcompoundshavingacarbonylgroup
andcontainingGamma-hydrogen,e.g.carboxylicacids(e.g.
R-CH2-CH2-CH2COOH).

20. C
O
H
2C OH
RCH
H
CH2
-e
-
C
O
H
2C OH
RCH
H
CH2
r
H
C
O
H
2C OH
RCH
CH2
H
C
O
H
2C OH
RCH
CH2
H
+
m/z = 60
 Therefore,longchaincarboxylicacidsusually
yieldpeakatm/z60intheirmassspectra.

21. Molecular ion abundance
Abundancereflectsthestabilityofmolecularionsformed.
Increasedunsaturationandnumberofrings,increasesthe
stabilityandabundances.
Resonanceswillincreaseabundances.
BranchingandincreasedchainlengthuptoC6orC8,
decreasestabilityandabundances.

22. Probability of ionization [I]
1.[I]valueofaπ-bondislowerthanthatofσ-bond.
2.[I]valueofconjugatedπ-bondislowerthanthatof
unconjugated.
3.[I]valueofnon-bondingelectronsonaheteroatom
islowerthanofπ-bond.

23. Common MS fragments of organic compoundsm/z lost Moiety Compounds exhibiting loss
1 H aldehydes
15 CH3 branched sites
16 O sulfoxides, nitro compounds
16 NH2 amides, aromatic amines
17 OH acids
18 H2O alcohols, aldehydes, ketones, ethers

24. m/z lost Moiety Compounds exhibiting loss
26 CN alkylcyanides
29 C2H5 or CHO alcohols
31 OCH3 or CH2OH methyl esters, alcohols
35 Cl halide-containing
45 OC2H5 or COOH ethyl esters or carboxylic
acids

25. Application of MS to organic compounds
Saturatedhydrocarbons-regularlyspacedclustersseparatedby14massunits
CleavagefavoredatbranchedCatoms:
Tertiary>secondary>primary
PositivechargeonbranchedC(carboniumion).
•Morestablecarbocationswillbemoreabundant.
•Ringcompoundshavestrongparentions.Intensityrelatedtostabilityofring.
•Compoundswithcarbonylbreakatthisgroup
m/z16largeforprimaryamines.

26. Mass spectra of alkanes
More stable carbocations will be more abundant

28. Mass Spectra of Alkenes
Resonance-stabilized cations favored

29. Application of MS to natural products

30. Fatty acids
Molecularionpeakofastraightchainmonocarboxylicacidisweak
butusuallydiscernible.
Themostcharacteristicpeak(sometimesthebasepeak)isatm/e60
duetoMcLaffertyrearrangement.

32. MS spectrum of stearic acid

33. Methyl ester of fatty acids
Themassspectrumofamethyl-esterisverysimilartothatof
correspondingcarboxylicacid.
Themethylesterismorevolatilethanthefreefattyacidsand
thereforetheeasiertoexamine.
m/e74;correspondingtothem/e60peakoffattyacidis
usuallybasepeakorpredominant.

35. MS spectrum of methyl stearate

36. Alkaloids (Papaverine)
Usingradicalsiteinitiation[α-cleavage]:N
H
3CO
H
3CO
H
3CO
H
3CO
-e
-
N
H
3CO
H
3CO
H
3CO
H
3CO
N
+
H
3CO
H
3CO
H
3CO
H
3CO

+

37. Volatile oils(Limonene)
UsingRDAfragmentation.+ =
m/z = 68
m/z = 68
+RDA
or
RDA-e
-




38. Volatile oils(Menthone)
UsingrH(McLaffertyrearrangement):O O
-e
-
H
OH OH
+
m/z = 112
r
H


39. Summary
Nitrogen rule
Mechanisms of fragmentation
Sigma bond cleavage [σ]
Charge site initiation
Radical site initiation [α-cleavage]
Application of MS to organic compounds

40. Ph.D.Ahmed M.Metwaly
Associateprofessor,Pharmacognosy
department,
FacultyofPharmacy,Al-Azhar University.
[email protected]
Email: [email protected] 40