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Mass Spectrometry Mass Spectrometry
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Mass Spectrometry Mass Spectrometry
Principles Mass Spectra
http://www.mrc-dunn.cam.ac.uk/facilities/mass_spectrometry.phphttp://www.mrc-dunn.cam.ac.uk/facilities/mass_spectrometry.php
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Principles Principles
The study of ionised molecules in the gas phase
Principles date back to ~ 1897 Based on accelerating ions in a vacuum Joseph J. Thompson used an early MS to discover the electron .
Received the Nobel Prize in 1906 [Physics].
For a “friendly” practical introduction see: http://www.asms.org/whatisms/p1.html and following pages
USES Molecular weight determination Structural characterisation Gas phase reactivity study Qualitative and Quantitative analysis of mixtures
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Principles Principles
Volatilisation Ionisation Separation Detection
Sample
Volatilisation
Ionisation
Separation
Detection
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Volatilisation Volatilisation
Gaseous and volatile samples are readily drawn into the reservoir.
Less volatile solid samples require heating.
The volatile sample diffuses from the reservoir into the ionization chamber via a leak - a pin-hole restriction in a gold foil.
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Ionisation (1) Ionisation (1)
Electron impact or EI (most common) Sample passes through an electron beam. energy of e- beam is increased until e- is ejected from the
target molecule (normally ~70eV) high energy causes substantial fragmentation
M + e [M]+. + 2e
Radical cation
[MOLECULAR ION]
Most MS are set up to detect positive ions.
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Ionisation (2) Ionisation (2) Chemical ionisation (CI) Softer than EI (used for more sensitive compounds). Sample is introduced with an excess of a carrier gas (eg
NH3) which is ionised by the electron beam.
Lower energy means less fragmentation
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Separation Separation
A stable controllable magnetic
field separates the ions
according to their momentum. Only ions of a single
mass/charge [m/z] ratio will
have the trajectory to be
detected. By varying the magnetic, field
ions with different m/z values
are focused on the detector
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Detection Detection
The ion current will cause emission of secondary electrons
from a metal plate detector. The Faraday cup detector suppresses secondary ion
formation. The -ve plate also suppresses secondary ion formation
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Mass Spectra Mass Spectra
Features Fragmentation
patterns
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Features Features
After ionisation: Ejection of an electron from the parent molecule gives the
molecular ion (generally M+, but depends on ionisation
technique) M+ fragments giving rise to other peaks
Gives rise to a mass spectrum: Plotted as intensity (%) vs m/z (amu) The tallest peak is termed the base peak and is given an
intensity of 100%. Other peak intensities are given relative to this
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Features Features
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Fragmentation patterns (1)Fragmentation patterns (1)Occurs via two major routes:
[M]+. A+ (even electron cation) + B. (radical)
OR
[M]+. C+. (cation radical) + D (molecule)
NB Only particles with positive charges are detected
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Fragmentation patterns (2)Fragmentation patterns (2)
Characteristic fragmentation patterns arise as: Weak bonds tend to break Formation of stable fragments (ions, radicals and
molecules) is favoured Some molecules can form cyclic transition states
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Fragmentation patterns Fragmentation patterns
Alkanes Alkenes and Aromatic compounds Heteroatoms Isotopes
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Alkanes (1) Alkanes (1) Carbocation is an sp2 hybrid with an empty p orbital.
Stability of the cation depends on the number of alkyl groups, due to inductive effects
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Alkanes (2) Alkanes (2)
+CH3 < CH3CH2CH2CH2+ < CH3CH2
+CHCH3 < (CH3)3C+
Increasing stability
C
H2C
CH3
H3C
CH3
CH2
CH3-e
C
H2C
CH3
CH3
CH2
CH3H3C
C
H2C
CH3
CH3
CH2
CH3H3C
C
H2C
CH3
H3C
CH3
CH2
CH3
-e
C
H2C
CH3
H3C
CH3
CH2
CH3
More stable
Less stable
Aliphatic carbon skeletons are readily cleaved at branches because it results in carbocations that are more stable
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Alkanes (3) Alkanes (3)
Stable fragments formed from
branched alkyl chain results in
less strong Molecular Ion (MI)
NB Size of peak is relative
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Alkenes and Aromatic compounds Alkenes and Aromatic compounds (1)(1)
Cleavage occurs β to double bonds because more stable carbocations are produced due to resonance stabilisation
Cyclic alkenes can undergo rearrangements
Double bonds easily migrate in MS and isomers may easily be difficult to identify
-e
+
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Alkenes and Aromatic compounds Alkenes and Aromatic compounds (2)(2)
Aromatic hydrocarbons strong molecular ion β cleavage favoured strongly by resonance stabilisation fragmentation of ring energetically disfavoured
C C- e-
C C CC
C C
C+
Resonance
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Alkenes and Aromatic compounds Alkenes and Aromatic compounds (3)(3)
Aromatic hydrocarbons Characteristic m/z = 91 and 65 from tropylium ion and
breakdown product.
Alkynes – strong MI, similar to alkenes, cleave at β site
CH2X CH2
Tropylium ion m/z = 91
H2C CH2
C5H5+
m/z = 65
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Heteroatoms (1)Heteroatoms (1)
Cleavage may occur α, β or γ to heteroatoms depending on the functional groups involved. BEWARE: α to the carbonyl is β to the heteroatom!
a) α cleavage promoted by electronegativity eg ethers
ROC- e-
ROC C O R
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Heteroatoms (2)Heteroatoms (2)b) β cleavage – promoted by resonance stabilisation
eg Carbonyl group
NB the most stable carbocation will be found in greatest abundance
R X C C C
X = O, N, S, halogen
R X C C C- e-
X C
C
X C
+
C C R
O
C C R
O- e-
C O C R O C R
Acylium ionR = alkyl,- OH or -OR
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Heteroatoms (3)Heteroatoms (3)c) McLafferty rearrangements give γ cleavage (β to carbonyl) occurs with carbonyl containing compounds depends on six membered transition state(i) Ketones, and carboxylic acids
1º carboxylic acids give a characteristic peak at m/z = 60
O
CC
C
CH
R
R = alkyl or OH
O
CC
H
R
O
CC
H
R
+C
C
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Heteroatoms (4)Heteroatoms (4)ii) Carboxylic esters
Can undergo two types of McLafferty rearrangement
O
CC
C
CH
RO
O
CC
H
RO
O
CC
H
RO
+C
C
O
CO
C
CH
R
O
CO
H
R
O
CO
H
R
+C
C
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Heteroatoms Heteroatoms
Oxygen heteroatoms Nitrogen heteroatoms Halogen heteratoms
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Oxygen heteroatoms Oxygen heteroatoms Group Formul
aMI α β γ Other
Alcohols R-OH Weak or absent
Yes* Main *H2O Elimination
Aldehydes
RCHO weak No acylium ion
McLafferty
Ketones RCOR’ Strong No acylium ion
McLafferty
Carboxylic Esters
RCOOR’ Weak Not really
acylium ion
McLafferty
Carboxylic Acids
RCOOH Weak Not really
Mostly McLafferty
Ethers ROR’ Fairly weak
Yes Yes Rearrang-ements
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Nitrogen heteroatoms Nitrogen heteroatoms
Weak or absent MI iminium ion (like acylium) gives most intense peak may also undergo cyclic rearrangements.
Nitrogen rule: Compounds with an odd number of nitrogens
have an odd molecular weight.
eg NH3 MW = 17;
CH3CH2NH2 MW = 45;
NH2CH2CH2NH2 MW = 60
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Halogen heteroatoms Halogen heteroatoms
MI is strongest for Iodides less so for Bromides etc. More branched halides have weaker MIs. Cl and Br Isotopes give visible patterns
Fragmentation may include (in order of likelihood):
a) Loss of halogen
b) Loss of HX
c) β cleavage (as for oxygen) – loss of CH2X
d) Rearrangements
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Isotopes (1)Isotopes (1)
It is important to use the correct method to calculate molecular
weight for MS. Average atomic masses take into account the
different abundances of isotopes. Need to use exact mass for
MS.
CARBON
1.1% of naturally occurring carbon is 13C: gives 1% abundance
m/z = 17 signal (M+H)+ in spectrum of Methane.
HALOGENS
Significant amounts of different isotopes, visible patterns within
spectrum.
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Isotopes (2)Isotopes (2)
Chlorine: Average Atomic weight = 35.453
Isotope 35Cl 37Cl
Atomic weight
35 37
Abundance 75.8 24.2
Ratio ~ 3 1
Rel
ativ
e ab
unda
nce
M
M+2
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Isotopes (3)Isotopes (3)Bromine: Average atomic weight = 79.904 Isotope 79Br 81Br
Atomic weight
79 81
Abundance 50.6 49.4
Ratio ~ 1 1
Rel
ativ
e ab
unda
nce
M M+2
m/z m/z
Isotopic pattern for
one bromine atom
Rel
ativ
e ab
unda
nce
M
M+2
M+4 eg
CH3Br Mass:
50% CH379Br = 94
50% CH381Br = 96
CH2Br2 Mass:
25 % CH279Br79Br = 172
25 % CH279Br81Br = 174
25 % CH281Br79Br = 174
25 % CH281Br81Br = 176
Isotopic pattern for
one bromine atom
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Characteristic signals in Characteristic signals in MSMS
Functional group Characteristic m/z
Alkanes 29, 43, 57, 71, 85
Alkenes Ions resulting from β cleavage
Amines 30, 44, 58, 72
Benzene 65, 77, 91
Halides Isotope peaks, loss of X and HX
Carbonyl, Ester, Acid Acylium ions and Mclafferty rearrangement products common