Download - Mass Spectrometry
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Mass Spectrometry
To here!From here…
Detection of ionic species to determine the molecular weight of
and obtain structural information on a molecule
Molecular Weight: 70 amu
Molecular Formula:
C5H10
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Mass spectrometry
electron beam+ 1e-
Molecular ion (M+)
+
CH3
+
CH2CH3
etcFragments
The more stable a fragment, the more likely it will form
ionicdetected
not ionicnot detected
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Fig. 12-1, p. 410
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Gas Chromatography –Mass Spectromety (GC-MS)
Liquid Chromatography –Mass Spectromety (LC-MS)
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CH3CH2CH3
Exact Mass: 44.0626Mol. Wt.: 44.0956
29
Base peakMolecular ion (M+)
Base Peak
Molecular ion-15
-CH3
CH4
Exact Mass: 16.0313Mol. Wt.: 16.0426
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Fig. 12-4, p. 413
Mass Spectrum of Hexane
Chemical Formula: C6H14
Exact Mass: 86.1095Molecular Weight:
86.1753
-15
-CH3
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Fig. 12-5, p. 413
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Fig. 12-6, p. 414
methylcyclohexane or ethylcyclopentane ?
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Isotope effects
Molecular weightBr = 79.9040
Br isotopic abundance79Br = 50.5 %81Br = 49.5 %
CH379Br
CH381Br
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Isotope effects
Molecular weightBr = 79.9040
Br isotopic abundance79Br = 50.5 %81Br = 49.5 %
CH379Br
CH381Br
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Isotope effects
Molecular weightCl = 35.453
Cl isotopic abundance35Cl = 75.4 %37Cl = 24.6 %35Cl : 37Cl Ratio = 3:1
CH335Cl
CH337Cl
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• -cleavage (cleavage of bond one removed from radical or radical cation)– Carbonyl compounds
CH3 H
O
C OH +CH3 H
OCH3
Fragmentation by -cleavage
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43
58
29 44
Acetone
H3C CH3
O
Exact Mass: 58.0419Mol. Wt.: 58.0791
H3C H
O
Exact Mass: 44.0262Mol. Wt.: 44.0526
Acetaldehyde
-15
-CH3
-15
-CH3
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OH
HH
OH
HH
• -cleavage (cleavage of bond one removed from radical or radical cation)– Carbonyl compounds
– Amines and alcohols
– Alkenes
CH3 H
O
C OH +CH3 H
OCH3
Fragmentation by -cleavage
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Fragmentation by -cleavage• -cleavage (cleavage of bond one
removed from radical or radical cation)– Carbonyl compounds
– Amines and alcohols
– Alkenes
CH3 H
O
C OH +CH3 H
OCH3
OH
HH
OH
HH
CH3
CH3 H
R
HH
CH3
CH3 H
R
HH
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Fragmentation by -cleavage• -cleavage (cleavage of bond one
removed from radical or radical cation)– Methyl benzene (toluene) compounds
CH
H
H
CH
H
H
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Fig. 12-8, p. 416
m/z = 102
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Determination of Formula:Rule of 13
Assumes CnHn and amu equivalent (13 for n=1) is present in all molecular fragment ions
Step 1: Divide M+ mass by 13, this gives n
Step 2: Any remainder represents count of additional Hʼs
example 1: for M+ = 7878 ÷ 13 = 6 → n = 6 → C6H6
example 2: for M+ = 9292 ÷ 13 = 7.077 → n = 77 x 13 = 91 → 1 extra H is presentFormula is C7H7+1 = C7H8
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Determination of Formula:Rule of 13
example 3: for M+ = 161161 ÷ 13 = 12.385 → n = 1212 x 13 = 156 → 5 extra Hʼs are presentFormula is C12H12+5 = C12H17
What about heteroatoms?Step 1: First derive formula as aboveStep 2: Next, modify using CnHm equivalents
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Problem
An unknown compound subjected to mass spectrometry gave the parent ion at m/z = 114. The parent ion quickly undergoes homolytic -cleavage to give fragment ions with m/z = 99 (100 %) and m/z = 45 (45 %). No other significant peaks are seen the in spectrum. The unknown does not react when treated with H2, Pd/C. The infrared spectrum of the unknown shows a strong, broad peak at 3500 cm-1. Show the reaction and the electron pushing mechanism that accounts for the fragmentation of the parent ion to give either of the observed fragments.
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MS Ionization Techniques
1. Electron Impact (EI)uses beam of electrons
2. Fast Atom Bombardment (FAB)uses beam of atoms
3. Chemical Ionization (CI)uses chemical reactions
4. Matrix Assisted Laser Desorption (MALDI)uses lasers to ionize molecules
5. Electrosprayuses compounds which are already ionic
High energyhard ionization
Low energysoft ionization