Mass Spectrometry

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Mass Spectrometry
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Molecular Weight 70 amu Molecular Formula
C5H10
Detection of ionic species to determine the
molecular weight of and obtain structural
information on a molecule
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Mass spectrometry
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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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Base peak Molecular ion (M)
-CH3
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Mass Spectrum of Hexane
Chemical Formula C6H14 Exact Mass
86.1095 Molecular Weight 86.1753
Fig. 12-4, p. 413
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Fig. 12-5, p. 413
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methylcyclohexane or ethylcyclopentane ?
Fig. 12-6, p. 414
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Isotope effects
Molecular weight Br 79.9040
Br isotopic abundance 79Br 50.5 81Br 49.5
CH379Br
CH381Br
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Fragmentation by a-cleavage

• a-cleavage (cleavage of bond one removed from
  radical or radical cation)
• Carbonyl compounds

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-CH3
-CH3
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Fragmentation by a-cleavage

• a-cleavage (cleavage of bond one removed from
  radical or radical cation)
• Carbonyl compounds
• Amines and alcohols
• Alkenes

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Fragmentation by a-cleavage

• a-cleavage (cleavage of bond one removed from
  radical or radical cation)
• Carbonyl compounds
• Amines and alcohols
• Alkenes

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Fragmentation by a-cleavage

• a-cleavage (cleavage of bond one removed from
  radical or radical cation)
• Methyl benzene (toluene) compounds

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m/z 102
Fig. 12-8, p. 416
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Determination of FormulaRule of 13
Assumes CnHn and amu equivalent (13 for n1) 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
H's example 1 for M 78 78 13 6 ? n 6
? C6H6 example 2 for M 92 92 13 7.077
? n 7 7 x 13 91 ? 1 extra H is
present Formula is C7H71 C7H8
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Determination of FormulaRule of 13
example 3 for M 161 161 13 12.385 ? n
12 12 x 13 156 ? 5 extra H's are
present Formula is C12H125 C12H17 What
about heteroatoms? Step 1 First derive formula
as above Step 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