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Mass Spectrometry

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Mass Spectrometry Spectroscopic technique used for: Determination of molecular mass Determination of partial or whole molecular structure Confirmation of suspected ... – PowerPoint PPT presentation

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Title: Mass Spectrometry


1
Mass Spectrometry
  • Spectroscopic technique used for
  • Determination of molecular mass
  • Determination of partial or whole molecular
    structure
  • Confirmation of suspected molecular structure
  • The instrument used is a mass spectrometer

2
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3
Mass Spectrum of Methane
Mass Spectrum of Decane
4
Electron-Impact Mass Spectra
  • The sample is vaporized in a vacuum and subjected
    to an electron beam of high energy
  • The energy of the beam is typically 70 eV (6700
    kJ/mol)
  • This easily exceeds that of chemical bonds
  • A radical-cation is produced

12.6 Introduction to Mass Spectrometry
5
  • Fragmentation Reaction radical cation comes
    apart
  • Fragment Ion Ionic product of fragmentation
  • Cation or radical cation

6
  • Each of the fragments are separated according to
    their mass-to-charge ratio (m/z)
  • Only ions appear in the mass spectrum
  • neutral molecules and radicals do not appear

7
The Mass Spectrum of Methane
  • Molecular ion (M) The ion derived from electron
    ejection only (no fragmentation)
  • Base peak The ion of greatest relative abundance
    in the spectrum
  • M and base peak are commonly different

8
MS of Decane
9
Isotopic Peaks
  • M1 peak a peak occurring in a mass spectrum
    that is one unit higher than the molecular mass
  • Due to isotopes
  • 12C 98.9
  • 13C 1.10
  • 14C 1.0 x 10-10
  • 13CH4 17 g/mol

12.6 Introduction to Mass Spectrometry
10
Isotopic Abundances
11
Isotopes of Bromine and Chlorine
  • The M and M2 peaks for compounds containing
    bromine or chlorine are diagnostic
  • 79Br and 81Br ? 11
  • 35Cl and 37Cl ? 31

12.6 Introduction to Mass Spectrometry
12
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13
More About Fragmentation
  • Radical ions frequently decompose into smaller
    fragments
  • Two fragmentations may occur
  • M ? radical cation
  • Cation with even of electrons and odd mass
  • M ? neutral molecule radical-cation
  • Cation with odd of electron and even mass

12.6 Introduction to Mass Spectrometry
14
Fragmentation
  • An example of M ? radical cation

12.6 Introduction to Mass Spectrometry
15
MS of Decane
12.6 Introduction to Mass Spectrometry
16
Fragmentation
  • An example of M ? neutral molecule
    radical-cation

12.6 Introduction to Mass Spectrometry
17
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18
Di-sec-butyl ether
19
12.6 Introduction to Mass Spectrometry
20
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21
Chemical-Ionization Mass Spectra
  • More useful than EI for the determination of
    molecular mass
  • EI frequently fails to show the M peak
  • Chemical ionization (CI) is a softer technique
    that ionizes the sample with gas-phase protons (a
    protic environment)
  • Commonly a sample is analyzed by both CI and EI
    mass spectrometry

12.6 Introduction to Mass Spectrometry
22
Chemical Ionization Mass Spectra
12.6 Introduction to Mass Spectrometry
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