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Lecture 6

Lecture 6. Mass Spectrometry Due: Lecture Problem 4. Spectral Analysis. NMR: Bond connectivities Types and kinds of nuclei IR: Functional groups. Now, let’s look at Mass Spectrometry : Determine the mass of the sample Look at fragmentation patterns determine “pieces”

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Lecture 6

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  1. Lecture 6 Mass Spectrometry Due: Lecture Problem 4

  2. Spectral Analysis • NMR: • Bond connectivities • Types and kinds of nuclei • IR: • Functional groups • Now, let’s look at Mass Spectrometry: • Determine the mass of the sample • Look at fragmentation patterns determine “pieces” • of the molecule.

  3. Mass Spectrometry • Basic Idea: • To ionize: bash the molecule with a high energy particle: • An electron (known as Electron Ionization) or • A proton (known as Chemical Ionization) • A molecular ion results: • Electron Ionization (EI): M + e- M+• • Chemical Ionization (CI): M + H+ MH+ • Send molecular ion through a magnetic field • Molecular ion travels and reaches a detector. The detector • only detects positively charged species  m/z ratio (mass/ • charge). • Obtain mass of molecular ion and fragments. The intensity of the signals • is proportional to the number of ions. The more intense the signal, the • more stable that ion is.

  4. The Mass Spectrometer

  5. Mass spectrum of cyclohexamine M+ = 99

  6. Molecular Ions Molecular ions of different compounds have different stabilities and, therefore, different fates. Some will reach the detector. Others will fragment before they reach the detector. Molecular ions of different compounds will have different intensities: • Typically strong in aromatics • Typically weak in alkanes, alcohols & others

  7. Molecular Ions M+ = 78 Benzene Strong M+ M+ = 86 Hexane Weak M+ M+ = 102 Hexanol M+ not seen

  8. Nitrogen Containing Compounds If odd # of N’s, then odd mass. If even mass, then either zero N’s or even # of N’s. M+ = 99 Cyclohexamine

  9. Molecular Ions Molecular Ions = sum of atomic weight(s) of all isotopes of all elements in a molecule. If you had 10,000 atoms of each type: • C: 9890 12C atoms 110 13C atoms • H: 9999 1H atoms 1 2H atoms • O: 9985 16O atoms, 4 17O atoms, 20 18O atoms • N: 9963 14N atoms 37 15N atoms • Cl: 7577 35Cl atoms 2423 37Cl atoms (3:1 35Cl:37Cl) • Br: 5069 79Br atoms 4931 81Br atoms (1:1 79Br:81Br)

  10. Molecular Ions: Cl & Br Compounds - Isotopes Significant M+2 peak => Cl or Br • Cl is 3 parts 35Cl and 1 part 37Cl • Br is 1:1 79Br and 81Br

  11. MS of Chlorobenzene M+ C6H535Cl = m/z 112 C6H537Cl = m/z 114 Note ratio: m/z 112:114 is 3:1 M + 2

  12. MS of Bromobenzene C6H579Br = m/z 156 C6H581Br = m/z 158 M+ = 156 M + 2 = 158 Note ratio: 156:158 1:1

  13. Mass Spectral Interpretation • Molecular ion bonds break to give fragment IONS (detected by MS) and NEUTRAL radicals/molecules (NOT detected by MS). • Spectra only show ions, not neutrals. • The greater the stability of the fragment ion, the more intense the peak. • Mass of neutral = m/z of M+- m/z of F+ (fragment ion)

  14. Mass Spectrum of Hexane

  15. Mass Spec Correlation Charts Common fragments and/or neutral losses: • 15 can only be methyl • 17 usually OH • 18 always H2O • 28 CO or CH2=CH2 • 29 CH3CH2 or CHO • 31 CH3O • 35/37 Cl (special isotope pattern too!) • 42 CH2=C=O • 43 CH3CO or C3H7 • 45 CH3CH2O or COOH • 79/81 Br (special isotope pattern too!)

  16. Mass Spec Correlation Charts Weights of common fragments or neutral losses:

  17. Sample Problem 1 Account for the peaks in the mass spectrum of 2-octanone at m/z values of 129 (M+1), 128 (M+), 113, 85, and 43.

  18. Sample Problem 2 The most intense peak in the mass spectrum of 2,2-dimethylbutane is at m/z = 57. Propose a structure of this fragment ion (daughter ion).

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