Introduction to Mass Spectrometry

1
Lecture 22

• Introduction to Mass Spectrometry
• Lecture Problem 7 Due
• This week in lab
• Work on 1st Synthetic
• Next week in lab
• Synthetic Experiment 2 PreLab Due
• Ch 9 Final Report Due

2
Spectral Analysis

• 1H NMR
• Bond connectivities
• Types and kinds of protons
• IR
• Functional groups

• Now, lets look at Mass Spectrometry
• Determine the mass of the sample
• Look at fragmentation patterns determine
  pieces
• of the molecule.

3
Mass Spectrum of Cyclohexamine
M 99
Fragments
4
Mass Spectrometry

• How to get the mass of the sample or pieces of
  the sample
• Ionize the sample with a high energy particle to
  give
• a molecular ion, M (a.k.a. parent ion).
• Note Ions may fragment which results in pieces
  of the sample.
• When fragmentation occurs, there is always a
  positive
• fragment (ion) and a neutral fragment.
• Only positive species are detected by the
  detector. Thus,
• only the mass of the molecular ion and the
  masses of
• positive ion fragments will be read by the
  detector and appear
• on the mass spectrum for the compound. Neutral
  fragments
• are not read by the detector and do not appear
  on the spectrum.

5
Mass Spectrometry

• Basic Idea
• To ionize bash the molecule with a high energy
  particle in a vacuum
• 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.

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The Mass Spectrometer
A quadrupole mass spectrometer. Ions oscillate
as they move through the electric fields on the
four rods. Resonant ions hit the
detector. Nonresonant ions are deflected and
dont hit the detector.
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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
• Strong in aromatics
• Weak in alkanes, alcohols others

8
Mass Spectrum of Hexane
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Molecular Ions
M 78
Benzene Strong M
M 86
Hexane Weak M
M 102
Hexanol M not seen
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Nitrogen Containing Compounds
If odd of Ns, then odd mass. If even mass,
then either zero Ns or even of Ns.
M 99
Cyclohexamine
11
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 (31
  35Cl37Cl)
• Br 5069 79Br atoms 4931 81Br atoms (11
  79Br81Br)

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Mass Spectrum of Hexane
87
Hexane with one 13C atom M 1