Mass Spectrometry Notes

Mass Spectrometry

Purpose of Mass Spectrometry

• Mass spectrometry produces spectra of masses from molecules in a sample and their fragments.
• It is used to:
  • Determine the elemental composition of a sample.
  • Determine the masses of particles and molecules.
  • Determine potential chemical structures of molecules by analyzing the fragments.
  • Identify unknown compounds by matching their mass spectra to known spectra.
  • Determine the isotopic composition of elements in a molecule.

Stages of Mass Spectrometry

• Ionizer: Converts some of the sample into ions.
• Mass analyzer: Separates the ions according to their mass-to-charge ratio.
• Detector: Records either the charge induced or the current produced when an ion passes by or hits a surface.

Mass Spectrometry Methods

Mass Spectrum

• Illustrative Example: $CO_2$
  • Molecular ion peak: $[CO_2]^+ = 44$
  • Fragment peaks:
    • $[CO]^+ = 28$
    • $[O]^+ = 16$
    • $[C]^+ = 12$

Mass Spectrum of Bromine ($Br_2$)

• Bromine has two isotopes: 50.69% $^{79}Br$ and 49.31% $^{81}Br$
• The mass spectrum shows:
  • $^{79}Br^+$
  • $^{81}Br^+$
  • $[^{79}Br^{81}Br]^+$
  • $[^{79}Br^{79}Br]^+$
  • $[^{81}Br^{81}Br]^+$
  • Molecular Ion Peaks
  • Fragments

Methyl Bromide ($CH_3Br$) Practice & Solutions

• Practice problem involving the mass spectrum of Methyl Bromide ($CH_3Br$).
• Solution includes peaks for:
  • $[CH_3^{81}Br]^+$
  • $[CH^{81}Br]$ and $[CH_3^{79}Br]^+$
  • $[CH_2^{81}Br]^+$
  • $[C^{81}Br]$ and $[CH_2^{79}Br]^+$
  • $[CH_3]^+$
  • $[CH^{79}Br]^+$
  • $[^{81}Br]^+$
  • $[C^{79}Br]^+$
  • $[^{79}Br]^+$

Methylene Chloride ($CH<em>2Cl</em>2$) Practice

• Practice problem involving the mass spectrum of Methylene Chloride ($CH<em>2Cl</em>2$).
• Chlorine is 75.77% $^{35}Cl$ and 24.23% $^{37}Cl$

Vinyl Chloride ($CH_2CHCl$) Practice

• Practice problem involving the mass spectrum of Vinyl Chloride ($CH_2CHCl$).
• Chlorine is 75.77% $^{35}Cl$ and 24.23% $^{37}Cl$

Spectra of Larger Molecules

• Spectra of large molecules have many fragments.
• The interpretation of these spectra is beyond the scope of this course.
• Example: Codeine, $C<em>{18}H</em>{21}NO_3$

Toluene ($C<em>7H</em>8$) Fragmentation

• Note that the molecular ion peak is NOT the dominant peak in the spectrum
• Key fragments include peaks at:
  • 92
  • 91
  • 77
  • 65
  • 51
  • 39

Mass Spectrometry in Forensics

• Mass spectrometry is used to confirm the identity of unknowns, such as illegal drugs.
• Unknowns are often not pure and must be separated from a mixture.
• Gas chromatography is used to separate the components of the mixture.
• Mass spectrometry "fingerprints" the components so that they can be matched to existing known spectra.