Forensic Applications of Mass Spectroscopy

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Forensic Applications of Mass Spectroscopy
2010-11-30 23:35:05
Forensic Chemistry

Mass Spectroscopy
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  1. Name some of the forensic applications of mass spectroscopy.
    • Drug Analysis (cocaine, hash, LSD)
    • Toxicology (poisons, alcohol, mace)
    • Arson (petroleum, alcohol based ignitable liquids)
    • Explosives (nitro-substituted benzenes, TATP)
    • Trace Analysis (polymers, adhesives, general unknowns)
  2. What is a mass spectrometer?
    An analytical instrument which creates charged particles from molecules, separate ions, and detect ions.
  3. A mass spectrometer separates ions according to which ratio?
    mass to charge (m/z)
  4. What are the 2 gas-phase methods of ionization?
    • Electron Impact
    • Chemical Ionization
  5. What are the 2 spray methods of ionization?
    • Electrospray
    • Atmospheric Pressure Chemical Ionization
  6. What are the 2 desorption methods of ionization?
    • Fast Atom Bombardment
    • Matrix-Assisted Laser Desorption Ionization
  7. How is the molecular ion formed in electron-impact?
    it is the result of a high energy electron colliding with a molecule forcing the loss of an electron from the molecule
  8. T/F: Molecular ions may degrade further to smaller fragment ions as a result of residual energy from the electron impact process.
  9. ______ of the filament in the source causes the emission f electrons which are accelerated towards the anode in the ______ ______ ionization technique.
    • Heating
    • electron impact
  10. Name some of the advantages of electron impact.
    • Well-established
    • Fragmentation libraries exist
    • Interface to GC
    • Non-Polar Samples
  11. Name some of the disadvantages of Electron Impact.
    • Parent Identification
    • Need volatile sample
    • Need thermal sample
    • Low mass compounds (<1000 amu)
  12. Define radicals
    an atom or group of atoms that has at least one unpaired electron and is therefore unstable and highly reactive
  13. Define neutrals
    a particle that has neither positive nor negative electric charge; a net electric charge of zero
  14. Define cations
    an ion or group of ions having a positive charge and characteristically moving toward the negative electrode in electrolysis; even electron species
  15. Define radical ion
    a charged compound that has an unpaired electron; it may be either a radical cation or radical anion; odd electron species
  16. What is the most common and simplest fragmentation?
    bond cleavage resulting in neutral radicals and a cation
  17. What is the less common fragmentation type?
    An even-electron neutral is lost and results in odd-electron radical cation fragment ion
  18. What is heteroatom participation?
    the presence of a functional group containing a heteroatom (N, S, O, X) can alter the fragmentation pattern
  19. Between what 2 things do the ion molecule reactions occur in chemical ionization?
    • ionized reagent gas molecules
    • volatile analyte neutral molecules
  20. In positive ion mode of chemical ionization, is a proton added or subtracted?
  21. What are the advantages of the chemical ionization technique?
    • parent ion presence
    • interface to GC
  22. What are the disadvantages of the chemical ionization technique?
    • no fragment library
    • need volatile sample
    • need thermal stability
    • low mass compounds (<1000 amu)
  23. What is the nitrogen rule?
    Ions without N atoms or those with an even number of N atoms have an even numbered mass, and the majority of the fragment ions have odd masses.

    Ions with an odd number of N atoms have an odd numbered mass and the majority of the fragment ions have even masses.
  24. T/F: The presence of bromine or chlorine in a molecule or ion are not easily detected by the intensity ratios of ions differing by 2 amu.
    False! They can be easily detected by the intensity ratios differing by 2 amu.
  25. What 2 ions are included in the 50:50 mixture of bromine?
    79Br and 81Br
  26. What 2 ions are included in the 75:25 mixture of chlorine?
    35Cl and 37Cl