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Mass Spectrometry

Mass Spectrometry. Electron Ionization and Chemical Ionization. Mass Spectrometer. All Instruments Have: Sample Inlet Ion Source Mass Analyzer Detector Data System. http://www.asms.org. Ionization Techniques. Gas-Phase Methods Electron Ionization (EI) Chemical Ionization (CI).

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Mass Spectrometry

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  1. Mass Spectrometry Electron Ionization and Chemical Ionization

  2. Mass Spectrometer • All Instruments Have: • Sample Inlet • Ion Source • Mass Analyzer • Detector • Data System http://www.asms.org

  3. Ionization Techniques Gas-Phase Methods • Electron Ionization (EI) • Chemical Ionization (CI) • Desorption Methods • Matrix-Assisted Laser Desorption Ionization (MALDI) • Fast Atom Bombardment (FAB) • Spray Methods • Electrospray (ESI) • Atmospheric Pressure Chemical Ionization (APCI)

  4. Electron Ionization http://www.noble.org/PlantBio/MS/ion_tech_main.html

  5. h λ= mv Electron Ionization

  6. Electron Ionization • Samples must be vaporized in the ion source • Typically 1 of 1000 molecules entering the source is ionized • 10-20 eV of energy is imparted to the molecule • ~10eV is enough to ionize most molecules • Up to 230 kcal/mol is left to cause fragmentation

  7. Electron Ionization

  8. Electron Ionization (low picomole) Advantages • Well-Established • Fragmentation Libraries • No Supression • Insoluble Samples • Interface to GC • Non-Polar Samples • Disadvantages • Parent Identification • Need Volatile Sample • Need Thermal Stability • No Interface to LC • Low Mass Compounds (<1000 amu) • Solids Probe Requires Skilled Operator

  9. Chemical Ionization http://www.noble.org/PlantBio/MS/ion_tech_main.html

  10. Chemical Ionization • Reagent gas is introduced into the source at ~0.5 torr • Reagent gas is preferentially ionized. Ions react mostly with neutral reagent gas • Reactions occurring depend on the nature of the reagent gas • Ions in the reagent gas plasma react with the analyte

  11. Chemical Ionization: Methane • Methane primarily forms CH4+• with CH2+• and CH3+ • CH4+• + CH4→ CH5+ + CH3 (m/z 17) • CH2+• + CH4→ C2H3+ + H2 + H• • C2H3+ + CH4→ C3H5+ + H2 (m/z 41) • CH3+ + CH4→ C2H5+ + H2 (m/z 29)

  12. Chemical Ionization: Methane

  13. Chemical Ionization: Methane • Ions other than saturated hydrocarbons react via proton transfer • CH5+ + M → MH+ + CH4 (or via C2H5+ or C3H5+) • For saturated hydrocarbons, hydride abstractions is common • CH5+ + RH → R+ + CH4 + H2 • For polar molecules, adducts can form • CH3+ + M → (M+CH3)+ • MH+, R+, and adducts are pseudomolecular ions.

  14. Chemical Ionization: Isobutane

  15. Chemical Ionization: Isobutane

  16. Chemical Ionization: Isobutane • Reacts through Proton Transfer • C4H9+ + M → MH+ + C4H8 • For saturated hydrocarbons, no reaction • For polar molecules, adducts can form • C4H9+ + M → (M+C4H9)+ • Lack of reaction with hydrocarbons can be used for selective detection of compounds in mixtures containing hydrocarbons • Less fragmentation is observed with isobutane. (molecular species is more reliably formed)

  17. EI vs. Methane vs. Isobutane EI Methane CI Isobutane CI

  18. EI vs. Methane vs. Isobutane

  19. Chemical Ionization: Ammonia Largely forms NH4+ and (NH4 + NH3)+

  20. Chemical Ionization: Ammonia • Reacts through Proton Transfer with basic molecules • NH4+ + M → MH+ + NH3 • Less basic molecules will form adducts • NH4+ + M → (M+NH4)+ • Many molecules with intermediate basicity, both ions will be observed • For saturated hydrocarbons, no reaction • Less fragmentation is observed with ammonia. (molecular species is most reliably formed)

  21. Chemical Ionization: Negative Ions • Low energy electrons are present in the CI plasma • These can attach to molecules with high electron affinities • There are two principal pathways • AB + e-→ AB-• (associative resonance capture) • AB + e-→ A• + B- (dissociative resonance capture) • Deprotonation can also occur if a basic ion is formed in the reagent gas plasma

  22. Chemical Ionization: Negative Ions • A common negative CI gas is a mixture of N2O/CH4 in a ratio 25/75 • N2O + e-→ N2O-• • N2O-•→ N2 + O-• • O-• + CH4 → CH3• + OH- • This mixture yields thermal electrons for electron capture and hydroxide for deprotonation of acidic molecules

  23. Proton Affinity ∆H˚acid CH4 543 1749 H2O 691 1634 NH3 854 1689 CH3OH 754 1592 C2H5OH 776 1583 IPA 793 1573 t-BuOH 803 1568 Aniline 921 1533 Chemical Ionization

  24. Chemical Ionization (low picomole) Advantages • Molecular Ion • Interface to GC • Insoluble Samples • Disadvantages • No Fragment Library • Need Volatile Sample • Need Thermal Stability • Quantitation Difficult • Low Mass Compounds (<1000 amu) • Solids Probe Requires Skilled Operator

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