Mass Spectrometry Electron Ionization and Chemical Ionization Mass

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Mass Spectrometry Electron Ionization and Chemical Ionization

Mass Spectrometry Electron Ionization and Chemical Ionization

Mass Spectrometer All Instruments Have: 1. Sample Inlet 2. Ion Source 3. Mass Analyzer

Mass Spectrometer All Instruments Have: 1. Sample Inlet 2. Ion Source 3. Mass Analyzer 4. Detector 5. Data System http: //www. asms. org

Ionization Techniques Gas-Phase Methods • Electron Ionization (EI) • Chemical Ionization (CI) Desorption Methods

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)

Electron Ionization http: //www. noble. org/Plant. Bio/MS/ion_tech_main. html

Electron Ionization http: //www. noble. org/Plant. Bio/MS/ion_tech_main. html

Electron Ionization λ= h mv

Electron Ionization λ= h mv

Electron Ionization • Samples must be vaporized in the ion source • Typically 1

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

Electron Ionization

Electron Ionization

Electron Ionization (low picomole) • • • Advantages Well-Established Fragmentation Libraries No Supression Insoluble

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

Chemical Ionization http: //www. noble. org/Plant. Bio/MS/ion_tech_main. html

Chemical Ionization http: //www. noble. org/Plant. Bio/MS/ion_tech_main. html

Chemical Ionization • Reagent gas is introduced into the source at ~0. 5 torr

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

Chemical Ionization: Methane • Methane primarily forms CH 4+ • with CH 2+ •

Chemical Ionization: Methane • Methane primarily forms CH 4+ • with CH 2+ • and CH 3+ • CH 4+ • + CH 4 → CH 5+ + CH 3 (m/z 17) • CH 2+ • + CH 4 → C 2 H 3+ + H 2 + H • • C 2 H 3+ + CH 4 → C 3 H 5+ + H 2 (m/z 41) • CH 3+ + CH 4 → C 2 H 5+ + H 2 (m/z 29)

Chemical Ionization: Methane

Chemical Ionization: Methane

Chemical Ionization: Methane • Ions other than saturated hydrocarbons react via proton transfer •

Chemical Ionization: Methane • Ions other than saturated hydrocarbons react via proton transfer • CH 5+ + M → MH+ + CH 4 (or via C 2 H 5+ or C 3 H 5+) • For saturated hydrocarbons, hydride abstractions is common • CH 5+ + RH → R+ + CH 4 + H 2 • For polar molecules, adducts can form • CH 3+ + M → (M+CH 3)+ • MH+, R+, and adducts are pseudomolecular ions.

Chemical Ionization: Isobutane

Chemical Ionization: Isobutane

Chemical Ionization: Isobutane

Chemical Ionization: Isobutane

Chemical Ionization: Isobutane • Reacts through Proton Transfer • C 4 H 9+ +

Chemical Ionization: Isobutane • Reacts through Proton Transfer • C 4 H 9+ + M → MH+ + C 4 H 8 • For saturated hydrocarbons, no reaction • For polar molecules, adducts can form • C 4 H 9+ + M → (M+C 4 H 9)+ • 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)

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

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

EI vs. Methane vs. Isobutane

EI vs. Methane vs. Isobutane

Chemical Ionization: Ammonia Largely forms NH 4+ and (NH 4 + NH 3)+

Chemical Ionization: Ammonia Largely forms NH 4+ and (NH 4 + NH 3)+

Chemical Ionization: Ammonia • Reacts through Proton Transfer with basic molecules • NH 4+

Chemical Ionization: Ammonia • Reacts through Proton Transfer with basic molecules • NH 4+ + M → MH+ + NH 3 • Less basic molecules will form adducts • NH 4+ + M → (M+NH 4)+ • 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)

Chemical Ionization: Negative Ions • Low energy electrons are present in the CI plasma

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

Chemical Ionization: Negative Ions • A common negative CI gas is a mixture of

Chemical Ionization: Negative Ions • A common negative CI gas is a mixture of N 2 O/CH 4 in a ratio 25/75 • N 2 O + e- → N 2 O- • • N 2 O- • → N 2 + O- • • O- • + CH 4 → CH 3 • + OH • This mixture yields thermal electrons for electron capture and hydroxide for deprotonation of acidic molecules

Chemical Ionization Proton Affinity ∆H˚acid CH 4 543 1749 H 2 O 691 1634

Chemical Ionization Proton Affinity ∆H˚acid CH 4 543 1749 H 2 O 691 1634 NH 3 854 1689 CH 3 OH 754 1592 C 2 H 5 OH 776 1583 IPA 793 1573 t-Bu. OH 803 1568 Aniline 921 1533

Chemical Ionization (low picomole) Advantages • Molecular Ion • Interface to GC • Insoluble

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