Mass Spectrometry
Electron Ionization
and
Chemical Ionization
Mass Spectrometer
All Instruments Have:
1. Sample Inlet
2. Ion Source
3. Mass Analyzer
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Detector
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Data System
http://www.asms.org
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/PlantBio/MS/ion_tech_main.html
Electron Ionization
λ=
h
mv
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
Electron Ionization
Electron Ionization
(low picomole)
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Advantages
Well-Established
Fragmentation Libraries
No Supression
Insoluble Samples
Interface to GC
Non-Polar Samples
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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/PlantBio/MS/ion_tech_main.html
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 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)
Chemical Ionization: Methane
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.
Chemical Ionization:
Isobutane
CH3
-HҐ
CH3
H3C
H
CH3
H 3C
CH3
m/z 57
-CH3Ґ
H3C
H
CH3
m/z 43
Chemical Ionization:
Isobutane
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)
EI vs. Methane vs. Isobutane
EI
O
O
Methane CI
Isobutane CI
EI vs. Methane vs. Isobutane
Chemical Ionization: Ammonia
Largely forms NH4+ and (NH4 + NH3)+
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)
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 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
Chemical Ionization
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
(low picomole)
Advantages
• Molecular Ion
• Interface to GC
• Insoluble Samples
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Disadvantages
No Fragment Library
Need Volatile Sample
Need Thermal Stability
Quantitation Difficult
Low Mass Compounds
(<1000 amu)
Solids Probe Requires
Skilled Operator