Mass Spectrometry Electron Ionization and Chemical Ionization
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 Matrix-Assisted Laser Desorption Ionization (MALDI) Fast Atom Bombardment (FAB) Spray Methods Electrospray (ESI) Atmospheric Pressure Chemical Ionization (APCI)
http://www.noble.org/plantbio/ms/ion_tech_main.html Electron Ionization
Electron Ionization 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) 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/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 H 3 C CH 3 CH 3 H -H CH 3 H 3 C CH 3 m/z 57 -CH 3 H 3 C H CH 3 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 O EI 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 Disadvantages No Fragment Library Need Volatile Sample Need Thermal Stability Quantitation Difficult Low Mass Compounds (<1000 amu) Solids Probe Requires Skilled Operator