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Ionization techniques, Mass spectra and MS-Instrumentation

Molecule ionization methods:. Ionization techniques, Mass spectra and MS-Instrumentation. Electron impact ( EI ) and Chemical ionization (CE) (GC-MS) 2. Electro spray ionization (ESI) (LC-ESI-MS) 3. Matrix assisted laser desorption (MALDI). Agilent GC-MS.

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Ionization techniques, Mass spectra and MS-Instrumentation

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  1. Molecule ionization methods: Ionization techniques, Mass spectra and MS-Instrumentation Electron impact (EI) and Chemical ionization (CE) (GC-MS) 2. Electro spray ionization (ESI) (LC-ESI-MS) 3. Matrix assisted laser desorption (MALDI)

  2. Agilent GC-MS

  3. M + e- M+. + 2e-

  4. Electron impact ionization (EI) ABC + e- ABC+. + e- + e-70 eV Molecular ion (1 eV) 55 eV + 0.1 eV ABC+.  AB+ + C. ABC+.  A. + BC+ AB+  A + B+ Mass spectrum

  5. Aromatic ketones

  6. Mass Analyzers • Quadrupole • Ion trap • Magnetic Sector • Linear Ion trap • Time of Flight (TOF) • Fourier Transform (FTMS)

  7. Determination of Bruttoformulars from precise Mass determination (Excact mass) Precise Mass (C3H8O) = 3×12.0000 + 8×1.00797 +15.9994 = 60.05754

  8. Molecular Mass and Nominal Mass • Atomic mass Br: 79Br (50.69%) m = 78.918 Da 81Br (49.31%) m = 80.916 Da Atomic mass Br = 0.5069*78.918 + 0.4931*80.916 = 79.904 Da 1 Da = 1/12 of the mass of 12C isotope. The nominal mass of a molecule or ion is the integer mass of the species with the most abundant isotope of the constituent atoms Nominal mass of Br = 79

  9. Number of C = 100×[M+1]/([M]×1.1)

  10. Nitrogen rule • If a compound contains an even number (0, 2, 4…) of nitrogen atoms, its molecular ion will be an even mass number. • An odd number (1, 3, 5 ..) of nitrogen atoms => The mass number of M+. will be odd.

  11. Basic advices • Identify the mole peak M+. • Nitogen rule: Determine number of N • A+2 elements: Cl, Br, S, Si • NC = 100×[M+1]/([M]×1.1) • Determine brutto formular • Calculate r + db = NC – NH/2 + NN/2 + 1 (halogens counts as H)

  12. Stevensons rule • The most probable fragmentation is the one that leaves the positive charge on the fragment with the lowest ionization energy • H3C+ < RCH2+ < R2CH+ < H2C=CH-CH2+< C6H6CH2+ Difficult Easy

  13. Ionization mechanisms in EI mass spectrometry Four mechanisms: sab rH

  14. s - cleavage

  15. a-cleavage

  16. a cleavage of alcohols

  17. a-cleavage

  18. b-cleavage

  19. rH : McLafferty rearangement

  20. Heptadecanoic acid methyl ester

  21. Ionization techniques, Mass spectra and MS-Instrumentation Molecule ionization methods: Electron impact (EI) and Chemical ionization (CE) (GC-MS) 2. Electro spray ionization (ESI) (LC-ESI-MS)

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