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Mass Spectrometry

Mass Spectrometry. From here…. To here!. Molecular Weight: 70 amu Molecular Formula: C 5 H 10. Detection of ionic species to determine the molecular weight of and obtain structural information on a molecule. Mass spectrometry. Fig. 12-1, p. 410. Gas Chromatography –

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Mass Spectrometry

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  1. Mass Spectrometry From here… To here! Molecular Weight: 70 amu Molecular Formula: C5H10 Detection of ionic species to determine the molecular weight of and obtain structural information on a molecule

  2. Mass spectrometry

  3. Fig. 12-1, p. 410

  4. Gas Chromatography – Mass Spectromety (GC-MS) Liquid Chromatography – Mass Spectromety (LC-MS)

  5. 29 Base Peak Molecular ion -15 Base peak Molecular ion (M+) -CH3

  6. -15 Mass Spectrum of Hexane Chemical Formula: C6H14 Exact Mass: 86.1095 Molecular Weight: 86.1753 -CH3 Fig. 12-4, p. 413

  7. Fig. 12-5, p. 413

  8. methylcyclohexane or ethylcyclopentane ? Fig. 12-6, p. 414

  9. Isotope effects Molecular weight Br = 79.9040 Br isotopic abundance 79Br = 50.5 % 81Br = 49.5 % CH379Br CH381Br

  10. Isotope effects Molecular weight Br = 79.9040 Br isotopic abundance 79Br = 50.5 % 81Br = 49.5 % CH379Br CH381Br

  11. Isotope effects Molecular weight Cl = 35.453 Cl isotopic abundance 35Cl = 75.4 % 37Cl = 24.6 % 35Cl : 37Cl Ratio = 3:1 CH335Cl CH337Cl

  12. Fragmentation by a-cleavage • a-cleavage (cleavage of bond one removed from radical or radical cation) • Carbonyl compounds

  13. Acetone 43 58 -15 Acetaldehyde 29 44 -15 -CH3 -CH3

  14. Fragmentation by a-cleavage • a-cleavage (cleavage of bond one removed from radical or radical cation) • Carbonyl compounds • Amines and alcohols • Alkenes

  15. Fragmentation by a-cleavage • a-cleavage (cleavage of bond one removed from radical or radical cation) • Carbonyl compounds • Amines and alcohols • Alkenes

  16. Fragmentation by a-cleavage • a-cleavage (cleavage of bond one removed from radical or radical cation) • Methyl benzene (toluene) compounds

  17. m/z = 102 Fig. 12-8, p. 416

  18. Determination of Formula:Rule of 13 Assumes CnHn and amu equivalent (13 for n=1) is present in all molecular fragment ions Step 1: Divide M+ mass by 13, this gives n Step 2: Any remainder represents count of additional Hʼs example 1: for M+ = 78 78 ÷ 13 = 6 → n = 6 → C6H6 example 2: for M+ = 92 92 ÷ 13 = 7.077 → n = 7 7 x 13 = 91 → 1 extra H is present Formula is C7H7+1 = C7H8

  19. Determination of Formula:Rule of 13 example 3: for M+ = 161 161 ÷ 13 = 12.385 → n = 12 12 x 13 = 156 → 5 extra Hʼs are present Formula is C12H12+5 = C12H17 What about heteroatoms? Step 1: First derive formula as above Step 2: Next, modify using CnHm equivalents

  20. Problem An unknown compound subjected to mass spectrometry gave the parent ion at m/z = 114. The parent ion quickly undergoes homolytic -cleavage to give fragment ions with m/z = 99 (100 %) and m/z = 45 (45 %). No other significant peaks are seen the in spectrum. The unknown does not react when treated with H2, Pd/C. The infrared spectrum of the unknown shows a strong, broad peak at 3500 cm-1. Show the reaction and the electron pushing mechanism that accounts for the fragmentation of the parent ion to give either of the observed fragments.

  21. High energy hard ionization Low energy soft ionization MS Ionization Techniques • Electron Impact (EI)uses beam of electrons • Fast Atom Bombardment (FAB)uses beam of atoms • Chemical Ionization (CI)uses chemical reactions • Matrix Assisted Laser Desorption (MALDI)uses lasers to ionize molecules • Electrosprayuses compounds which are already ionic

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