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Chapter 12 Mass Spectrometry

. Chapter 12. 2. INTRODUCTION TO MASS SPECTROMETRY. MASS SPECTROMETER: AN INSTRUMENT THAT GENERATES IONS FROM MOLECULES AND MEASURES THEIR MASSES . THE ESSENTIAL COMPONENTS OF A MASS SPECTROMETER:. SAMPLE INLET. IONSOURCE. ION ACCELERATOR. IONANALYSER. IONDETECTOR. signal. COMPUTER. MASS SPECTRUM.

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Chapter 12 Mass Spectrometry

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    1. Chapter 12 Mass Spectrometry

    2. Chapter 12 2 INTRODUCTION TO MASS SPECTROMETRY

    3. Chapter 12 3 Mass Spectrometry Molecular weight can be obtained from a very small sample. It does not involve the absorption or emission of light. A beam of high-energy electrons breaks the molecule apart. The masses of the fragments and their relative abundance reveal information about the structure of the molecule. =>

    4. Chapter 12 4 WAYS TO PRODUCE IONS Electron impact (EI) - vapour of sample is bombarded with electrons: M + e 2e + M.+ fragments Chemical ionization - sample M collides with reagent ions present in excess e.g. CH4 + e CH4.+ CH5+ M + CH5+ CH4 + MH+ Fast Atom/Ion Bombardment Laser Desorption & Matrix-Assisted Laser Desorption(MALDI) - hit the sample with a laser beam Electrospray Ionisation (ESI) - a stream of solution passes through a strong electric field (106 V/m)

    5. Chapter 12 5

    6. Chapter 12 6 Electron Impact Source

    7. Chapter 12 7 Typical Reactions during Electron Impact

    8. Chapter 12 8 Chemical Ionization MS Sources

    9. Chapter 12 9 Field Ionization /Field Desorption Sources Apply large electric fields to carbon dendrites on a tungsten wire Field Ionization – gas is passed over ionization source Field Desorption – dipped in solution containing sample and placed back in spectrometer

    10. Chapter 12 10

    11. Chapter 12 11 MASS ANALYSERS Magnet (1912) - ions are deflected in mag field Quadrupole (1953) - ions travel down a flight path between four parallel rods to which varying electric potentials are applied Time of Flight (1955) - ions are timed from source to detector Quadrupole Ion Trap (1960) - ions are trapped in & selectively released from a circular cavity made up of three electrodes

    12. Chapter 12 12 Magnetic Sector Analyzer

    13. Magnetic-Sector Mass Spectrometry

    14. Magnetic Sector Physics

    15. Chapter 12 15 Mass Spectrometer

    16. Chapter 12 16 Time-of-Flight MS

    17. Chapter 12 17 Quadrupole Mass Ion Filter

    18. Chapter 12 18 The Mass Spectrum Masses are graphed or tabulated according to their relative abundance.

    19. Chapter 12 19 The GC-MS

    20. Chapter 12 20 High Resolution MS Masses measured to 1 part in 20,000. A molecule with mass of 44 could be C3H8, C2H4O, CO2, or CN2H4. If a more exact mass is 44.029, pick the correct structure from the table:

    21. Chapter 12 21 Interpretation of Mass Spectra Select a candidate peak for the molecular ion (M+) Examine spectrum for peak clusters of characteristic isotopic patterns Test (M+) peak candidate by searching for other peaks correspond to reasonable losses Look for characteristic low-mass fragment ions Compare spectrum to reference spectra

    22. Chapter 12 22 Isotopic Abundance

    23. Chapter 12 23 Molecules with Heteroatoms Isotopes: present in their usual abundance. Hydrocarbons contain 1.1% C-13, so there will be a small M+1 peak. If Br is present, M+2 is equal to M+. If Cl is present, M+2 is one-third of M+. If iodine is present, peak at 127, large gap. If N is present, M+ will be an odd number. If S is present, M+2 will be 4% of M+. =>

    24. Chapter 12 24 Isotopic Abundances

    25. Chapter 12 25

    26. Chapter 12 26

    27. Chapter 12 27

    28. Chapter 12 28 Mass Spectrum with Sulfur

    29. Chapter 12 29 Mass Spectrum with Chlorine

    30. Chapter 12 30

    31. Chapter 12 31 Mass Spectrum with Bromine

    32. Chapter 12 32 Mass Spectra of Alkanes More stable carbocations will be more abundant.

    33. +CH3 m/e = 15 CH3 | +C - CH3 m/e = 57 | CH3 CH3 - CH2 + m/e = 29

    34. Chapter 12 34 Mass Spectra of Alkenes Resonance-stabilized cations favored.

    35. Chapter 12 35 Mass Spectra of Alcohols Alcohols usually lose a water molecule. M+ may not be visible.

    36. Chapter 12 36 End of Chapter 12

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