1. MC 13.3 Spectroscopy, Pt III 1 Introduction to Mass Spectrometry (cont) Principles of Electron-Impact Mass Spectrometry:  A mass spectrometer produces a spectrum of masses based on the structure of a molecule  A mass spectrum is a plot of the distribution of ion masses corresponding to the formula weight of a molecule and/or fragments derived from it  The x-axis of a mass spectrum represents the masses of ions  The y-axis represents the relative abundance of each ion  The pattern of ions obtained and their abundance is characteristic of the structure of a particular molecule continue…..

2. 2 Introduction to Mass Spectrometry (cont) Principles of Electron-Impact Mass Spectrometry (cont):  If the only ion that is present is the molecular ion, mass spectrometry provides a way to measure the molecular weight of a compound and is often used for this purpose.  However, the molecular ion often fragments to a mixture of species of lower m/z continue…..

3. MC 13.3 Spectroscopy, Pt III 3 Introduction to Mass Spectrometry (cont) The Mass Spectrometer Performs Several Functions: 1)Elevated temperatures and reduced pressures convert solids and liquids to gases 2)Gaseous molecules are ionized to positively charged species as they interact with a high energy electron beam 3)Electric and magnetic fields separate these positively charged ions into a spectrum according to their mass-to-charge ratio 4)A mass detector connected to a computer measures, records and stores the spectrum continue…..

4. MC 13.3 Spectroscopy, Pt III 4 Introduction to Mass Spectrometry (cont) The Mass Spectrometer: continue…..

5. MC 13.3 Spectroscopy, Pt III 5 Introduction to Mass Spectrometry (cont) The Mass Spectrometer: continue…..

6. 6 Interpreting Mass Spectral Data Some molecules undergo very little fragmentation:  Benzene is an example. The major peak corresponds to the molecular ion 100 80 60 40 20 0 20406080100 120 m/z m/z = 78 Relative intensity continue…..

7. 7 Interpreting Mass Spectral Data (cont) Isotopic Clusters:  The natural distribution of isotopes gives rise to specific ion clusters All H are 1 H and all C are 12 C H H H H H H 78 93.4% H H H HH H One C is 13 C 79 6.5% H H H H H H One H is 2 H 79 0.1% continue…..

8. 8 Interpreting Mass Spectral Data (cont) Isotopic Clusters in Chlorobenzene: 37 Cl 114 35 Cl 112 20406080100 120 m/z 100 80 60 40 20 0 Relative intensity Relative Abundance of Chlorine Isotopes 35 Cl 37 Cl = 100 32.5 continue…..

9. 9 Interpreting Mass Spectral Data (cont) Isotopic Clusters in Chlorobenzene (cont): 20406080100 120 m/z 100 80 60 40 20 0 Relative intensity 77 H H H H H +  No m/z 77, 79 pair;  Therefore ion responsible for m/z 77 peak does not contain Cl continue…..

10. 10 Interpreting Mass Spectral Data (cont) Alkanes undergo extensive fragmentation: m/z Decane 142 100 80 60 40 20 0 Relative intensity 20406080100 120 99 43 57 71 85 CH 3 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 3 continue…..

11. 11 Interpreting Mass Spectral Data (cont) Propylbenzene fragments at the benzylic position: 20406080100 120 m/z Relative intensity 120 91 100 80 60 40 20 0 CH 2 — CH 2 CH 3 91 continue…..

12. MC 13.3 Spectroscopy, Pt III 12 Interpreting Mass Spectral Data (cont) Molecular Formula: A Clue to Structure:  One of the first pieces of information we try to obtain when determining a molecular structure is the molecular formula  However, we can gain some information from the molecular weight  Mass spectrometry makes it relatively easy to determine molecular weights. continue…..

13. MC 13.3 Spectroscopy, Pt III 13 Interpreting Mass Spectral Data (cont) Exact Molecular Weights:  The exact mass of certain nuclides is shown below end…..