Introduction to Mass Spectrometry cont Principles of ElectronImpact

Introduction to Mass Spectrometry (cont) The Mass Spectrometer Performs Several Functions: 1) Elevated temperatures

Introduction to Mass Spectrometry (cont) The Mass Spectrometer: MC 13. 3 Spectroscopy, Pt III

Interpreting Mass Spectral Data Some molecules undergo very little fragmentation: Ø Benzene is an

Interpreting Mass Spectral Data (cont) Isotopic Clusters: Ø The natural distribution of isotopes gives

Interpreting Mass Spectral Data (cont) Isotopic Clusters in Chlorobenzene: 35 Cl Relative Abundance of

Interpreting Mass Spectral Data (cont) Isotopic Clusters in Chlorobenzene (cont): Ø No m/z 77,

Interpreting Mass Spectral Data (cont) Alkanes undergo extensive fragmentation: CH 3—CH 2—CH 2—CH 3

Interpreting Mass Spectral Data (cont) Propylbenzene fragments at the benzylic position: CH 2 —

Interpreting Mass Spectral Data (cont) Molecular Formula: A Clue to Structure: Ø One of

Interpreting Mass Spectral Data (cont) Exact Molecular Weights: Ø The exact mass of certain

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Introduction to Mass Spectrometry (cont) Principles of Electron-Impact Mass Spectrometry: Ø A mass spectrometer produces a spectrum of masses based on the structure of a molecule Ø 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 Ø The x-axis of a mass spectrum represents the masses of ions Ø The y-axis represents the relative abundance of each ion Ø The pattern of ions obtained and their abundance is characteristic of the structure of a particular molecule MC 13. 3 Spectroscopy, Pt III continue…. .

Introduction to Mass Spectrometry (cont) Principles of Electron-Impact Mass Spectrometry (cont): Ø 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. Ø However, the molecular ion often fragments to a mixture of species of lower m/z continue…. .

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 MC 13. 3 Spectroscopy, Pt III continue…. .

Introduction to Mass Spectrometry (cont) The Mass Spectrometer: MC 13. 3 Spectroscopy, Pt III continue…. .

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

Interpreting Mass Spectral Data (cont) Isotopic Clusters: Ø The natural distribution of isotopes gives rise to specific ion clusters H H H H 79 79 78 H H H H H 93. 4% 6. 5% 0. 1% All H are 1 H and all C are 12 C One C is 13 C One H is 2 H continue…. .

Interpreting Mass Spectral Data (cont) Isotopic Clusters in Chlorobenzene: 35 Cl Relative Abundance of Chlorine Isotopes 35 Cl Relative intensity 37 Cl = 37 Cl 100 32. 5 100 112 80 60 114 40 m/z 20 0 20 40 60 80 100 120 continue…. .

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

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

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

Interpreting Mass Spectral Data (cont) Molecular Formula: A Clue to Structure: Ø One of the first pieces of information we try to obtain when determining a molecular structure is the molecular formula Ø However, we can gain some information from the molecular weight Ø Mass spectrometry makes it relatively easy to determine molecular weights. MC 13. 3 Spectroscopy, Pt III continue…. .

Interpreting Mass Spectral Data (cont) Exact Molecular Weights: Ø The exact mass of certain nuclides is shown below MC 13. 3 Spectroscopy, Pt III end…. .