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 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…..
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…..
MC 13.3 Spectroscopy, Pt III 4 Introduction to Mass Spectrometry (cont) The Mass Spectrometer: continue…..
MC 13.3 Spectroscopy, Pt III 5 Introduction to Mass Spectrometry (cont) The Mass Spectrometer: continue…..
6 Interpreting Mass Spectral Data Some molecules undergo very little fragmentation: Benzene is an example. The major peak corresponds to the molecular ion m/z m/z = 78 Relative intensity continue…..
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 % H H H HH H One C is 13 C % H H H H H H One H is 2 H % continue…..
8 Interpreting Mass Spectral Data (cont) Isotopic Clusters in Chlorobenzene: 37 Cl Cl m/z Relative intensity Relative Abundance of Chlorine Isotopes 35 Cl 37 Cl = continue…..
9 Interpreting Mass Spectral Data (cont) Isotopic Clusters in Chlorobenzene (cont): m/z 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 Interpreting Mass Spectral Data (cont) Alkanes undergo extensive fragmentation: m/z Decane Relative intensity CH 3 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 3 continue…..
11 Interpreting Mass Spectral Data (cont) Propylbenzene fragments at the benzylic position: m/z Relative intensity CH 2 — CH 2 CH 3 91 continue…..
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…..
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…..