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Introduction to Mass Spectrometry

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Lecture 22 Introduction to Mass Spectrometry Lecture Problem 7 Due This week in lab: Work on 1st Synthetic Next week in lab: Synthetic Experiment #2 PreLab Due – PowerPoint PPT presentation

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Title: Introduction to Mass Spectrometry


1
Lecture 22
  • Introduction to Mass Spectrometry
  • Lecture Problem 7 Due
  • This week in lab
  • Work on 1st Synthetic
  • Next week in lab
  • Synthetic Experiment 2 PreLab Due
  • Ch 9 Final Report Due

2
Spectral Analysis
  • 1H NMR
  • Bond connectivities
  • Types and kinds of protons
  • IR
  • Functional groups
  • Now, lets look at Mass Spectrometry
  • Determine the mass of the sample
  • Look at fragmentation patterns determine
    pieces
  • of the molecule.

3
Mass Spectrum of Cyclohexamine
M 99
Fragments
4
Mass Spectrometry
  • How to get the mass of the sample or pieces of
    the sample
  • Ionize the sample with a high energy particle to
    give
  • a molecular ion, M (a.k.a. parent ion).
  • Note Ions may fragment which results in pieces
    of the sample.
  • When fragmentation occurs, there is always a
    positive
  • fragment (ion) and a neutral fragment.
  • Only positive species are detected by the
    detector. Thus,
  • only the mass of the molecular ion and the
    masses of
  • positive ion fragments will be read by the
    detector and appear
  • on the mass spectrum for the compound. Neutral
    fragments
  • are not read by the detector and do not appear
    on the spectrum.

5
Mass Spectrometry
  • Basic Idea
  • To ionize bash the molecule with a high energy
    particle in a vacuum
  • An electron (known as Electron Ionization)
  • or
  • A proton (known as Chemical Ionization)
  • A molecular ion results
  • Electron Ionization (EI) M e- ? M
  • Chemical Ionization (CI) M H ? MH
  • Send molecular ion through a magnetic field
  • Molecular ion travels and reaches a detector.
    The detector
  • only detects positively charged species ? m/z
    ratio (mass/
  • charge).
  • Obtain mass of molecular ion and fragments. The
    intensity of the signals
  • is proportional to the number of ions. The more
    intense the signal, the
  • more stable that ion is.

6
The Mass Spectrometer
A quadrupole mass spectrometer. Ions oscillate
as they move through the electric fields on the
four rods. Resonant ions hit the
detector. Nonresonant ions are deflected and
dont hit the detector.
7
Molecular Ions
  • Molecular ions of different compounds have
    different
  • stabilities and, therefore, different fates.
  • Some will reach the detector.
  • Others will fragment before they reach the
    detector.
  • Molecular ions of different compounds will have
    different
  • intensities
  • Strong in aromatics
  • Weak in alkanes, alcohols others

8
Mass Spectrum of Hexane
9
Molecular Ions
M 78
Benzene Strong M
M 86
Hexane Weak M
M 102
Hexanol M not seen
10
Nitrogen Containing Compounds
If odd of Ns, then odd mass. If even mass,
then either zero Ns or even of Ns.
M 99
Cyclohexamine
11
Molecular Ions
  • Molecular Ions sum of atomic weight(s) of all
    isotopes of all elements in a molecule. If you
    had 10,000 atoms of each type
  • C 9890 12C atoms 110 13C atoms
  • H 9999 1H atoms 1 2H atoms
  • O 9985 16O atoms, 4 17O atoms, 20 18O atoms
  • N 9963 14N atoms 37 15N atoms
  • Cl 7577 35Cl atoms 2423 37Cl atoms (31
    35Cl37Cl)
  • Br 5069 79Br atoms 4931 81Br atoms (11
    79Br81Br)

12
Mass Spectrum of Hexane
87
Hexane with one 13C atom M 1
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