CHM 238 - Ch 12 - McMurray

CHM 238 - Ch 12 - McMurray - Chapter 12 Mass Spectrometry...

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Chapter 12 Mass Spectrometry and Infrared (IR) Spectroscopy.
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Spectroscopic Methods: Mass Spectrometry (MS) : Molecular size, formula, presence of some elements, … Infrared Spectroscopy (IR) : Molecular motion, functional groups, … Nuclear Magnetic Resonance Spectroscopy (NMR) : Molecular framework, … Ultraviolet-Visible Spectroscopy (UV-vis) : Conjugation of π -electrons, …
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MS Instrument-Process: Three steps: Ionization : converts molecule M to M .+ . Mass analyzer : separates ions by mass/charge ( m/z ) . Detector : observes and counts ions . Sample Display Ionization EI, CI ESI MALDI Mass analyzer Magnetic Sector Time of flight Quadrupole Detector Photomultiplier Electron multiplier Micro-channel plate M M m/z m/z
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Ionization: The molecules to be analyzed must be converted into gas phase ions in order to be “weighed”. Generally, they are converted to cations, anions have also been used. Hard ionization (EI) induces the loss of an electron (M M + ). Soft ionization (CI, ESI, MALDI) gives a protonated molecule (M MH + ).
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Ionization: Electron Impact (EI) In the vapor phase, molecules of our analyte are hit with high energy electrons. The electrons go though the molecule sometimes knocking another electron out of the molecule. Molecule RH M Organic molecule e - 2e - Molecular ion RH + M + Radical cation Mass analyzer
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Magnetic Sector Mass Analyzer: In a magnetic field, traveling charges are force to turn. Heavier ions turn more slowly than lighter ions. B Magnetic Field heavier ions lighter ions Detector M +
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Mass Spectrum: Important Terms m/z : mass of ion divided by its charge. Mass spectrum : bar graph of m/z on x-axis and the relative abundance on the y-axis. Base peak : the m/z bar with the greatest abundance, is assigned 100%. The other m/z peaks are given valued at their percentage of the base peak’s abundance. Parent peak : also called the molecular ion, is m/ z peak that corresponds to the radical-cation of the molecule.
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Example Mass Spectrum of Pentan-2-ol Base Peak Parent Peak
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Example Mass Spectrum In examining the mass spectrum, you should have noticed that there are a lot of other peaks besides the parent peak. These other peaks, of smaller m/z , are referred to as fragments . The ionized molecule is often unstable and breaks into smaller fragments. As we will see later, fragmentation can be useful in determining the structure of a molecule.
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Interpretation of Mass Spectra From the parent peak, the molecular mass is obtained, so that only compounds with that molecular mass would potential choices if it were an unknown substance.
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