08 - MS 1 - Mass Spectroscopy We wish to learn about the...

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1 Mass Spectroscopy We wish to learn about the molecular weight, or at least, molecular fragments Electron Impact MS: •Cause ionization (loss of an electron) •Accelerate in an electric field •Send through a magnetic field and determine the mass/charge ratio Information gained: • Mass of the molecule (need to see the molecular ion peak-highest mass peak in the spectrum except for isotopes) • Fragmentation patterns tell you something about the structure • Carbon count (M+1) • High resolution MS gives molecular formula (quadrupole, e.g.) • Nitrogen rule
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2 • Base peak is the most intense (100%) • Molecular ion peak can be missing (use soft techniques to get this) • Fragment ions correspond to stable carbocations---but other less stable C + ions can be seen (high energy in EI MS) • In EI, compound must be somewhat volatile • %m+1 = 100 [(m+1)/m] ~ 1.1x #carbons + 0.38 #number of nitrogens The kinetic energy of an accelerated ion is: 1/2m v 2 = e V where m is the mass of the ion,
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08 - MS 1 - Mass Spectroscopy We wish to learn about the...

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