Introduction to Mass Spectrometry (MS)

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• Introduction to Mass Spectrometry (MS)
• A mass spectrometer produces a spectrum of masses
  based on the structure of a molecule.
• The x-axis of a mass spectrum represents the
  masses of ions produced (m/z)
• The y-axis represents the relative abundance of
  each ion produced
• The pattern of ions obtained and their abundance
  is characteristic of the structure of a
  particular molecule

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• Ionization (the formation of ions)
• A molecule is bombarded with a beam of high
  energy electrons
• An electron is dislodged from the molecule by the
  impact, leaving a positively charged ion with an
  unpaired electron (a radical cation)
• This initial ion is called the molecular ion
  (M.) because it has the same molecular weight as
  the analyte

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• Fragmentation
• Excess vibrational energy is imparted to the
  molecular ion by collision with the electron beam
  - this causes fragmentation
• The fragmentation pattern is highly
  characteristic of the structure of the molecule

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Common Isotope Abundances
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3 Classes of Isotopes

• M - only a single isotope
• EX F, P, I
• M1 - two isotopes with significant relative
  abundance differing by 1 mass unit
• EX H, C, N
• M2 - two isotopes with significant relative
  abundance differing by 2 mass units
• EX O, S, Cl, Br

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• Determination of Molecular Formulas Weights
• The Molecular Ion and Isotopic Peaks
• The presence of heavier isotopes one or two mass
  units above the common isotope yields small peaks
  at M.1 and M.2
• Example In the spectrum of methane one expects
  an M.1 peak of 1.17 based on a 1.11 natural
  abundance of 13C and a 0.016 natural abundance
  of 2H

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Four Basic Rules

• If M is even, then the unknown contains an even
  number of Nitrogen atoms (zero is an even number)
• The abundance of M1 indicates the number of
  Carbon atoms
• of C relative abundance/1.1
• The abundance of the M2 peak indicates the
  presence of O (0.2), S (4.4), Cl (33) or Br
  (98)
• The remaining unknown mass can be attributed to
  Hydrogen

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m/z intensity abundance
78 (M) 10.00 100
79 .3 3
80 3.3 33

• Is the molecular ion even?
• Yes, there must be either an even number of N,
  or no Nitrogen atoms.
• How many Carbon atoms are there?
• Carbons 3 / 1.1 3 carbon atoms
• Is a O, S, Cl or Br present?
• A M2 peak of 33 indicates the presence of
  chlorine
• How many Hydrogen atoms are there?
• 78 (1 35) (3 12) (H 1)
• 78 71 H
• of Hydrogen atoms 7

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m/z intensity abundance
86 (M) 10.00 100
87 .56 5.6
88 .04 .4

• Is the molecular ion even?
• Yes, there must be either 0, 2, 4 Nitrogen
  atoms
• How many Carbon atoms are there?
• Carbons 5.6 / 1.1 5 carbon atoms
• if there are 2 N atoms then the FW would be
  (512) (214) 88
• Therefore, there are no nitrogen atoms
• Is an O, S, Cl or Br present?
• A M2 peak of .4 indicates O
• How many Hydrogen atoms are there?
• 86 (5 12) (O1) (H 1)
• 86 76 H
• of Hydrogen atoms 10
• of Hydrogen atoms 10

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Determination ofMolecular Formula

• distinguish between compounds of same MW
• C5H10O4 or C10H14

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Determination ofMolecular Formula

• distinguish between compounds of same MW
• C5H10O4
• 13C 5 1.11 5.55
• 2H 10 0.016 0.16
• 17O 4 0.04 0.16
• -------
• 135peak/134peak 5.87

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Determination ofMolecular Formula

• distinguish between compounds of same MW
• C10H14
• 13C 10 1.11 11.1
• 2H 14 0.016 0.22
• -------
• 135peak/134peak 11.32

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The Numbers Approach

• If compound with formula CwHxNyOz , relative
  intensities of M, M1, and M2 ions will be given
  by

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• High-Resolution Mass Spectrometry
• Low-resolution mass spectrometers measure m/z
  values to the nearest whole number
• High-resolution mass spectrometers measure m/z
  values to three or four decimal places
• The high accuracy of the molecular weight
  calculation allows accurate determination of the
  molecular formula of a fragment
• Example
• One can accurately pick the molecular formula of
  a fragment with a nominal molecular weight of 32
  using high-resolution MS

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• The exact mass of certain nuclides is shown below

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http//www.cem.msu.edu/reusch/VirtualText/Spectrp
y/MassSpec/masspec1.htm
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• Fragmentation by Cleavage at a Single Bond
• Cleavage of a radical cation gives a radical and
  a cation but only the cation is observable by MS
• In general the fragmentation proceeds to give
  mainly the most stable carbocation
• In the spectrum of propane the peak at 29 is the
  base peak (most abundant) 100 and the peak at 15
  is 5.6

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• Fragmentation Equations
• The M. Ion is formed by loss of one of its most
  loosely held electrons
• If nonbonding electron pairs or pi electrons are
  present, an electron from one of these locations
  is usually lost by electron impact to form M.
• In molecules with only C-C and C-H bonds, the
  location of the lone electron cannot be predicted
  and the formula is written to reflect this using
  brackets

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• Example The spectrum of hexane

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• Example spectrum of neopentane
• Fragmentation of neopentane shows the propensity
  of cleavage to
• occur at a branch point leading to a
  relatively stable carbocation
• The formation of the 3o carbocation is so favored
  that almost no
• molecular ion is detected

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• Carbocations stabilized by resonance are also
  formed preferentially
• Alkenes fragment to give resonance-stabilized
  allylic carbocations
• Carbon-carbon bonds next to an atom with an
  unshared electron pair break readily to yield a
  resonance stabilized carbocation
• ZN, O, or S R may be H

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• Carbon-carbon bonds next to carbonyl groups
  fragment readily to yield resonance stabilized
  acylium ions

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• Alkyl substituted benzenes often lose a hydrogen
  or alkyl group to yield the relatively stable
  tropylium ion
• Other substituted benzenes usually lose their
  substitutents to yield a phenyl cation

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• Fragmentation by Cleavage of 2 Bonds
• The products are a new radical cation and a
  neutral molecule
• Alcohols usually show an M.-18 peak from loss of
  water

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• Cycloalkenes can undergo a retro-Diels Alder
  reaction (section 13.11) to yield an alkadienyl
  radical cation

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