Mass Spectrometry

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Mass Spectrometry

• Chemistry 223
• Fall 2008

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First Steps 19th Century

• J.J Thompson was investigating the kanal strahlen
  from cathode tubes positive ions formed from
  residual gas in the tube
• He used channels in plates to focus the stream
  and demonstrated that the beam could be deflected
  by both magnetic and electrostatic fields
• Device was used to shown that there were two
  isotopes of neon

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Uses of Mass Spectrometry

• Molecular weight determination
• Molecular formula determination
• Low resolution methods
• High resolution methods
• Structure analysis fragmentation
• Heavy isotope incorporation studies
• Sequence determination in biopolymers
• Analysis of geological materials
• Atomic physics, physical chemistry

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Block Diagram of the Instrument
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Fragmentation
The instrument is designed to accelerate and
analyze only positively charged ions (repeller
plates are positively charged). These ions are
detected using one of several detectors
(photographic plate, electron multiplier, etc).
The basic equation assumes that only singly
charged pieces are accelerated and detected, so
the units are m/z or mass /charge ratio.
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Mass Spectrum
Base peak
Fragment ions
Molecular ion peak
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Ionization Methods

• Electron impact ionization (EI)
• Use high energy electrons to dislodge an electron
  from a molecule to create a molecular ion (50
  -100V)
• Chemical ionization (CI) (soft ionization)
• Use gas molecules (ammonia, methane) to promote
  ionization
• Atmospheric Pressure Ionization (API)
• Desorption ionization (LD, MALDI, FAB)
• Use energetic source to both vaporize and ionize
  a large molecule with a very low vapor pressure

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Ionization Methods
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Electron Impact Ionization
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Mass Analyzers

• Time of Flight (TOF)
• Sector Fields
• Magnetic
• Electrical
• Double focusing
• Quadrupoles
• Ion traps
• Quadrupole ion trap (QIT)
• Ion cyclotron resonance (ICR)

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Quadrupole Mass Analyzer
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(No Transcript)
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Agilent Quadrupole GC-MS
Quadrupole Analyzer
GC
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Ion Trap Procedure
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Ion Trap Mass Analyzer
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Griffin Instruments Model 300CIT Instrument
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Basic Fragmentation Patterns
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Mass Spectral Peak Interpretation
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Mass Spectral Interpretation
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Mass Spectral Interpretation
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Halogen Containing Compounds
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Odd Molecular Weight - Nitrogen
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Bromine Compounds
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Information Obtained from MS

• The exact mass of an analyte
• The molecular formula of the analyte by using the
  exact MW (six SFs) and Beynons Table.
• The presence of certain functional groups or
  characteristic chemical segments
• m/z 43 (methyl ketone or similar compound)
• m/z 91 (tropylium ion methyl benzene)
• Match total spectrum to mass spec database of
  fragmentation patterns (NIST Database of 200,000
  compounds)

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How Do You Use MS Data

• Look for molecular ion (M?) and try to get a
  molecular weight
• Look at the exact mass and use tabular data to
  obtain a potential molecular formula
• Look for the presence of common fragment ions
  do they fit other data?
• Look for the presence of halogens
• Look at neutral fragments that would be lost
  between two peaks