Making Liquids and Solids into Ions

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Making Liquids and Solids into Ions
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Mass Spectroscopy

• Mass Spectrometry is an analytical spectroscopic
  tool primarily concerned with the separation of
  molecular (and atomic) species according to their
  mass
• Mass Spectrometry has a number of uses for
  example geologists use it to date rocks,
  pharmaceutical companies make use of mass
  spectrometry to determine the structure of novel
  compounds.

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Mass Spectroscopy

• Sample is first Vaporized before passing through
  an ion chamber
• Then bombarded with a stream of high-energy
  electrons.
• This results in the formation of a molecular ion
  as well as a number of fragments
• These ions are then fed into an electric field
  where they are accelerated and passed through a
  narrow slit, producing a narrow beam of positive
  ions.

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Mass Spectroscopy

• This beam is then subjected to a magnetic field
  causing the ions to be deflected a particular
  distance depending on the mass to charge ratio
• The ions deflected are detected by a particular
  instrument, which then sends messages to a
  recorder resulting in the formation of a mass
  spectrum.

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Pneumatically-Assisted Electrospray
Ionization(ESI)

• Liquid is directed through a capillary tube, at
  the end of which, a voltage is applied.
• This voltage at the outlet of the metal capillary
  creates excess charge in the liquid by redox
  reactions
• If the capillary is positively charged,
  oxidation at the outlet enriches the liquid in
  cations.
• This production of charged liquid is called
  pneumatic nebolization.

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ESI Cont

• At the end of the capillary tube a coaxial flow
  of N2 and a strong electric field create a fine
  aerosol mist of charged droplets.
• The solvent evaporates, which makes the droplets
  even smaller and the concentration of charge in
  the droplet even greater.
• Eventually the repulsion of charges in the
  droplets is so great that the droplet explodes
  into smaller lesser charged droplets.

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ESI Cont

• This is Called the Coulombic explosion.
• The process is repeated (solvent evaporation,
  shrinking, and explosion) until individually
  charged naked analyte ions are formed
• These ions then enter the mass spectrometer.
• The ions that eventually vaporize from the
  aerosol droplets are those that were already in
  the liquid solution.

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Electrical Discharge/Electron Ionization

• Electrons are produced from a Tungsten or Rhenium
  filament.
• Typical filament current is 1x10-4Amps
• Electrons are accelerated towards the ion source
  chamber, that is held at positive potential.
• The electrons acquire an energy equal to the
  voltage between the filament and the source
  chamber
• - typically 70 electron volts (70 eV)
• The electron beam will strike the electron trap
  producing the trap current
• Done to stabilize the electron beam.

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Electron Ionization

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Electron Ionization Cont

• A magnet is positioned across the ion chamber to
  produce a magnetic flux parallel to the electron
  beam.
• This causes the electron beam to spiral from the
  filament to the trap,
• Gaseous molecules are introduced into the path of
  the electron beam then ionized by electronic
  interactions with the beam

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Electron Ionization Cont

• The positive ion repeller voltage and the
  negative excitation voltage work together to
  produce an electric field in the source chamber
  such that ions will leave the source through the
  ion exit slit
• The ions are directed through the various
  focusing and centering lenses and are focused
  onto the source exit slit.
• The ions can then be mass analyzed

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Matrix-Assisted Laser Desorption/Ionization
(MALDI)

• Especially useful for polar compounds, which some
  other ionization methods cannot handle.
• The primary requirement for successful LD is that
  the energy transfer from the laser beam to the
  analyte should take place in as short a time as
  possible to prevent decomposition.
• One restriction is that it can only be used with
  time-of-flight mass spectroscopy, because of the
  short duration of ion burst following the laser
  pulse.
• Matrix is used to circumvent the restrictive mass
  limitations of the technique.

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Mechanism of Ion Desorption

• Formation of solid solution
• The Analyte molecules are distributed throughout
  the matrix so that they are isolated from one
  another
• The matrix is dried to form a homogenous solid
  solution

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Mechanism of Ion Desorption Cont

• Matrix excitation
• A pulsed laser beam is directed at the plate
• Some of the laser energy is absorbed by the
  matrix, causing rapid vibrational excitation.
• This causes part of the solid solution to
  disintegrate.
• Clusters develop of a single analyte molecule
  surrounded by matrix molecules.
• The matrix molecules evaporate leaving excited
  analyte molecules

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Mechanism of Ion Desorption Cont

• Analyte ionization
• The analyte molecules are either protonated or
  deprotonated by photo-excited matrix, forming
  cations or anions.

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Fast Atom Bombardment

• Works best for organic compounds with polarity
  and either acidic and or basic functional groups.
• Basic groups run well in positive ionization
  mode.
• Acidic centers run well in negative ionization
  mode.
• Classes that use FAB are peptides, proteins,
  fatty acids, organometallics, surfactants,
  carbohydrates, antibiotics, and gangliosides.

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FAB

• The particle beam is a neutral inert gas,
  typically Ar or Xe
• Fast moving beam containing these atoms is
  directed toward the sample.
• Sample is dissolved in a liquid matrix, which
  coats the target of the beam.
• Beam collides with the sample and matrix
  molecules, producing positive and negative
  sample-related ions that can be accelerated into
  the mass spectrometer.

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FAB Matrix

• Dissolve sample to be analyzed
• Facilitate in the ionization of the sample
• Be of low volatility
• Should not undergo a chemical reaction with the
  sample
• Common matrices- thioglycerol, glycerol,
  3-nitrobenzyl alcohol.

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FAB Output

• After sample is dissolved in matrix, it is
  admitted into the mass spectrometer.
• Several scans are taken
• Data can be plotted in various ways