Electrospray Ionization ESI

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Electrospray Ionization (ESI) Gaskell J. Mass
Spectrom. 1997, 32, 677. Kebarle J. Mass
Spectrom. 2000, 35, 804.
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• electrospray permits the direct introduction of
  ions from solution into the mass spectrometer. A
  solution of analyte is pumped (small quantities)
  through a capillary which is held at a high
  potential.

• the high electric field between the end of the
  tube and an extraction plate causes the
  emerging liquid to leave the end of the capillary
  as a mist of fine droplets mixed with the vapor
  (nebulization)

taylor cone

• occurs at atmospheric pressure

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N2 gas nitrogen curtain
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Mechanism of Gaseous Ion Formation

• ions exist in solution, if not, electrospray
  doesnt work, it is not an ion formation
  technique rather than a technique for extracting
  ions from the solution-phase into the gas-phase
  free of solvent for mass spectral analysis

Charge Residue Mode.
evaporation and fission
fission
ion evaporation
evaporation
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Ion-Induced Desorption Model
evaporation
fission
field-assisted ion desorption

• the ion-induced desorption model is more widely
  accepted

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positive ESI mass spectrum of myoglobin

• soft process, little to no fragmentation due to
  the ESI process

• each peak refers to myoglobin in a different
  charge state (?)

• can solve for M and n

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• the charge states of the gaseous ions generally
  represent the charge states in the condensed
  phase. These are sometimes modified by
  ion/molecule collisions. Ions such as large
  biomolecules are highly charged.
• the transfer of ions to the gas phase is not an
  energetic process. Ions are cold, in fact the
  desolvation process further cools ions.
• non-covalent interactions can be preserved when
  the species enters the gas phase. This is
  significant for the application of ESI to the
  study of biological molecules such as proteins.

pH2.6
pH3.0
pH5.2
ESI mass spectra of bovine cytochrome c
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• Electrospray ionization (ESI) has opened up the
  techniques of mass spectrometry to such a
  diversity of molecules that it has completely
  revolutionized mass spectrometry.
• Types of molecules which are now amenable to
  mass spectrometry include biological molecules
  (proteins, DNA, RNA, oligonucleotides, hormones
  etc.), polymers, non-volatile inorganic and
  organometallic molecules and salts, as long as
  they exist as ions in solution
• It has also revolutionized the sales of mass
  spectrometers as the can be considered to be an
  analytical technique for biochemistry (big ).
• There are many many fundamental and applied
  applications of ESI and they can be stumbled
  upon simply by opening up a journal and browsing
  the contents.

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Nanospray Electrospray using a much smaller
capilliary so that much less solution is used and
much less sample is required
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Atmospheric Pressure Chemical Ionization (APCI)
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• unlike electrospray, ions do not need to exist
  in solution
• this technique, then, can be used for non-polar
  (or non-ionizing) compounds such as hydrocarbons
  or steroids or polymers that are soluble

• the analyte solution is introduced into a
  pneumatic nebulizer and converted to a thick fog
  which is passed through a high temperature region
  and is desolvated

• they are then carried along a corona discharge
  where ionization occurs

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• the main ions produced in the atmosphere will be
  N2 and O2 which in turn ionize the solvent
  molecules with react with the analyte and
  typically proton-transfer or adduct formation

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• like ESI, is easily coupled to chromatographic
  separation techniques

• presence of acidic group leads to deprotonation

• no acidic group therefore only adduct formation