Plasma Desorption Ionization

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Plasma Desorption Ionization

• Methods in complex biological molecule ionization

Jeffrey Maas
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Complex Bio-molecules

• Mass Spectrometry is limited, because it
  requires volatilization of the sample.
• Problem
• Low vapor pressure leads to few ions for
  detection.
• Solution
• Heating sample increases vapor pressure.
• Opposition
• Heating sample also increases decomposition.

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History

• Four methods for ionization of large molecules
• Increase sensitivity for low vapor pressure
  McIver
• New instrument (ICR)
• Chemical alteration of polar groups to non-polar
  substituents Watson
• Additional complicated steps
• Deposit analyte on a non-reacting surface to
  increase vapor
• pressure Buehler
• Only verifed for a small group of peptides
• Heating in the presence of a strong electric
  field reduces heat of vaporization Beckey
• High decomposition of molecules
• Observed Phenomena leading to PDMS
• Vaporization rate gt Decomposition rate(T gt 400K)
  - Buehler
• Heating at fast rates lowers decomposition -
  Torgerson

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PDMS spontaneous fission of 252Cf

• Ionization of large bio-molecules achieved by
  PDMS
• Ions heavier than Kr with energy gt 100MeV can
  penetrate films up to 10µm thick
• 252Cf fission most often produces the pair
• 142Ba18 at 79MeV
• 106Tc22 at 106MeV
• 252Cf can heat the sample locally up to 10,000ºK
• Rapid heating occurs on a time scale in which
  most of the energy is not coupled into
  vibrational motion.
• Deposits 10MeV through Coulomb interaction in
  10-13s corresponding to 16 watts/fission fragment
  or 10 terawatts/cm2!

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PDMS

• Process using PDMS
• Sample is deposited over a 1cm2 region on Ni foil
  1µm thick
• Foil is aligned with 252Cf source for maximum
  energy transfer
• Two fission fragments occur from the 252Cf
• First travels towards a start detector
• Second travels into the Mass Spectrometer
• Ionization occurs through ion-pair formation
• An electric lens accelerates the ions into the
  TOF
• Standard TOF analysis at this point

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Details of the Process
Ions in the Plasma
Fission Fragments
(Which would be mounted on a thin foil)
Californium-252 Plasma Desorption Mass
Spectroscopy R. D. Macfarlane D. F. Torgerson,
Science, New Series, Vol. 191, No. 4230. (Mar. 5,
1976), pp. 920-925. Figure 2
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Details of the Process

• Time of flight set up with PDMS

(Used for ion orbit)
Californium-252 Plasma Desorption Mass
Spectroscopy R. D. Macfarlane D. F. Torgerson,
Science, New Series, Vol. 191, No. 4230. (Mar. 5,
1976), pp. 920-925. Figure 1
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Unique Timing Aspects of PDMS

• Typically used with TOF MS due to the creation of
  fission fragment pairs
• Two fragments give timing ability with precision
  of 10-9s
• Continuous operation
• for trace detection

PDMS. (2006, June 21). In Mass Spectrometry Wiki.
Retrieved 9am, September 26, 2007, from
http//mass-spec.lsu.edu/mswiki/index.php/Plasma_d
esorption_mass_spectrometry
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Results of PDMS

• Positive and negative ions produced in equal
  amounts for further determination
• Low fragmentation of parent ion
• Ability to detect trace amounts by continuous
  operation
• Each event at the detector can be calibrated to
  the sister ion produced at the point of fission
• PDMS was short lived due to the realization of
  MALDI which improved upon the concepts described
  here.

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Remember

• PDMS generates a plasma by striking a thin Ni
  foil with a sample deposited on it and
  subsequently breaking the dimer.
• PDMS is used with TOF because it has a unique
  timing feature.
• PDMS can be used with other applications due to
  the low energy ions created.
• PDMS in this configuration uses a radioactive
  source for nuclear fission fragment emission.
• PDMS can be observed by any means of bombarding a
  sample with high energy ions. It is the
  combination of a rapid thermal desorber, with a
  little energy left over for ionization.

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References

• 1) R. T. McIver, E. B. Ledford. Jr., J. S.
  Miller.
• Anal.Chem. 47, 692 (1974).
• 2) J. T. Watson and K. Biemann,
• ibid. 37, 844 (1965).
• 3) R. J. Beuhler, E. Flanigan, L. J. Greene, L.
  Friedman.
• J. Am. Chem. Soc. 96, 3990 (1974)
• 4) H.D. Beckey, A. Heindrichs, H.U. Winkler.
• Int. J. Mass Spectrom. Ion Phys. 3, 9 (1970)
• 5) D.F. Torgerson, R.P. Skowronski, R.D.
  Macfarlane.
• Biochem. Biophys. Res. Commun. 60, 616 (1974)