Alternative Ionization Means

Pure or neat compounds can be introduced into the ion source having been deposited in the tip of a rod or solids probe. With heat, the sample sublimates or evaporates into the gas phase. In most cases, ionization follows by the means described here. But in some cases, ionization occurs simultaneous with the sublimation or evaporation.

  • Atmospheric Pressure Photo-ionization (APPI)
    • Direct or dopant-assisted photon ionization of analytes with ionization potential below 10 eV (the primary photon energy output by a krypton gas lamp). The ionization potential for solvents commonly used in LC are above 10 eV. APPI is one of the primary API alternatives in the lab since it extends the ionization range to more non-polar analytes than either ESI or APCI can ionize.
  • Matrix-assisted laser desorption (MALDI)
    • Soft ionization for intact proteins, peptides, and most other biomolecules (oligonucleotides, carbohydrates, natural products, and lipids) and analysis of heterogeneous samples (analysis of complex biological samples such as proteolytic digests)
    • High energy photons interact with a sample embedded in an organic matrix typically with sub-pico mole sensitivity.
    • First introduced in 1988 by Tanaka, Karas, and Hillenkamp.
  • Fast atom bombardment (FAB)
    • An early form of soft ionization using a stream of cesium ions to “sputter” ions from a sample dissolved in a glycerol, or similar, matrix.
  • Desorption 
    • Plasma Desorption nuclear fission fragments interact with a solid sample deposited on a metal foil.
    • Secondary Ion MS (SIMS) high velocity ions impact a thin film of sample deposited on a metal plate or contained in a liquid matrix (liquid SIMS).
    • Field Desorption: a high field gradient is imposed on a sample deposited on a support.
    • Desorption ESI (DESI): along with closely related techniques like DART (direct analysis real time), ASAP (atmospheric solids analysis probe) and others recently introduced to the market these tend to create ions secondary to some interaction on a surface. In DESI, an energized liquid stream is aimed at a sample deposited in a flat surface, causing secondary ionization to occur at atmospheric pressures.

See MS – The Practical Art, LCGC

  • Incipient Technologies: Desorption and Thermal Desorption Techniques, Vol. 25, No. 10 December 2007
    • Why this is important: Describes and compares techniques such as DESI, DART and ASAP with realistic appraisals for their use.

  • Alternatives in the Face of Chemical Diversity, Vol. 25, No. 4, April 2007
    • Why this is important: Explores prospects for applying GC and other not-so-typical techniques to contemporary instruments designed for atmospheric work.

  • Ionization Revisited, Vol. 24, No. 12, December 2006
    • Why this is important: An overview of the major ionization techniques in use today with references.

Also:

  • Balogh, M.P., The Commercialization of LC-MS During 1987-1997: A Review of Ten Successful Years, LC/GC, Vol. 16 no 2 135-144, February 1998.

  • Van Berkel, G.J., Pasilis, S. P., Ovchinnikova, O., Established and emerging atmospheric pressure surface sampling/ionization techniques for mass spectrometry, Journal of Mass Spectrometry, Vol. 43, no. 9, 1161-1180 2008

  • Knox, J.H., Practical Aspects of LC Theory, Journal of Chromatographic Science, 15, (1977) 357

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