Field desorption

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Field desorption (FD)/field ionization (FI) refers to an ion source for mass spectrometry first reported by Beckey in 1969. In field ionization, a high-potential electric field is applied to an emitter with a sharp surface, such as a razor blade, or more commonly, a filament from which tiny "whiskers" have been grown. This produces a very high electric field in which electron tunneling can result in ionization of gaseous analyte molecules. FI produces mass spectra with little or no fragmentation, dominated by molecular radical cations M+. and occasionally protonated molecules [M+H]+.

In field desorption, a thin film of analyte is applied directly to the emitter. Small crystals of solid materials can also be deposited directly on the emitter. The emitter is then heated slowly by passing a high current through the emitter, which is maintained at a high potential (e.g. 5 kilovolts). As the emitter is heated, low-vapor-pressure materials are desorbed and ionized by electron tunneling or by alkali metal cation attachment.

Many earlier applications of FD/FI to analysis of polar and nonvolatile analytes such as polymers and biological molecules have largely been supplanted by newer ionization techniques. However, FD/FI remains one of the only ionization techniques that produce simple mass spectra with molecular information from hydrocarbons and certain other analytes. The most commonly encountered application of FD/FI at the present time is the analysis of complex mixtures of hydrocarbons such as petroleum fractions.


[edit] References

  • Beckey H.D. Field ionization mass spectrometry. Research/Development, 1969, 20(11), 26
  • Prokai, L. Field Desorption Mass Spectrometry Marcel Dekker, New York, 1990.
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