Field desorption

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Schematic of field desorption ionization with emitter at left and mass spectrometer at right

Field desorption (FD)/field ionization (FI) refers to an ion source for mass spectrometry first reported by Beckey in 1969.[1] 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 formed. This results in a very high electric field which can result in ionization of gaseous molecules of the analyte. Mass spectra produced by FI have little or no fragmentation because FI is a soft ionization method. They are dominated by molecular radical cations M+. and less often, protonated molecules [M+H]^{+}\,.

Mechanism

In FD, the analyte is applied as a thin film directly to the emitter, or small crystals of solid materials are placed onto the emitter. Slow heating of the emitter then begins, by passing a high current through the emitter, which is maintained at a high potential (e.g. 5 kilovolts). As heating of the emitter continues, low-vapor-pressure materials get desorbed and ionized by alkali metal cation attachment.

Applications

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 can produce simple mass spectra with molecular information from hydrocarbons and other particular analytes. The most commonly encountered application of FD/FI at the present time is the analysis of complex mixtures of hydrocarbons such as that found in petroleum fractions.

Liquid injection

The recently developed liquid injection FD ionization (LIFDI) [2] technique "presents a major breakthrough for FD-MS of reactive analytes":[3] Transition metal complexes are neutral and due to their reactivity, do not undergo protonation or ion attachment. They benefit from both: the soft FD ionization and the safe and simple LIFDI transfer of air/moisture sensitive analyte solution. This transfer occurs from the Schlenk flask to the FD emitter in the ion source through a fused silica capillary without breaking the vacuum.

References

  1. ↑ Beckey H.D. Field ionization mass spectrometry. Research/Development, 1969, 20(11), 26
  2. ↑ HB Linden, Liquid injection field desorption ionization: a new tool for soft ionization of samples including air sensitive catalysts and non-polar hydrocarbons, Eur. J. Mass Spectrom. 2004, 10, 459-468
  3. ↑ JH Gross, Mass Spectrometry - A Textbook, Springer 2004, page 362

Bibliography

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