Mass Spectrometry

pp 355-380

Field Ionization and Field Desorption

  • Jürgen H. GrossAffiliated withInstitute of Organic Chemistry, University of Heidelberg

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The first observation of the desorption of positive ions from surfaces by high electrostatic fields was made by means of a field ion microscope. [1, 2] The mass spectrometric analysis of some field-ionized gases followed. [2–4] However, it was Beckey putting forth the development of a focusing field ionization ion source. [5] In these early experiments electric field strengths of about 108 V cm-1 (1 VÅ-1) were generated at sharp tungsten tips. [2, 4, 5] The method of field ionization (FI) was soon extended to analyze volatile liquids [6–11] and solids introduced by evaporation from a sample vial in close proximity to the ionizing tip or wire electrode. [12]. FI, still immature in the mid-1960s, had soon to compete with chemical ionization (CI, Chap. 7). [13] The major breakthrough came from its further development to field desorption (FD), because FD circumvents the evaporation of the analyte prior to ionization. [14, 15] Instead, the processes of ionization and subsequent desorption of the ions formed are united on the surface of the field emitter. The specific charm of FI-MS and of FD-MS in particular arises from their extraordinary softness of ionization yielding solely intact molecular ions in most cases, and from the capability of FD to handle neutral as well as ionic analytes. [16–25] FD-MS had its flourishing period from the mid-1970s to the mid-1980s and suffered from the advent of fast-atom bombardment (FAB) and later electrospray ionization (ESI) mass spectrometry. [21] However, it appears that the unique capabilities of FD-MS are currently being re-discovered. [26–29]