Field ionization emitter activation

Furthermore, the radicals formed upon field-induced hydrogen abstraction can lead to polymerization products on the emitter surface. The mechanism of this field polymerization helped to elucidate the phenomenon of activation of field emitters, i.e., the growth of microneedles on the emitter surface under the conditions of field ionization of certain polar organic compounds. [59]... 

In field ionization (as an experimental configuration) field ionization (the process) is the major pathway of ion generation. In field desorption from activated emitters, the analyte may also undergo field ionization. Presuming that the molecules are deposited in layers on the shanks of the whiskers or between them, this requires that i) analytes of low polarity are polarized by action of the electric field, ii) become mobile upon heating, and iii) finally reach the locations of ionizing electric... 

Beckey, H.D. Hilt, E. Schnlten, H.-R. High Temperature Activation of Emitters for Field Ionization and Field Desorption Spectrometry. J. Phys. E Sci. Instrum. 1973,6, 1043-1044. 

Field desorption (FD) was introduced by Beckey in 1969 [76]. FD was the first soft ionization method that could generate intact ions from nonvolatile compounds, such as small peptides [77]. The principal difference between FD and FI is the sample injection. Rather than being in the gas phase as in FI, analytes in FD are placed onto the emitter and desorbed from its surface. Application of the analyte onto the emitter can be performed by just dipping the activated emitter in a solution. The emitter is then introduced into the ion source of the spectrometer. The positioning of the emitter is cmcial for a successful experiment, and so is the temperature setting. In general, FI and FD are now replaced by more efficient ionization methods, such as MALDI and ESI. For a description of FD (and FI), see Reference 78. 

Alternatives to activated tungsten wire emitters are also known, but less widespread in use. Cobalt and nickel [44,47] as well as silver [48] can be electrochemi-cally deposited on wires to produce activated FD emitters. Mechanically strong and efficient emitters can be made by growing fine silicon whiskers from silane gas on gold-coated tungsten or tantalum wires of 60 pm diameter. [45] Finally, on the fracture-surface of graphite rods fine microcrystallites are exposed, the sharpness of which provides field strengths sufficient for ionization. [49]... 

Figure 14.2. Arrangement of an activated emitter in a liquid injection field desorption ionization (LIFDI) system. Courtesy of Linden ChroMasSpec GmbH, Leeste, Germany.

Field (desorption) ionization (FDI) was first described by Beckey in 1969 [4]. In FDI, the sample solution is deposited on a lO-qm-diameter FDI emitter, which is activated to provide for whiskers or microneedles on the surface [5, 6]. The emitter is kept at a high potential (> 5 kV) in the high-vacuum ion source, mostly of a sector instmment (see Fig. 7.1). By passing through a current, slow heating of the emitter is achieved. As heating of the emitter continues, nonvolatile analytes can be desorbed and ionized by various mechanisms. High local electrical fields at the... 

Soltmann B, Sweeley C C, Holland J F 1977 Electron impact ionization mass spectrometry using field desorption activated emitters as solid sample probes. Anal Chem 49 1164-1166... 

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