Field desorption emitter activation

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... 

Figure 1. Electron micrograph of field desorption emitter prepared by activation of 10-n tungsten wire in a benzonitrile atmosphere. Distance between posts is 5 mm.

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. 

H. R. Schulten and H. D. Beckey. Field Desorption Mass Spectrometry with High Temperature Activated Emitters. Org. Mass Spectrom., 6(1972) 885-895. 

Giessmann, U. Heinen, H.J. Rollgen, F.W. Field Desorption of Nonelectrolytes Using Simply Activated Wire Emitters. Org. Mass Spectrom. 1979,14, 177-179. 

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 mass spectra were obtained on a Varian MAT 731 instrument (Florham Park, NJ) fitted with the combined EI/FI/FD ion source. Emitters were prepared in the Varian apparatus according to Schulten and Beckey (3J, or were pretreated before activation by soaking in a saturated salt solution (9 ). 

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... 

The reactivity of a surface depends on many factors. These include the adsorption energies of chemical species and their dissociation behavior, their diffusion on the surface, the adatom-adatom interactions, the active sites where a chemical reaction can occur, and the desorption behavior of a new chemical species formed on the surface. The site specificity depends on at least three factors the atomic configuration of the surface, the electronic structures of the surface, and the localized surface field. In atom-probe experiments, the desorption sites can be revealed by a timegated image of an imaging atom-probe as well as by an aiming study with a probe-hole atom-probe, the electronic structure effect of a chemical reaction can be investigated by the emitter material specificity, and the surface field can be modified by the applied field. 

The experimental technique for the trace analysis of metals simply involves the production of an emitter of acceptable quality. In general, 10 /im tungsten wires are activated at high temperature with benzonitrile in a multiple activation device. As the result of such an activation process, the tungsten wire is covered with dendrites of partially ordered pyrocarbon. Due to the small radii of curvature of the tips of the microneedles, the field strength is enhanced to a. level suitable for FDMS. These emitters are mechanically stable, which is important for repeated use they can also be chemically and thermally strained. This property is a prerequisite for the pyrolysis of the organic matrix and desorption of the metal cations, and last not least, the surface area of the emitter is sufficient for sample application. 

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