Activated Emitters

Alternative wire emitters other than activated tungsten are less commonly used. Cobalt and nickel [50,53] as well as silver [54] can be electrochemically deposited 

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] 

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A number of other up-conversion processes are known. The blue emission from a Yb3+/Tm3+ couple in which the active emitters are defect Tm3+ centers is mainly due to the efficient excitation ET process from Yb3+ centers. Two-frequency up-conversion has been investigated using Pr3+ defects in a fluoride glass matrix. Illumination with one pump wavelength results in GSA, while simultaneous irradiation with a second pump wavelength further excites the GSA centers via ESA. The doubly excited defects emit red light. Up-conversion and visible output only takes place at the intersection of the two beams. 

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. 

Fig. 8.7. FD probe, (a) Emitter holder of a JEOL FD probe tip, (b) a drop formed of 1-2 pi analyte solution placed onto the activated emitter by means of a microliter syringe.

Fig. 8.9. Probe for LIFDI. A fused silica capillary delivers the sample to the backside of the activated emitter. Here, the counter electrode is part of the FD probe. By courtesy of Linden CMS, Leeste, Germany.

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 2. Electron micrograph of activated emitter showing dendrite growth along...

Figure 3. Electron micrograph of region at dendrite tips on activated emitter...

Figure 4. Electron micrograph of activated emitter to which sample has been applied by dipping emitter into a solution and allowing the solvent to air dry.

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

The sample can be easily applied by dipping the activated emitter into a solution or suspension of the substance. The amount of sample, i.e. organic matrix plus metal, deposited on the emitter should be in the range of 10 ng to a few ng. Diluted solutions which are supplied by a microsyringe can be concentrated most effectively by evaporation of the solvent under the control of a stereomicroscope as illustrated in Fig. 11. Furthermore, this method enables the transfer and deposition of known amounts of sample solution onto the emitter surface and thus provides essential experimental conditions for quantitative determinations. 

Fig. 11. Syringe technique the sample solution or suspension is supplied with a 10 microsyringe to the activated emitter wire. The microscope photographs show a magnification of approximately 1 35 ...

The equivalent electrical circuit, rearranged under the influence of an apphed physical field, is considered as a parallel resonant circuit coupled to another circuit such as an antenna output circuit Thus, in Figure 15.4c, Wj, Cd, La, and Ra correspond to the circuit elements each Wd represents active emitter-coupled oscillator and Cd, Ld, and Rd, represent passive capacitive, inductive, and resistive elements respectively. The subscript d is related to the particular droplet diameter, that is, the droplet under consideration. Now, again the initial electromagnetic oscillation is represented by... 

A typical upconversion material is composed of two components, an inorganic host matrix and doping lanthanide ions (Figure 3). For achieving efficient ETU, effective energy transfer between the host matrix-embedded ions is essentially required, so the doped ions can also be divided into two types, activator (emitter) and sensitizer (absorber). 

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... 

In FD, the sample is deposited directly onto carbon dendrites serving on the anode as activated emitters. For hydrocarbon-type anionic, cationic, and nonionic surfactants, FD usually produces molecular or quasimolecular ions free of fragmentation. For amphoteric nonfluorinated surfactants, molecular ions have been obtained together with fragment ions providing structural information [95-97], which showed that perfluoroalkanesulfonates are desorbed as high-mass clusters under FD conditions. 

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