FD emitter

Note The field anode is usually referred to as field emitter, FI emitter, or FD emitter. The properties of the field emitter are of key importance for FI- and FD-MS. The electrode on the opposite side is called field cathode or simply counter electrode. 

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 1. Field desorption (FD) mass spectrum of a purified C(o sample (M, 780) recorded using a JEOL AX505H spearometer. Experimental conditions were as follows 30 mA FD emitter current 3 keV ion energ) 9 keV extraction energy. Similar spectra were recorded using fast atom bombardment.

This plot correlates the sample amount applied to the FD emitter with the peak area of the evaporation profile. The range of sample amounts covered by this curve extends from 1000 to 0.3 pg of cesium corresponding to 7.5 pmol to 2.3 fmol. 

The organic solvents investigated were of commercially available quality (carbon tetrachloride, Uvasol, E. Merck AG, Darmstadt, West Germany methanol, analyzed reagent, J. T. Baker Chemicals B.V., Deventer, The Netherkmds). Drinking water was taken from the tap in our laboratory and seawater from the North Sea near the coast of The Netherlands. The aerosol was directly sampled on a FD emitter for 2 h on the roof of our laboratory on May 13, 1977. Samples of body fluids were taken from volunteers and analyzed without further treatment. 

To obtain these data the FD ion currents were recorded on a homebuilt single focusing mass spectrometer of low resolution equipped with a FD source with micromanipulator . The FD ion source employed is schematically described in Fig. 12. The FD emitter (at + 8 kV) is positioned at a distance of 2 mm from the counter electrode (at — 4 kV). Only the first lens (at approximately + 2 kV) is used for the focusing of the ion beam whereas all other reflection plates are at ground potential. With an entrance slit width of 0.1 mm and an exit slit width of 0.5 mm, a resolution of about 300 (10% valley definition) is achieved. This experimental set-up considerably simplifies the operation of the FD mass spectrometer because it allows... 

The ions are detected using a channel electron multiplier (Valvo) and a combined counter/ratemeter registering unit (Ortec). The channel electron multiplier is operated at —3 kV. A linear emitter heating current (EHC) programmer is employed for the desorption of the samples. In all cases the EHC is raised at 0.19 mA/s from 0 to 100 mA (see Fig. 14). All measurements for the calibration curve of [Cs] (see Fig. 13) are made with one FD emitter starting from small sample amounts. 

The stereoscan micrograph of the FD emitter used in Fig. 16 shows the branched structure of carbonaceous needles grown on the 10 pm tungsten wire at high enlargement. The micrograph-was taken after a 4-hour exposure to natural air in the jet collector, at a flow rate of 3 1 min" Numerous solid particles were caught in the dendritic emitter. 

Fig. 16. Scanning-electron micrograph of a FD emitter after direct impact of a natuial aerosol from the top. Single aerosol particles are seen to be caught in the branched structure of the carbonaceous microneedles ...

In a first pilot study analyzing 1 fil human blood which was applied directly to the FD emitter sodium, potassium, calcium, rubidium and cesium were found in one analytical run . Thus, it could be expected that FDMS using photographic detection should also be applicable to biological or medical samples without pretreatment. 

The experiments were carried out with a Varian MAT 731 double focusing mass spectrometer. The combined EI/FI/FD source commercially available from Varian and a home-made source for FD only were used. The mono FD source is superior in two respects first, the adjustment of the FD emitter is fast and easily performed by means of the micromanipulator and second, the source is much less sensitive to contaminations because of its simple and open construction. The 514 nm line of a tunable argon ion laser. Spectra Physics model 166, was used for indirect heating of the emitter. Fig. 21 shows the experimental set-up. 

Fig. 21. Set-up of a mass spectrometer equipped with combined electrtm impact/FD ion source and laser for indirect heating of the FD emitter and the sample. The laser beam passes through a quartz window and strikes the emitter wire perpendicular to the direction of the ion beam produced. Simultaneously, observation of the emitter and measurement of the emitter temperature by a pyrometer are possible ...

However, in both FI and FD, there are other neutral molecules on or close to the surface of the emitter and, in this region, ion/molecule reactions between an initial ion and a neutral (M(H)) can produce protonated molecular ions ([M + H]+), as seen in Equation 5.2. 

For nonvolatile or thermally labile samples, a solution of the substance to be examined is applied to the emitter electrode by means of a microsyringe outside the ion source. After evaporation of the solvent, the emitter is put into the ion source and the ionizing voltage is applied. By this means, thermally labile substances, such as peptides, sugars, nucleosides, and so on, can be examined easily and provide excellent molecular mass information. Although still FI, this last ionization is referred to specifically as field desorption (FD). A comparison of FI and FD spectra of D-glucose is shown in Figure 5.6. 

Sometimes, in FD, the emitter electrode is heated gently either directly by an electrode current or indirectly by a radiant heat source to aid desorption of ions from its surface. 

As field desorption (FD) refers to an experimental procedure in which a solution of the sample is deposited on the emitter wire situated at the tip of the FD insertion probe, it is suited for handling lubricants as well as polymer/additive dissolutions (without precipitation of the polymer or separation of the additive components). Field desorption is especially appropriate for analysis of thermally labile and high-MW samples. Considering that FD has a reputation of being difficult to operate and time consuming, and in view of recent competition with laser desorption methods, this is probably the reason that FD applications of polymer/additive dissolutions are not frequently being considered by experimentalists. 

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. 

The standard MAT 90 ion source is used for optimized FD/FI mode by means of the newly designed FD/FI probe. Conversion from electron impact (El), chemical ionization (Cl) or fast ion bombardment (FAB) to FD/FI operation does not require the exchange of the ion source. The FD/FI probe accommodates both the field emitter and the extraction electrodes, mounted at the probe tip. Both are introduced as a unit into the ion source through the ionization volume exchange lock without breaking vacuum. The fast and simple changeover illustrates the versatility of the Finnigan MAT 90 with no compromise on the performance. 

Fig. 8.2. FI/FD ion source with potentials applied to emitter and lenses. Reproduced from Ref. [33] by permission. American Association for the Advancement of Science, 1979.

Fig. 8.3. Schematic of an FI/FD ion source (a) in H mode, (b) in FD mode. The distance between emitter and counter electrode is shown exaggerated for clarity. Adapted from Ref. [34] by permission. Springer-Verlag, Heidelberg, 1991.

In practice, moderate heating of the emitter at constant current serves to reduce adsorption to its surface during FI measurements. Heating at a constant rate (1-8 mA min ) is frequently employed to enforce desorption of analytes from the emitter in FD-MS. To avoid electric discharges resulting from too massive ion de-... 

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. 8.8. FD probe inserted into the vacuum lock. FD probes are generally inserted in axial position to leave the vacuum lock of the DIP free for FI use. The emitter wire is now oriented vertically to comply with the beam geometry of the magnetic sector analyzer.

Numerous analytes could be good candidates for FD-MS, but undergo immediate decomposition by reacting with ambient air and/or water under the conditions of conventional emitter loading. Inert conditions such as emitter loading in a glove box does not really avoid the problem, because the emitter still needs to be mounted to the probe before insertion into the vacuum lock. Furthermore, the tuning of an FD ion source usually has to be optimized for the emitter actually in use which is not practicable with the sample already loaded onto its surface. 

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.

Note In FI-MS, the ionization efficiency is very low, because of the low probability for a neutral effusing from any inlet system towards the field emitter to come close enough to the whiskers. Consequently, FI-MS produces very low ion currents. The application of FI-MS is therefore restricted to samples that are too volatile for FD-MS or require gas chromatographic separation before. 

Multiply charged ions of minor abundance are frequently observed in FI and FD mass spectra. Their increased abundance as compared to El spectra can be rationalized by either of the following two-step processes i) Post-ionization of gaseous M ions can occur due to the probability for an M ion to suffer a second or even third ionization while drifting away from the emitter surface. [69,70] Especially ions generated in locations not in line-of-sight to the counter electrode pass numerous whiskers on their first 10-100 pm of flight ... 

Although electrospray ionization and matrix-assisted laser desorption ionization allow to transfer much larger ions into the gas phase, it is FD that can be regarded the softest ionization method in mass spectrometry. [27,74] This is mainly because the ionization process itself puts no excess energy into the incipient ions. Problems normally arise above 3000 u molecular weight when significant heating of the emitter causes thermal decomposition of the sample. 

Example D-Glucose may be evaporated into the ion source without complete decomposition as demonstrated by its FI spectrum (Fig. 8.13). FD yields a spectrum with a very low degree of fragmentation that is most probably caused by the need for slight heating of the emitter. The occurrence of ions, m/z 180, and [Mh-H]" ions, m/z 181, in the FD spectrum suggests that ion formation occurs via field ionization and field-induced proton transfer, respectively. However, thermal... 

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