Field ionization wire emitters

Molecules can lose an electron when subjected to a high electric potential resulting in field ionization (FI) [366,534,535]. High fields can be created in an ion source by applying a high voltage between a cathode and an anode called a field emitter. A field emitter consists of a wire covered with microscopic carbon dendrites, which greatly amplify the effective field at the carbon points. 

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]... 

Beckey, H.D. Krone, H. Rollgen, F.W. Comparison of Tips, Thin Wires, and Sharp Metal Edges As Emitters for Field Ionization Mass Spectrometry. J. Sci. Instrum. 1968, 7, 118-120. 

Field ionization occurs when gas-phase sample molecules are inteijected in a strong electrical field that is on the order of 10 Vcm The field distorts the electron cloud around the sample molecule and lowers the barrier for the removal of an electron. The quantum mechanical tunneling of this electron from the molecule to the conduction bands of the emitter produces M+ ions [10]. The heart of the FI ion source is an emitter electrode made fi om a sharp metal object such as a razor blade or thin wire. The emitter electrode is placed approximately 1 mm away from the cathode. The field is produced by applying a high potential (10 to 20 kV) to the tip of the emitter electrode. FI is a very soft ionization technique that produces primarily a molecular ion signal. It is applicable to volatile samples only. 

Field desorption (FD), pioneered by Beckey in 1969 [4], was the first and clearly the most successful of the early desorption ionization techniques. In the FD experiment, very high electric fields were used to extract ions from sample-coated thin wire emitters. FD spectra normally contained molecular weight information however, structural fragments were often absent and signal inslability resulted in data acquisition difficulties. As an added complication, FD was experimentally difficult and the method by which ions were formed was not well understood. 

In field ionization sources, ions are formed under the influence of a large electric field (10 7cm). Such fields are produced by applying high voltages (10 to 20 kV) to specially formed emitters consisting of numerous fine tips having diameters of less than 1 pm. The emitter often takes the form of a fine tungsten wire (-10 pm... 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

FAB is most often compared to the soft ionization method known as field desorption (FD) mass spectrometry, a technique in which the sample, deposited on an emitter wire coated with microcrystalline carbon needles, is desorbed under the influence of a high electric field gradient. As usual, bioorganic systems are best represented by both techniques (21, 33). Though FAB is the easier of the two, they are complementary, FAB being particularly suited for the case of extreme thermal lability and FD for the case of chemical lability or matrix interference. Cerny et al. (33) compare the two techniques for the study of coordination complexes and conclude FD is better for molecular-ion determination, while FAB provides better fragmentation information, which is useful in elucidating structures. 

Fig. 32. Field emission microscope for adsorption studies. A—gas bottle B—break off seal C—inverted ionization gauge (also serves as selective getter) D—Granville-Phillips valve E—ionization gauge F—grounding rings G—double Dewar H—emitter assembly (tip mounted on hairpin support wire, equipped with potential leads for measuring resistance) I—anode terminal J—willemite screen settled onto tin-oxide conductive coating K—ground glass port L—trap.

This ionization technique is extensively utilized in biomedical and environmental research because of its applicability to a wide range of samples from inorganic salts to polar metabolites. The sample is normally adsorbed on dendritic needles grown on a thin tungsten wire (the so-called emitter). When a high electric field is applied to this adsorbed sample layer, ionization will occur ( 1 V/A). Similar to field... 

In the field desorption technique (FD) the sample is placed on a specially prepared emitter wire and ionized in a high voltage electrostatic field. In this way little or no fragmentation occurs and mass peaks are obtained even from practically non-volatile peptides [34]. 

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