Field Desorption FD

2 Field Desorption (FD) Stable molecular ions are obtained from a sample of low volatility, which is placed on the anode of a pair of electrodes, between which there is an intense electric field. Desorption occurs, and molecular and quasimolecular ions are produced with insufficient internal energy for extensive fragmentation. Usually the major peak represents the [M + H]+ ion. 

Synthetic polymers with molecular weights on the order of 10,000 Da have been analyzed, but there is a much lower molecular weight limit for polar biopolymers here the FAB procedure and others (see below) are superior. 

The radical calions are accelerated to 6-10 keV to give radical cations of high translational energy (Xe) +, 

The compound of interest is dissolved in a high-boiling viscous solvent such as glycerol a drop is placed on a thin metal sheet, and the compound is ionized by the high-energy beam of xenon atoms (Xe). Ionization by translational energy minimizes the amount of vibrational excitation, and this results in less destruction of the ionized molecules. The polar solvent promotes ionization and allows diffusion of fresh sample to the surface. Thus ions are produced over a period of 20-30 min, in contrast to a few seconds for ions produced from solid samples. 

The molecular ion itself is usually not seen, but adduct ions such as [M + H]+ are prominent. Other adduct ions can be formed from salt impurities or upon addition of salts such as NaCl or KC1, which give [M + Na]+ and [M + K]+ additions. Glycerol adduct peaks are prominent and troublesome in the spectrum. Fragment ions are prominent and useful. 

Another method, developed to a much greater extent, is that of field desorption. This method involves the desorption of the sample which is deposited on the filament, activated as above, and which emerges in ionic form. Again, internal energy remains very low and is highly sensitive to thermal effects. 

When studying mixtures, it is necessary to increase the emitter current slowly, in order to induce the sequential desorption of the components of the mixture. Thus, in addition to these molecular species, there also appear decomposition ions (probably by pyrolysis) of the molecular species (protonated, cationized or not) which were emitted initially. 

Molecular ions formed under low pressure may have an odd number of electrons (M ) or may be protonated (MH ). There are several possibilities of detecting them, including a search for doubly charged ions (M ), which often appear and which may be used as a molecular test. It is possible [77] to use cationization when it is difficult to obtain. molecular ions and [M + cation] ions may thus appear, as well as doubly charged ions, such as [M-l-Na] . The choice of an alkali cation leads to charge localization on the metal, thus reducing the 

This method, introduced by Beckey [79], has been successfully utilized by different groups studying high-molecular-mass molecules which are difficult to volatilize. A number of reviews exists [80-82], including Schulten s most recent one [83]. 

The field of application of this method is quite large, especially at the level of all classes of biological macromolecules, e.g., polysaccharides, antibiotics, natural metal complexes, etc. This is shown by the observation of ions with masses of 4000 units produced by peptides composed of 29 amino acid residues [84]. The method seems to be sensitive in the ng and pg range of sample detection. 

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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. 

The process of field ionization presupposes that the substance under investigation has been volatilized by heat, so some molecules of vapor settle onto the tips held at high potential. In such circumstances, thermally labile substances still cannot be examined, even though the ionization process itself is mild. To get around this difficulty, a solution of the substance under investigation can be placed on the wire and the solvent allowed to evaporate. When an electric potential is applied, positive or negative ions are produced, but no heating is necessary to volatilize the substance. This technique is called field desorption (FD) ionization. 

Laali and Lattimer (1989 see also Laali, 1990) observed arenediazonium ion/crown ether complexes in the gas phase by field desorption (FD) and by fast atom bombardment (FAB) mass spectrometry. The FAB-MS spectrum of benzenediazonium ion/18-crown-6 shows a 1 1 complex. In the FD spectrum, apart from the 1 1 complex, a one-cation/two-crown complex is also detected. Dicyclo-hexano-24-crown-6 appears to complex readily in the gas phase, whereas in solution this crown ether is rather poor for complexation (see earlier in this section) the presence of one or three methyl groups in the 2- or 2,4,6-positions respectively has little effect on the gas-phase complexation. With 4-nitrobenzenediazonium ion, 18-crown-6 even forms a 1 3 complex. The authors assume charge-transfer complexes such as 11.13 for all these species. There is also evidence for hydride ion transfer from the crown host within the 1 1 complex, and for either the arenediazonium ion or the aryl cation formed from it under the reaction conditions in the gas phase in tandem mass spectrometry (Laali, 1990). 

Recent attention has focused on MS for the direct analysis of polymer extracts, using soft ionisation sources to provide enhanced molecular ion signals and less fragment ions, thereby facilitating spectral interpretation. The direct MS analysis of polymer extracts has been accomplished using fast atom bombardment (FAB) [97,98], laser desorption (LD) [97,99], field desorption (FD) [100] and chemical ionisation (Cl) [100]. 

Solid/liquid probe Field desorption (FD) Radio frequency (RF) Image currents... 

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. 

Mass spectrometry (MS) in its various forms, and with various procedures for vaporization and ionization, contributes to the identification and characterization of complex species by their isotopomer pattern of the intact ions (usually cation) and by their fragmentation pattern. Upon ionization by the rough electron impact (El) the molecular peak often does not appear, in contrast to the more gentle field desorption (FD) or fast-atom bombardment (FAB) techniques. An even more gentle way is provided by the electrospray (ES) method, which allows all ionic species (optionally cationic or anionic) present in solution to be detected. Descriptions of ESMS and its application to selected problems are published 45-47 also a representative application of this method in a study of phosphine-mercury complexes in solution is reported.48... 

Fragmentation of amino acid-derived spirophosphorane 128 has been analyzed using field desorption (FD), El, and Cl mass spectrometry <1997RCM1825, 1997CCL629>. In spiro-crypta cyclophosphazene derivatives 129, the major decomposition pathway involved the initial cleavage of a P-Cl bond rather than cleavage of an exocyclic P-N bond as is normally seen for cyclophosphazenes <2004JST139>. 

The introduction of soft ionization techniques, such as plasma desorption (PD),[1] field desorption (FD)[2] and fast atom bombardment (FAB),[3] marked the beginning of a new era for MS. In fact, they allowed MS to extend its applications to wide classes of nonvolatile, polar, thermally unstable and high molecular weight analytes. This opened up new horizons for MS in many unexpected fields, such as biology, biomedicine and biotechnology, in which this methodology had not previously found any possible application. 

Volatile or volatilizable compounds may be introduced into the spectrometer via a pinhole aperture or molecular leak which allows a steady stream of sample molecules into the ionization area. Non-volatile or thermally labile samples are introduced directly by means of an electrically heated probe inserted through a vacuum lock. Numerous methods of sample ionization are available of which the most important are electron impact (El), chemical ionization (CY), field ionization (FI), field desorption (FD), fast atom bombardment (FAB), and radio-frequency spark discharge. 

Field desorption (FD) is similar in principle to FI. It enables ions to be produced directly from solid samples which are deposited from solution onto an anode fitted to a probe that can be inserted into the instrument via a vacuum lock. It is even more gentle than Cl and FI, producing molecular ions and virtually no fragmentation. However, the ionization process decays very rapidly so spectra must be scanned quickly and cannot be re-recorded without introducing more sample. 

Field desorption FD Desorption/ionization by strong electric field Nonvolatile molecular ions First soft method Large molecules... 

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. 

Lattimer, R.P. Field ionization (Fl-MS) and Field Desorption (FD-MS), in Mass Spectrometry of Polymers, Montaudo, G. Lattimer, R.P., editors CRC Press Boca Raton, 2001 pp. 237-268. 

For compounds that are not thermally stable enough for the direct inlet probe, field desorption (FD) is the next resort. 

FIGURE 2.17. Mass spectra of leucine, (a) Electron impact (El). (b) Chemical ionization (Cl), (c) Field desorption (FD). 

In field desorption (FD), the sample is deposited onto the emitter, a high voltage is applied, and a current is passed through the emitter to heat up the filament. Mass spectra are acquired as the emitter current is gradually increased and the sample is evaporated from the emitter into the gas phase. Tlie analyte... 

Conventional electron impact or chemical ionization mass spectrometry requires that volatilization precede ionization and this is clearly a limiting factor in the analysis of many biochemically significant compounds. A newer ionization technique, field desorption (FD) (1, 2 ) removes this requirement and makes it possible to obtain mass spectrometric information on thermally unstable or non-volatile organic compounds such as glycoconjugates and salts. This development is particularly significant for those concerned with the analysis of glycolipids and we have therefore explored the suitability of field desorption mass spectrometry (FDMS) for this class of compounds. We have evaluated experimental procedures in order to enhance the efficiency of the ionization process and to maximize the information content of spectra obtained by this technique. 

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