Desorption, field

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. 

When placed in a strong electrostatic field (10 —10 V cm ) a molecule can lose an electron and form a positive charged ion. In the earlier technique of field ionization mass spectrometry the sample was first vaporized prior to 

Field desorption spectra are usually simple since the field imparts little excess internal energy to the desorbed ions. The mass spectra often contain only the molecular ion (M ) or protonated molecular ion ([M + 

Which of these species is formed depends on the nature of the sample. Lipophilic or aprotic molecules tend to form M ions whereas [M + H] ions form from polar molecules such as peptides and carbohydrates by intermolecular proton transfer. The intensity of [M + H] ions can often be enhanced in FD as well as in other desorption ionization techniques by treatment with acid. Because of the ambiguity as to whether or [M + H] is formed, precise molecular weight determination of an unknown sample by FD can be a problem. By mixing the sample with a metal salt, for example lithium chloride, [M +cation] ions may be produced, defining the molecular weight unambiguously 18). 

The introduction of field desorption (FD) as a method for the analysis of non-volatile molecules is principally due to Beckey [14], 

Based on FI, already described, FD has been developed as the first method that combines desorption and ionization of the analyte. There is no need for evaporation of the analyte prior to ionization. Consequently, FD is particularly suitable for analysing high-molecular-mass and/or thermally labile compounds. 

The technique is demanding and requires an experienced operator. It has now been largely replaced by other desorption techniques. However, it remains an excellent method to ionize high-molecular-mass non-polar compounds such as polymers. 

Top FAB mass spectrum of a mixture of five peptides. The m/z of the protonated molecular ion (M + H)+ of each of them is observed. Bottom product ion tandem mass spectrum of the (M + H)+ ion with m/z 872, giving the sequence of this peptide alone. The values within the frame are the masses of the various possible fragments for the indicated sequence. 

The two closely related techniques of field desorption [108] and field ionisation [105] are appropriate for 

Ionisation mechanism Technique Ions formed Ionisation mode Type of sample 

FD-MS is a very effective technique for determining molecular weights of thermally labile and nonvolatile compounds, such as polymer additives which do not give good molecular ion spectra during electron impact or chemical ionisation [108], In order to enhance the structural information of the technique, MS/MS approaches must be used [96], Hyphenated chromatography-FD/FT-MS techniques appear to be restricted to on-line GC-MS. 

Although FD was one of the earliest forms of soft ionisation, poor sensitivity and limited applicability have restricted the impact of the approach in the mainstream of mass spectrometry. More recently, many of the application areas of FD and FI have been appropriated by FAB-MS, which is generally considered to be a technique that requires less expertise alternatively, laser desorption is frequently being applied. FD-MS is only used in a handful of laboratories worldwide. The technique has recently been reviewed [107], and is subject of various monographs [108,112], 

Applications Rather intractable samples, such as organic polymers, are well suited to FD, which avoids the need for volatilisation of the sample. Since molecular ions are normally the only prominent ions formed in the FD mode of analysis, FD-MS can be a very powerful tool for the characterisation of polymer chemical mixtures. Application areas in which FD-MS has played a role in the characterisation of polymer chemicals in industry include chemical identification (molecular weight and structure determination) direct detection of components in mixtures off-line identification of LC effluents characterisation of polymer blooms and extracts and identification of polymer chemical degradation products. For many of these applications, the samples to be analysed are very complex 

CONDENSED-PHASE IONIZATION TECHNIQUES IONIZATION OF SOLID-STATE SAMPLES 

As in the case of the FI process (Section 2.6), ionization of the sample molecules in FD also involves the loss of an electron via quantum-mechanical tunneling to produce M+ ions, which after conversion to [M - - H]+ and [M - - Na] types of ions are desorbed from the emitter surface under the influence of a strong electric field. Negative-ion formation requires the capture of an electron by the sample molecule fi om the negatively charged emitter. Because almost no vibrational excitation occurs in the ionized molecule, fragmentation is barely 

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Gomer R 1994 Field emission, field ionization, and field desorption Surf. Sc/299/300 129-52... 

Todd C J and Rhodin T N 1974 Adsorption of single alkali atoms on tungsten using field emission and field desorption Surf. Sc/. 42 109-21... 

The main difference between field ionization (FI) and field desorption ionization (FD) lies in the manner in which the sample is examined. For FI, the substance under investigation is heated in a vacuum so as to volatilize it onto an ionization surface. In FD, the substance to be examined is placed directly onto the surface before ionization is implemented. FI is quite satisfactory for volatile, thermally stable compounds, but FD is needed for nonvolatile and/or thermally labile substances. Therefore, most FI sources are arranged to function also as FD sources, and the technique is known as FI/FD mass spectrometry. 

Practical Considerations of Field lonization/Field Desorption... 

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. 

In field ionization (or field desorption), application of a large electric potential to a surface of high curvature allows a very intense electric field to be generated. Such positive or negative fields lead to electrons being stripped from or added to molecules lying on the surface. The positive or negative molecular ions so produced are mass measured by the mass spectrometer. 

Field desorption. The formation of ions in the gas phase from a material deposited on a solid surface (known as an emitter) that is placed in a high electrical field. Field desorption is an ambiguous term because it implies that the electric field desorbs a material as an ion from some kind of emitter on which the material is deposited. There is growing evidence that some of the ions formed are due to thermal ionization and some to field ionization of material... 

Beckey, H.D., Principles of Field Ionisation and Field Desorption Mass Spectrometry, Pergamon Press, Oxford, 1977. 

Prokai, L., Field Desorption Mass Spectrometry, Marcel Dekker, New York, 1990. 

El = electron ionization. Cl = chemical ionization TSP = thermospray FAB = fast atom bombardment FD = field desorption,... 

Mass Spectrometry. Field desorption mass spectrometry has been used to analy2e PPO (179). Average molecular weight parameters (M and could be determined using either protonated (MH + ) or cation attachment (MNa + ) ions. Good agreement was found between fdms and data supphed by the manufacturer, usually less than 5% difference in all cases up to about 3000 amu. Laser desorption Fourier transform mass spectrometry was used to measure PPG ion and it was claimed that ions up to m/2 9700 (PEG) can be analy2ed by this method (180). 

Alternative ( soft ) ionization techniques are not usually required for aromatic isothiazoles because of the stability of the molecular ions under electron impact. This is not the case for the fully saturated ring systems, which fragment readily. The sultam (25) has no significant molecular ion under electron impact conditions, but using field desorption techniques the M + lY ion. is the base peak (73X3861) and enables the molecular weight to be confirmed. 

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). 

Field desorption mass spectrometry radiopharmacological agents, 6,976 Filter dyes photography, 6, 104 Fissile material criticality... 

The field desorption mass spectra of 154-158 always showed the ions [M+ 1]+, [M + 2] + and [M + 3] + in addition to the molecular ions which were the base peaks. No fragment peaks were seen in the latter spectra. 

Various ionization methods were used to bombard phenol-formaldehyde oligomers in mass spectroscopic analysis. The molecular weights of resole resins were calculated using field desorption mass spectroscopy of acetyl-derivatized samples.74 Phenol acetylation was used to enable quantitative characterization of all molecular fractions by increasing the molecular weights in increments of 42. 

Bob Ardrey obtained a first degree in Chemistry from the University of Surrey where he went on to obtain his doctorate stndying the chemistry of trans-2,3-dichloro-l,4-dioxan and the stereochemistry of its reaction prodncts using primarily mass spectrometry and nnclear magnetic resonance spectroscopy. He then carried ont post-doctoral research at King s College, London, into the development of emitters for field-desorption mass spectrometry. 

Field desorption An ionization method in which sample is deposited on a wire to which a high voltage is applied. 

Evans, N. et al.. Applications of high-pressure liquid chromatogrphy and field desorption mass spectrometry in studies of natural porphyrins and chlorophyll derivatives, J. Ghromatogr, 115, 325, 1975. 

Field desorption mass spectrometry [1606], C nuclear magnetic resonance, and fourier-transform infrared spectroscopy [1337] have been used to characterize oil field chemicals, among them, scale inhibitors. Ion... 

J. Shen and A. S. Al-Saeed. Study of oil field chemicals by combined field desorption/collision-activated dissociation mass spectrometry via linked scan. Ana/y Chem, 62(2) 116-120, 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... 

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