Field desorption methods

To obtain a molecular ion, mild ionisation techniques are employed, i.e. electron impact (E. I.) with low ionization energy, chemical ionisation (C.I.) or field desorption methods (F.D.). The manner in which the various physical methods have been employed to elucidate the structure of the most important aminoglycoside antibiotics... 

Another technique for thermically labile substances is the field-desorption method. 

Field desorption methods are based on the same mechanism as that of the field ionization ones, but in this case, the sample is deposited directly on the surface carrying the field. The method has been successfully applied to polymers with masses up to 10,000 g mol" [37]. 

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). 

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. 

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

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... 

The term desorption ionization indicates those ionization techniques in which the production of ions is based on a desorption process. This consists of the rapid addition of energy to a sample in a condensed phase (i.e. liquid or solid) with subsequent production and emission of stable ions in the gas phase. These are generally even electron species that fragment only to a limited extent. The development of desorption methods has amplified the impact and utility of MS in a lot of fields, such as biology, biochemistry and proteomics. 

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 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. 

A study of a series of C-nitroso compounds, including monomers as well as dimers, by field desorption has demonstrated the superiority of this technique to this class of compounds118. All the compounds display intense molecular ions118. The method has a significant potential for studies of the equilibrium between mixed and pure C-nitroso compounds, since the amount of pure and mixed dimers present in a solution apparently can be visualized by the relative abundances of the respective molecular ions see Scheme 43. Determination of the concentrations versus time may resolve the kinetics of the dimer formation118. 

The ability to recover monolayers and subject them to meaningful analysis has become practical only in recent years because of the development of new methods of trace analysis. High-performance liquid chromatography and vapor phase chromatography allow separation and identification of such small quantities (54a). Attenuated total reflectance techniques for infrared analysis (56) and field desorption mass spectrometry (68) have been applied to the trans-... 

Very large branched alkanes, such as 24,24-diethyl-19,29-dioctadecylhepta-tetracontane, CgvHng, for example, pose difficulties to obtaining useful mass spectra and even 15 eV El does not anymore allow for the detection of their molecular ions. [80] Beyond C40 alkanes, especially in case of mixtures such as hydrocarbon waxes or polyethylenes of low molecular weight, field desorption and matrix-assisted laser desorption/ionization are the ionization methods of choice (Chaps. 8, 10). 

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]... 

Olson, K.L. Rinehart, K.L. Field Desorption, Field ionization, and Chemical ionization Mass Spectrometry. Methods Car-bohyd. Chem. 1993, 9, 143-164. 

The purpose of the MS techniques is to detect charged molecular ions and fragments separated according to their molecular masses. Most flavonoid glycosides are polar, nonvolatile, and often thermally labile. Conventional MS ionization methods like electron impact (El) and chemical ionization (Cl) have not been suitable for MS analyses of these compounds because they require the flavonoid to be in the gas phase for ionization. To increase volatility, derivatization of the flavonoids may be performed. However, derivatization often leads to difficulties with respect to interpretation of the fragmentation patterns. Analysis of flavonoid glycosides without derivatization became possible with the introduction of desorption ionization techniques. Field desorption, which was the first technique employed for the direct analysis of polar flavonoid glycosides, has provided molecular mass data and little structural information. The technique has, however, been described as notorious for the transient... 

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