Soft ionization field desorption

This situation is even more true as the molecular (or quasimolecular) ions arise under conditions of soft ionizations (chemical ionization, field ionization, field desorption, FAB, SIMS, etc.). 

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. 

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. 

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. 

Electrospray (ESI) is an atmospheric pressure ionization source in which the sample is ionized at an ambient pressure and then transferred into the MS. It was first developed by John Fenn in the late 1980s [1] and rapidly became one of the most widely used ionization techniques in mass spectrometry due to its high sensitivity and versatility. It is a soft ionization technique for analytes present in solution therefore, it can easily be coupled with separation methods such as LC and capillary electrophoresis (CE). The development of ESI has a wide field of applications, from small polar molecules to high molecular weight compounds such as protein and nucleotides. In 2002, the Nobel Prize was awarded to John Fenn following his studies on electrospray, for the development of soft desorption ionization methods for mass spectrometric analyses of biological macromolecules. ... 

ToF mass spectrometers as dynamic instruments gained popularity with the introduction of matrix assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) as effective pulsed ion sources for the soft ionization of large biomolecules (up to 10s dalton) due to their high ion transmission.38 ToF mass spectrometers, quadrupole analyzers and/or magnetic sector fields can be combined in tandem mass spectrometers (MS/MS) for the analysis of organic compounds. 

Future work will involve the use of soft-ionization mass spec-trometric techniques such as field desorption and fast atom bombardment to try to identify these compounds. 

Analytical pyrolysis is defined as the characterization of a material or a chemical process by the instrumental analysis of its pyrolysis products (Ericsson and Lattimer, 1989). The most important analytical pyrolysis methods widely applied to environmental samples are Curie-point (flash) pyrolysis combined with electron impact (El) ionization gas chromatography/mass spectrometry (Cp Py-GC/MS) and pyrolysis-field ionization mass spectrometry (Py-FIMS). In contrast to the fragmenting El ionization, soft ionization methods, such as field ionization (FI) and field desorption (FD) each in combination with MS, result in the formation of molecule ions either without, or with only very low, fragmentation (Lehmann and Schulten, 1976 Schulten, 1987 Schulten and Leinweber, 1996 Schulten et al., 1998). The molecule ions are potentially similar to the original sample, which makes these methods particularly suitable to the investigation of complex environmental samples of unknown composition. 

Linden, H. B. (2004). Liquid injection field desorption ionization A new tool for soft ionization of samples including air sensitive catalysts and non-polar hydrocarbons. Eur. Mass Spectrom. 10, 459 168. 

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. 

Another important development in the field of biopolymer analysis is the introduction of matrix-assisted laser desorption ionization (MALDl), which is a rather recent soft ionization technique that produces molecular ions of large organic molecules. In combination with time-of-flight (TOP) mass spectrometry, it was proposed as a valuable tool for the detection and characterization of biopolymers, such as proteins, peptides, and oligosaccharides, in many types of samples.The use of these recently developed techniques has not decreased the use of chromatography in determinations of biopolymers. Some efforts on the adaptation of the separation abilities of HPLC to the high potential of MALDl-TOF for the sensitive determination of additives in biocomposites are currently being carried out. 

The resolution of this latter problem passed through the development of the so-called soft ionization methods, in which ions are directly produced from the solid or liquid state. Field ionization (FI) and field desorption (FD) were two of the first alternative ionization methods. A few years later, other techniques were developed. Examples of these include desorption/chemical ionization (D/CI), secondary ion mass spectrometry (SI-MS), and fast atom bombardment (FAB). 

Chemical, electron field desorption, laser desorption, photon, plasma desorption, spark, and thermal ionization are all used as primary ionization processes. Secondary ionization is the term used to describe a process in which ions are ejected from a surface as a result of bombardment by a primary beam of atoms or ions. If low energy or soft ionization techniques are used, the mass of the target molecule can be determined. Advances in soft ionization techniques have extended the use of MS to the direct measurement of peptide and protein mass. Ionization at higher energy results in more extensive fragmentation of target molecules. 

We have summarized in the previous pages the recent developments (prior to 1980 ) of new soft ionization techniques and of various peripherals. These aspects often responded to the need to obtain molecular ions, in order to determine the molecular masses of these often thermo-labile biological molecules, leading to the development of soft ionization methods and to obtain sufficient vaporization (or desorption) of the sample studied. Among other methods, field desorption (and DCI), the use of lasers and radioactive decomposition of Cf have been introduced. 

Polysaccharides are often thermo-labile, even when soft ionization methods such as field desorption are utilized. Pyrolysis effects may occur, leading to a low abundant molecular ion, in spite of the slight energy excess. This generates a spectrum which occasionally is complex. 

An impetus in the field of mass spectrometry (MS) analysis occurred in the early 1990s with the invention of two novel and soft ionization methods, electrospray ionization (ESI) by John Fenn and matrix-assisted laser desorption ionization (MALDI) by Koichi Tanaka, who both shared the Nobel Prize in Chemistry in 2002. A second impetus, which is more diffuse, is currently occurring and consists of miniaturization. Whereas the intrinsic sensitivity of mass spectrometers has roughly remained the same for a couple of decades, the amount of material required for recording one spectrum has... 

Mass spectrometry has proven to be an important tool for quickly and reliably identifying the number of monomeric units of 1 that are incorporated into an oligomer. A variety of ionization techniques have been used on resveratrol oligomers with widely varying results. Standard electron impact techniques have occasionally resulted in failure to observe a molecular ion [100] however, the use of other soft ionization methods has helped circumvent this problem. Some of the more commonly employed techniques that have repeatedly been used with a high degree of success include electrospray [106], field desorption [47,65], and fast atom bombardment [39,42,53,67,68,92,99,103] ionization. 

Saponins are too polar, too thermally unstable and not volatile enough to directly provide ions that are suitable for analysis. Until recently, derivatives (Me or TMS ethers) had to be prepared in order to obtain spectra. In the past ten years however, new techniques of soft ionization have emerged which have proved useful in the analysis of saponins. They are Field Desorption (FD), Fast Atom Bombardment (FAB), Laser Field Desorption (LD) and Californium Plasma Desorption (CPD). 

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