Ionization methods soft’ techniques

In the past several years, a number of new ionization methods in mass spectrometry have been introduced. These new techniques have extended mass spectrometric analysis to a wide variety of labile (thermally unstable), highly polar, and higher molecular weight materials. Field ionization (FI) and field desorption (FD) are two of the pioneering techniques in this list of alternative ionization methods. FI-MS, which was introduced for organic molecules in 1954, was the first soft ionization method. (Soft ionization refers to processes that produce high relative abundances of molecular, or quasimolecular, ions.) FD-MS, which was invented in 1969, was the first desorption/ionization method. (Desorption/ionization refers to processes in which die vaporization/ desorption, and ionization steps occur essentially simultaneously.)... 

Soft ionization methods can be classified into different groups (a) those occurring in the gas phase (b) spray ionization techniques (c) desorption ionization techniques. 

ESI has become the most commonly used interface for LC/MS. It was recognized by John Fenn and co-workers as an important interface for LC/MS immediately after they developed it as an ionization technique for MS. ESI transforms ions in solution to ions in the gas phase and may be used to analyze any polar molecule that makes a preformed ion in solution. The technique has facilitated the ionization of heat-labile compounds and high-molecular-weight molecules such as proteins and peptides. ESI is a continuous ionization method that is particularly suitable for use as an interface with FiPLC. It is the most widely accepted soft-ionization technique for the determination of molecular weights of a wide variety of analytes and, has made a significant impact on drug discovery and development since the late 1980s. 

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. ... 

The fast atom bombardment ionization (FAB) technique is a soft ionization method, typically requiring the use of a direct insertion probe for sample introduction in which a high energy beam of Xe atoms, Cs+ ions, or massive glycerol-NH4+ clusters sputter the sample and matrix from the probe surface (Figure 8). 

A considerable amount of information has been accumulated during the review period with respect to fragmentation studies of flavonoid aglycones and their glycosides using ionization techniques such as El and CID (Figure 2.17). Tandem mass spectrometry with soft ionization methods such as FAB, ESI, and APCI have been used for the structural characterization of a variety of flavonoids, and both deprotonation ° ° and... 

We will now concentrate upon some soft ionization methods — so called because they give rise mainly to the peak associated with the molecular ion and very little fragmentation. Another benefit of these techniques is that, unlike both EI and CI, they do not require the sample to be volatile and so permit the analysis of biomolecules, which are generally large, sensitive and polar. 

The Laser Desorption Ionization (LDI) was investigated by Franz Hillenkamp and Michael Karas [2, 3], LDI involved sample bombardment with short and intense pulses from a laser light to effect both desorption and ionization of the analyte molecules. It has become a soft desorption ionization method for mass spectrometric analyses of biological macromolecules and small molecular compounds. The MALDI technique was improved by Koichi Tanaka. 

The mass spectra of GeH4 and SnTLj obtained by field ionization15 are simpler with the EH3+ fragments as the base peaks. Since this is a soft ionization method, fragmentation is considerably reduced. For example, no metal cation fragment is observed and the abundance of the EH+ is also considerably less than that observed by electron ionization. This technique also allows for the observation of Gel I5+ and SnI I5+, a fact that is rare and has prevented the characterization of the proton affinity of these substrates (see below). 

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. 

Field ionization (FI) is a method that uses very strong electric fields to produce ions from gas-phase molecules. Its use as a soft ionization method in organic mass spectrometry is principally due to Beckey [8], Like El or Cl, FI is only suitable for gas-phase ionization. Therefore, the sample is introduced into the FI source by the same techniques that are commonly used in El and Cl sources, for example using a direct probe that can be heated or the eluent from a gas chromatograph. 

Because the soft ionization methods used for oligosaccharides produce few fragments, collision-induced dissociation (CID) or post-source decay (PSD) must be used for structural study. These two techniques have been applied to deprotonated, protonated or alkaliated... 

A possible solution to the above problems would be the triple-dimensional analysis by using GC x GC coupled to TOFMS. Mass spectrometric techniques improve component identification and sensitivity, especially for the limited spectral fragmentation produced by soft ionization methods, such as chemical ionization (Cl) and field ionization (FI). The use of MS to provide a unique identity for overlapping components in the chromatogram makes identification much easier. Thus MS is the most recognized spectroscopic tool for identification of GC X GC-separated components. However, quadru-pole conventional mass spectrometers are unable to reach the resolution levels required for such separations. Only TOFMS possess the necessary speed of spectral acquisition to give more than 50 spectra/sec. This area of recent development is one of the most important and promising methods to improve the analysis of essential oil components. 

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). 

As long as one deals with small clusters, beam analysis is possible by combining spectroscopy with expansion modeling. It is possible to use, for example, the soft ionization methods to obtain a better idea of the relative concentration of different clusters in the beam. Soft ionization can be achieved either by direct photoionization or by applying the multiphoton ionization methods (see Cheshnovsky and Leutwyler as a recent example). This technique does not solve completely the fragmentation problem, since if the positively charged species formed is not stable, it will fall apart. However, combining it with sp>ectroscopy, satisfactory results can be obtained. 

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