Mass spectrometry soft-ionization methods

Low ionizing potentials or soft ionization methods are necessary to observe the parent ions in the mass spectra of many S-N compounds because of their facile thermal decomposition. Mass spectrometry has been used to investigate the thermal breakdown of S4N4 in connection with the formation of the polymer (SN). On the basis of the appearance potentials of various S Ny fragments, two important steps were identified ... 

Chapter 12, Structure Determination Mass Spectrometiy and Infrared Spectroscopy—A new Section 12.4 discusses mass spectrometry of biological molecules, focusing on time-of-flight instruments and soft ionization methods such as MAI.DI. 

In soft ionization methods the excess energy deposited onto the ionized molecule is very small and stable even-electron ions are formed. This leads to easy determination of the molecular weight of the analyte, but as fragmentation is absent or it occurs to a very low extent, structural information is missing in the mass spectrum. However, one can obtain structural information by causing ion fragmentation out of the source by means of tandem mass spectrometry experiments (see below). 

Stereochemical aspects in mass spectrometry have aroused more and more interest. El mass spectra of stereoisomers are practically indistinguishable. However, the use of soft ionization methods (chemical ionization, field ionization, etc.) accompanied by tandem mass spectrometry allows important and reliable conclusions on the molecular structures to be drawn. 

The development of soft ionization methods (electrospray ionization and matrix-assisted laser desorption ionization, and others not discussed here) has contributed to the remarkable progress seen in mass spectrometry applied to biochemistry and molecular biology research progress, and is beginning to find applications in archaeology. 

Nonetheless, during the first decades of analytical mass spectrometry low energy El spectra have been the only way to minimize fragmentation, and thereby to increase the relative intensity of a weak molecular ion peak. Nowadays, El mass spectra are preferably complemented with spectra obtained from so-called soft ionization methods (Chaps. 7-11). 

In El mass spectrometry, the molecular ion peak can be increased to a certain degree by application of reduced electron energy and lower ion source temperature (Chap. 5.1.5). However, there are compounds that thermally decompose prior to evaporation or where a stable molecular ion does not exist. The use of soft ionization methods is often the best way to cope with these problems. 

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... 

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. 

Peptides. One of the applications that has benefited most from the development of soft ionization methods for mass spectrometry has been the analysis of polar biological materials. Peptides, in particular, have been extensively studied by a variety of soft ionization methods, primarily FAB, with excellent results being obtained for peptides in the 1000-10000 dalton range. LD-FTMS also has proved to be very successful for analysis of simple peptides and an examination of these results should help delineate the analytical potential of LD-FTMS in this important area. 

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. 

Mass spectrometry and particularly ESI can be used for the study of the non-covalent interactions of proteins. Indeed, ESI is a sufficiently soft ionization method to allow, under appropriate circumstances, the observation of non-covalent complexes of proteins formed in solution and transferred in the gas phase [122-127]. 

The molecular masses of the complex compounds are conveniently determined by soft-ionization methods, such as fast atom bombardment mass spectrometry (FAB-MS), in the positive and/or negative mode. This method can easily be applied to the ionization of non-volatile, non-derivatized, polar compounds, and is particularly suited to the analysis of saponins. Extremely valuable is the use of high resolution FAB-MS (HRFAB-MS), which provides the exact mass of high molecular weight compounds, and consequently their molecular formulae. 

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). 

Mass spectrometry is widely used for the qualitative analysis of unknowns samples, and in particular, for the identification and characterization of biological macromolecules. Recent decades have seen the introduction and optimization of the so-called soft ionization methods that provide intact, vapor-phase biomolecular ions for separation and detection. This chapter considers MS fundamentals, ionization methods, and applications to biological macromolecules. Conventional mass spectrometers used for low volatile molecular weight samples that are introduced in the vapor phase are called single-focusing mass spectrometers, and use an electron-impact ion source.1 Figure 15.1 shows a diagram of this type of instrument. 

Many researchers still consider mass spectrometry [1] to be a rather destructive method. The soft ionization methods - despite the revolution they caused in the life sciences - are often not appropriately highlighted in lecture courses on mass spectrometry, so prejudice has it that mass spectrometry almost unavoidably causes fragmentation even of covalent bonds. Consequently, the examination of noncovalent complexes is too often considered to be futile and successful only in some very special cases. However, the soft ionizaton methods developed in the 1980s reduce fragmentation to a minimum and even noncovalent, weakly-bound complexes can be ionized without complete destruction. Technically, the problem of intact ionization of weakly-bound complexes can be solved in many cases [2]. 

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... 

Much of the success of these so-called soft ionization methods results from the low levels of vibronic excitation imparted to the molecules during the ionization process. Thus molecular ions are less likely to break down and they are mostly preserved intact, even for labile and potentially unstable compounds. Structural information can be obtained by collision-induced dissociation (CID) of these preponderant molecular ions in a tandem mass spectrometer, which can lead directly to the sequence of amino acids in a peptide or the sequence and branching of sugar units in an oligosaccharide. This technique is also referred to as mass spectrometry/mass spectrometry (MS/MS). Today mass spectrometry is capable of determining the amino acid sequence and posttranslational modifications of virtually any protein that can be purified in sulficient quantity. 

Nearly all known ionization methods of mass spectrometry (including electron impact, laser desorption and fast atom bombardment) were already successfully applied to lipids. However, many ionization techniques are not very suitable for the analysis of complex PL mixtures as they provide considerable amounts of fragment ions. Therefore, only three soft-ionization methods play nowadays a major role in lipid analysis. Beside atmospheric pressure chemical ionization (APCI) (Byrdwell 2001), electrospray ionization (ESI) (Pulfer and Murphy... 

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. 

The exact m/z value of the molecular or quasimolecular ion reveals the ion s elemental composition and, thus, allows for the compositional analysis of the sample under study. If the molecular ions are unstable and decompose completely, the resulting fragmentation patterns can be used as a fingerprint for the identification of the sample. Fragment ions also provide important information about the primary structure (i.e., coimectivity or sequence) of the sample molecules.With soft ionization methods that produce little or no fragments, fragmentation can be induced by employing tandem mass spectrometry (MS/MS). ... 

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.)... 

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