Soft-ionization technologies

The state-of-the-art biological mass spectrometric soft-ionization technologies (matrix-assisted laser desorption/ionization (MALDI) electrospray ionization (ESI)) facilitate the routine characterization of proteins. Those two ionization methods are integrated with several differention analyzers to form a variety of mass spectrometers. 

Currently, high-performance liquid chromatography (HPLC) combined with atmospheric pressure ionization (API) triple-quadrupole mass spectrometry (MS) is the predominate quantitative technique used in modem pharmaceutical bioanalysis. The key technological achievement in API-MS was the efficient ionization in a liquid stream and transference of ions from atmosphere to vacuum. Of the API approaches developed, electrospray ionization (ESI) is the most commonly used. ESI provides an efficient means of soft ionization amenable to most molecules encountered in a dmg discovery setting. An alternative soft ionization approach is the use of desorption ionization (DI) techniques. The major distinguishing feature of DI techniques is that ions are typically produced from dried samples. 

Mass spectrometry is an emerging technology and now a core resource in all areas of viral research [1,2]. It is becoming a fundamental tool for work in areas such as protein characterization [3-6], structural virology [7-12], drug discovery [12,13], and clinical chemistry [14]. Most of the increased interest in this technique has been due to the development of matrix-assisted laser desorp-tion/ionization [15] (MALDI) and electrospray ionization (ESI) [16], which are highly sensitive and soft ionization techniques. 

B. Crathome et al. Organic compounds in water analysis using coupled-column high perfoimirroe liquid chromatography and soft ionization mass spectrometry. Environmental Science and Technology 1984. 

On the other hand, the studies of mixtures necessitating the use of soft ionization techniques, such as SIMS [271], PDMS [272] and LD [273], are possible. Indeed, they can be coupled with liquid chromatography. Thus, quantification should be one of the objectives of these new technologies in the future [274]. However, the search for zero [275] is moving, and in particular the sensitivities obtained with the negative ions are already promising. 

Recent advances of mass spectrometry and chromatography, and their hyphenated technologies in lipidomics analyses have been reviewed by Li and coworkers [38]. Soft ionization techniques such hquid secondary ion mass spectrometry (LSI-MS) [39] and FAB [40] allowed the mass spectrometric analyses of polar thermally labile molecules of masses up to a few thousand daltons. Later, MALDI [41] and electrospray ionization (ESI) [42] permitted the direct analysis of native biomolecules within the megadalton masses without derivatization. 

Matrix-assisted laser desorption/ionization (MALDI) is one of the two soft ionization techniques besides electrospray ionization (ESI) which allow for the sensitive detection of large, nonvolatile and labile molecules by mass spectrometry. Over the past 27 years, MALDI has developed into an indispensable tool in analytical chemistry, and in analytical biochemistry in particular. In this chapter, the reader will be introduced to the technology as it stands now, and some of the underlying physical and chemical mechanisms as far as they have been investigated and clarified to date will be discussed. 

The mass spectrometry (MS) of nudeic acids (NAs) has a history similar to that of proteins and other biomolecules. Although earlier work had been made possible by field desorption and fast atom bombardment (FAB) and other ionization techniques, the field did not really take off until the soft ionization techniques of electrospray ionization (ESI) [1] and MALDI [2] became available. Even with the benefits of these new ionization techniques, however, the analysis of NAs turned out to be substantially more difficult than that of proteins and, as a result, MS-based technologies are not nearly as prominent in genetics and genomics as they are in proteomics. The widespread utilization of MS for NA analysis has been further limited by the competition of powerful techniques that can rely on amplification, hybridization, and fluorescence detection. MALDI-MS has found its place in these fields only more recently, and its performance must consistently be measured against that of the competing techniques in any given analytical task. 

The marketed spectra databases are sold by institutes, constructors of mass spectrometers, editors on CD-ROM, or by download. Publicity put aside, the two most well known are, by far, the NIST database (from the American National Institute of Standard and Technology) and the WILEY database (marketed by the famous scientific editor). These are databases that index mass spectra recorded in electron ionization at 70 eV. Generally speaking, the few marketed databases devoted to chemical ionization are not very reliable. With this soft ionization mode, the characteristics of spectra depend greatly on conditions of tanperature and pressure withiu the source. For that reason, spectral reproducibility in Cl from one mass spectrometer to another is generally mediocre. 

Despite the first prediction [34] of a measurable PECD effect being a few decades old, it is only in the last few years that experimental investigations have commenced. Practical experiments have needed to await advances in experimental technology, and improvements in suitable sources of circularly polarized radiation in the vacuum ultraviolet (VUV) and soft X-ray (SXR) regions needed for single-photon ionization have been been key here. In the meantime, developments in other areas, principally detectors, also contribute to what can now be accomplished. 

In the late 1980s, two technology breakthroughs in soft desorption/ionization of large biomolecules occurred with the introduction of electrospray ionization and laser desorption... 

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