Plasma desorption applications

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. 

Californium plasma desorption ( Cf-PD) dates back to 1973 [4-6,22,154-156] and was the first method to yield quasimolecular ions of bovine insulin. [157] Practically, Cf-PD served for protein characterization, a field of application which is now almost fully transferred to MALDI or ESI (Chaps. 10,11). [158]... 

Earlier DI techniques include fast atom bombardment (FAB), secondary ionization mass spectrometry (SIMS), plasma desorption, and field desorption. Since their applications are primarily qualitative, they will not be discussed here. 

The application of molecular SIMS as a sensitive ionization source for nonvolatile and thermally labile molecules compares favorably with other new ionization methods in mass spectrometry such as field desorption (FD), Californium-252 plasma desorption (PD), fast heavy ion induced desorption (FHIID), laser desorp-... 

The book is organized into three sections. The first contains the most recent views on fundamental aspects of particle bombardment. Discussions of 252Cf plasma desorption and laser desorption mass spectrometry have been included for comparison. The second section addresses the issues involved in instrument design, covering work on liquid metal and FAB ion guns. The last part presents representative applications of these bombardment methods. 

The general principle of mass spectrometry (MS) is to produce, separate and detect gas phase ions. Traditionally, thermal vaporization methods are used to transfer molecules into the gas phase. The classical methods for ionization are electron impact (El) and chemical ionization (Cl). Most biomolecules, however, undergo severe decomposition and fragmentation under the conditions of both methods. Consequently, the capabilities of mass spectrometry have been limited to molecules the size of dinucleotides [1]. Analysis of oligonucleotides with a mass range of up to 3000 Da became feasible with the development of plasma desorption (PD) methods [2]. However, until the invention of soft ionization techniques such as ESI- and MALDI MS, mass spectrometric tools were not widely considered for routine applications in biological sciences. 

Californium-252 ( Cf) plasma desorption (PD) ionization, introduced in 1974 by Torgerson and co-workers [15], soon found applications for the analysis of large nonvolatile, polar, and thermally labile molecules [16,17]. Because of the pulsed nature of the plasma beam, PD is well adapted to TOF mass spectrometry. The basic concept of PD-TOFMS is illustrated in Figure 2.5. In this technique, the sample is deposited as a solid film on a thin aluminum foil or other suitable... 

Several mass spectrometric techniques including fast atom bombardment (FAB), plasma desorption (PD), matrix-assisted laser desorption/ionization (MALDI), and electrospray (ES) mass spectrometry (MS) are presently available for the analysis of peptides and proteins (Roepstorff and Richter, 1992). Of these techniques, mainly PDMS has gained footing in protein laboratories because the instrumentation is relatively cheap and simple to operate and because, taking advantage of a nitrocellulose matrix, it is compatible with most procedures in protein chemistry (Cotter, 1988 Roepstorff, 1989). Provided that the proper care is taken in the sample preparation procedure most peptides and small proteins (up to 10 kDa) are on a routine basis amenable to analysis by PDMS. Molecular mass information can be obtained with an accuracy of 0.1% or better. Structural information can be gained by application of successive biochemical or chemical procedures to the sample. 

A wide variety of soft ionization methods is available, some of which are extensively used such ESI and MALDI, while some others are only applied for specific applications such as FDI and secondary ion mass spectrometry (SIMS). Some of the older techniques can be considered to be outdated and obsolete such as TSI, Cf plasma desorption ionization (PDI), and FAB, because more powerful and/or more user-friendly alternatives have been developed. The various condensed-phase ionization techniques are briefly discussed in this section. The use of metal-ion cationization in chemical and desorption ionization was already reviewed in 1992 [3]. 

Roepstorff P, Nielsen PF, Klarskov K, Hojrup P. Applications of plasma desorption mass spectrometry in peptide and protein chemistry. Biomed Environ Mass Spectrom. 1988 16 9-18. 

Mass Spectrometry (MS). MS is one of the key techniques used in structure determination of carbohydrates and analyses via electron impact (E.I.) and chemical ionization (C.I.) methods are performed routinely on low molecular weight permethylated or peracetylated carbohydrates. Recently, MS procedures have found wider application in the structure elucidation of less volatile higher molecular weight oligosaccharides as a result of instrumental developments (in particular, desorption methods of ionization, based on fast atom bombardment (FAB) (93), field desorption (FD), laser desorption (LD), plasma desorption (PD), and secondary ion (SI) mass spectrometry) and improvements in derivatization techniques. For example, a series of malto-oligosaccharides, starch and other glycans have been examined with LD FD-MS (94,95) whilst FAB techniques have been employed for studies of cello- and malto-oligosaccharides (96) and branched cyclo-dextrins (97). 

See also Fragmentation in Mass Spectrometry Ion Molecule Reactions in Mass Spectrometry Ion Structures in Mass Spectrometry Ionization Theory Laser Applications in Electronic Spectroscopy Metastable Ions Plasma Desorption Ionization in Mass Spectrometry Quadrupoles, Use of in Mass Spectrometry. 

See also Biochemical Applications of Mass MS-MS and MS Organometallics Studied Using Spectrometry Fragmentation in Mass Spectrometry Mass Spectrometry Peptides and Proteins Studied Ion Energetics in Mass Spectrometry Ionization The- Using Mass Spectrometry Plasma Desorption loniza-ory IR Spectroscopy Sample Preparation Methods tion in Mass Spectrometry Spectroscopy of Ions. 

Sample preparation for the common desorption/ionisation (DI) methods varies greatly. Films of solid inorganic or organic samples may be analysed with DI mass spectrometry, but sample preparation as a solution for LSIMS and FAB is far more common. The sample molecules are dissolved in a low-vapour-pressure liquid solvent - usually glycerol or nitrobenzyl alcohol. Other solvents have also been used for more specialised applications. Key requirements for the solvent matrix are sample solubility, low solvent volatility and muted acid - base or redox reactivity. In FAB and LSIMS, the special art of sample preparation in the selection of a solvent matrix, and then manipulation of the mass spectral data afterwards to minimise its contribution, still predominates. Incident particles in FAB and LSIMS are generated in filament ionisation sources or plasma discharge sources. 

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