Particle bombardment fragmentation

FAB and LSIMS are matrix-mediated desorption techniques that use energetic particle bombardment to simultaneously ionize samples like carotenoids and transfer them to the gas phase for mass spectrometric analysis. Molecular ions and/or protonated molecules are usually abundant and fragmentation is minimal. Tandem mass spectrometry with collision-induced dissociation (CID) may be used to produce abundant structurally significant fragment ions from molecular ion precursors (formed using FAB or any suitable ionization technique) for additional characterization and identification of chlorophylls and their derivatives. Continuous-flow FAB/LSIMS may be interfaced to an HPLC system for high-throughput flow-injection analysis or on-line LC/MS. 

A major thrust of recent work on FTMS of large biomolecules has dealt with questions concerning the lifetime of molecular ions formed by high-energy particle bombardment. Chait and Field have reported that a large fraction of the molecular ions of chlorophyll A formed by 252Cf fission fragment ionization decomposes with lifetimes of less than a few... 

By bombarding a surface consisting of species A with primary ions, the surface coverage of A is reduced. Particles of A can he removed hy desorption, hy driving them into a deeper layer or, for molecular species, hy fragmentation. The ratio of the number of sputtered particles to the number of primary ions is given by the disappearance yield Y (A) ... 

Nuclear Fission The splitting of heavier atom like that of U-235 into a number of fragments of much smaller mass, by suitable bombardment with sub-atomic particles with liberation of huge amount of energy (due to mass defect) is called nuclear fission. For example. 

The FAB matrix is essentially a nonvolatile liquid material, such as those illustrated in Scheme 1, that serves to constantly replenish the surface with new sample as the incident ion beam bombards the surface. The matrix also serves to minimize sample damage from the high-energy particle beam by absorbing most of the incident energy and is believed to facilitate the ionization process. The spectrum produced often includes matrix peaks along with some fragments and a peak for the protonated or cationized (i.e., M + Na+) molecular ion. 

A problem with all mass spectroscopy of large molecules is how to get them into the vapor phase so that they can be ionized and their fragmentation patterns determined. Simple heating may cause excessive degradation and formation of ions not corresponding to the desired substance. Two useful methods that involve only intense short-term local heating of the sample appear to have promise in this connection. One method uses a burst from a powerful infrared laser to volatilize part of the sample, and the other uses bombardment by heavy and energetic particles from fission of californium-252 nuclei to raise the local temperature of the sample to about 10,000°. The latter technique both volatilizes and ionizes the sample molecules. 

Neutral gaseous molecules entering the ionization area are bombarded with electrons to smash the compound of interest to yield positively charged ions. Ionization is often followed by a series of spontaneous competitive decomposition reactions (fragmentation) which produce additional ions. The instrument operates under a high vacuum to prevent absorption of the charged particles by air molecules. 

Mass spectrometry, especially in the form of fast atom bombardment, has become a potent tool in the elucidation of the structure of phospholipids. In this technique, the phospholipid sample in amounts in the range of 15-100 ng (or more), dissolved in chloroform-methanol (1 1, v/v), is mixed with a glycerol or thioglycerol matrix and placed on a sample mount in the spectrometer. A fast atom gun is then aimed, at a 90° angle, at the sample and the resulting ionized particles are directed to a mass analyzer unit. Under these conditions, most of the energy of the beam is limited to the surface of sample matrix. Consequently, the bulk of the sample is unaltered and can be recovered by solvent extraction. In the case of phospholipids, a mass ion, MH+, is produced also, some fragments are useful for structural analysis. A more... 

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