Molecular ion, in mass spectrometry

The stability of isothiazole derives from the fact that it has an aromatic delocalized ir-electron system. The NMR chemical shifts, which depend, inter alia, on ring currents, and the high stability of the molecular ions in mass spectrometry, are typical of aromatic compounds, and X-ray measurements confirm the partial double bond character of all the bonds of the ring. 

Molecular ion In mass spectrometry, the species formed by loss of an electron from a molecule. 

Shimizu, T, Watanabe-Ezoe, Y., Yamaguchi, S., Tsukatani, H., Imasaka,T, Zaitsu, S., Uchimura,T, Imasaka,T. (2010) Enhancement of molecular ions in mass spectrometry using an ultrashort optical pulse in multiphoton ionization. Anal. Chem.,82,3441-3444. 

Three methods are being used to obtain Ip (a) the spectroscopic determination of the convergence limit of a Rydberg series1 (b) the measurement of the spectral threshold for the appearance of an electron current in the irradiated gas2-4 (c) the direct identification of the positive molecular ion by mass spectrometry with the determination of the spectral threshold of its appearance. 

Varied Methane Cations. The methane molecular ion (methane radical cation, CH4+ ), the parent ion in mass spectrometry, and the methane dication (CH42+) are of great significance and have been studied both experimentally and theoretically.800 802 Recent advanced studies have shown that the methane radical cation, CH4+ has a fivecoordinate planar structure as suggested in early calculations by Olah and Klopman.800... 

Methods for producing ions in mass spectrometry with a chromatographic sample inlet MM = molecular mass... 

The ions in mass spectrometry may be formed in a variety of ways. One method for converting molecules to ions (ionization) in a mass spectrometer is to place a sample under high vacuum and bombard it with a beam of high-energy electrons ( 70 eV, or 6.7 X 10 kj mop ). This method is called electron impact (El) ionization mass spectrometry. The impact of the electron beam dislodges a valence electron from the gas-phase molecules, leaving them with a -El charge and an unshared electron. This species is called the molecular ion (M ). We can represent this process as follows ... 

According to early theoretical calculations Kloptnan and I carried out in 1971, the parent molecular ions of alkanes, such as CH4, observed in mass spectrometry, also prefer a planar hypercarbon structure. 

Fragmentation pattern (Section 13 22) In mass spectrometry the ions produced by dissociation of the molecular ion... 

Molecular ion (Section 13 22) In mass spectrometry the species formed by loss of an electron from a molecule Molecular orbital theory (Section 2 4) Theory of chemical bonding in which electrons are assumed to occupy orbitals in molecules much as they occupy orbitals in atoms The molecular orbitals are descnbed as combinations of the or bitals of all of the atoms that make up the molecule Molecularity (Section 4 8) The number of species that react to gether in the same elementary step of a reaction mechanism... 

The mass of an electron is very small compared with the total mass of the molecule. Consequently, the relative molecular mass of a molecule (M,.) is almost the same as that of the derived molecular ion (M +). For practical purposes in mass spectrometry, = M/+, and is written, M +. 

Metastable ions yield valuable information on fragmentation in mass spectrometry, providing insight into molecular structure. In electron ionization, metastable ions appear naturally along with the much more abundant normal ions. Abundances of metastable ions can be enhanced by collisionally induced decomposition. 

Some of the target molecules gain so much excess internal energy in a short space of time that they lose an electron and become ions. These are the molecular cation-radicals found in mass spectrometry by the direct absorption of radiation. However, these initial ions may react with accompanying neutral molecules, as in chemical ionization, to produce protonated molecules. 

Matrix-assisted laser desorption mass spectrometry (MALDI-MS) is, after electrospray ionization (ESI), the second most commonly used method for ionization of biomolecules in mass spectrometry. Samples are mixed with a UV-absorbing matrix substance and are air-dried on a metal target. Ionization and desorption of intact molecular ions are performed using a UV laser pulse. 

Electrospray ionization, in contrast to the majority of other ionization methods employed in mass spectrometry, produces predominantly multiply charged ions of the intact solute molecule. This effectively extends the mass range of the mass spectrometer and allows the study of molecules with molecular weights well outside its normal range. 

Klemm and coworkers studied the spectra of several thienopyridine sulfones (72-78) and found that it is possible to distinguish by mass spectrometry between a sulfone function and a combination of two sulfoxide or JV-oxide functions in the same molecule . For instance, compound 77 forms 78 losing the 7V-oxygen atom, since except for the molecular ion their mass spectra are very similar. Compound 78 rearranges prior to fragmentation to the two possible cyclic sulfinates (80 and 81), which then fragment further by losing SO and CNO, respectively. 

Enhanced molecular ion implies reduced matrix interference. An SMB-El mass spectrum usually provides information comparable to field ionisation, but fragmentation can be promoted through increase of the electron energy. For many compounds the sensitivity of HSI can be up to 100 times that of El. Aromatics are ionised with a much greater efficiency than saturated compounds. Supersonic molecular beams are used in mass spectrometry in conjunction with GC-MS [44], LC-MS [45] and laser-induced multiphoton ionisation followed by time-of-flight analysis [46]. 

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