Cations, in mass spectrometry

Electron impact ionization (commonly used for the production of radical cations in mass spectrometry). 

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. 

Electron ionization (earlier called electron impact) (see Chapter 2, Section 2.1.6) occupies a special position among ionization techniques. Historically it was the first method of ionization in mass spectrometry. Moreover it remains the most popular in mass spectrometry of organic compounds (not bioorganic). The main advantages of electron ionization are reliability and versatility. Besides that the existing computer libraries of mass spectra (Wiley/NIST, 2008) consist of electron ionization spectra. The fragmentation mles were also developed for the initial formation of a radical-cation as a result of electron ionization. 

By analogy with their behavior in mass spectrometry, branched hydrocarbons are cleaved when oxidized in CH3 CN/TEABF4 at —45 °C. The resulting acetamides of the fragments (Table 6) are formed by cleavage of the initial radical cation at the C,C bond between the secondary and tertiary C atom, to afford after a second electron transfer, carbocations, which react in a Ritter reaction with acetonitrile [29]. 

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... 

Ionization Methods/Processes. The recent development of several new ionization methods in mass spectrometry has significantly improved the capability for the analysis of nonvolatile and thermally labile molecules [18-23]. Several of these methods (e.g., field desorption (FD), Californiun-252 plasma desorption (PD), fast heavy ion induced desorption (FHIID), laser-desorption (LD), SIMS, and fast atom bombardment (FAB) or liquid SIMS) desorb and ionize molecules directly from the solid state, thereby reducing the chance of thermal degradation. Although these methods employ fundamentally different excitation sources, similarities in their mass spectra, such as, the appearance of protonated, deprotonated, and/or cationized molecular ions, suggest a related ionization process. 

The. V-pixvl function, when substituted with electron-donating groups, has also proved useful since the stabilized cations derived from this function are readily detected by mass spectrometry at the femtomolar level by either laser desorption ionization or MALDI techniques <2005EPP1506959, 2005SL2453>. The readily available sulfoxide derivatives also function as efficient mass-tags in mass spectrometry applications <2005CC3466>. 

Hydrogen and chlorine atoms have an odd number of electrons and are radicals. The methyl radical is the simplest organic radical. It has one more electron than a carbocat-ion and one fewer than a carbanion. The last example is a radical cation, which results from the loss of one electron from a normal molecule. Radical cations are important in mass spectrometry (see Chapter 15). 

Vinyl cations and radical cations can be generated from the corresponding alkenes (equation 30). The appearance potentials and fates of the species thus generated are more of interest in mass spectrometry than in carbonium ion chemistry, and have recently been reviewed by Loudon and Maccoll (1970). 

Some such reactions have a simpler rate expression rate = [R1 COC1J in which the alcohol does not appear at all. Evidently, no collision between the acid chloride and the alcohol is required for this reaction to go. What actually happens is that the acid chloride decomposes by itself to give a reactive cation (a cation you have already seen in mass spectrometry) with the loss of the good leaving group Cl-. 

The absence of compounds that contain the simple H+ ion has been discussed here. However, the solvated species are not the only species known that contain H+ in some form. A series of cations have been identified in mass spectrometry that result from the attachment of a hydrogen molecule to H+. The simplest of these is the H3+ ion that has a trigonal planar structure. The electrostatic interaction of a proton with H2 occurs at the shared pair of electrons that can be shown as follows ... 

Charged radical cations and anions are often indicated by the symbol, formula, or structure with a superscript dot followed by a plus or minus sign. However, in mass spectrometry, the reverse is used. Therefore, use the order of dots and signs for charges that is appropriate for the context. 

In mass spectrometry, the molecular ion is a cation radical. Further fragmentations of the molecular ion can be of two types - those that produce a cation plus a radical, and those that produce a different cation radical plus a neutral atom. In all cases, the fragment bearing the charge - whether cation or cation radical - is the one that is detected. 

Recent advances in mass spectrometry have rendered it an attractive and versatile tool in industrial and academic research laboratories. As a part of this rapid growth, a considerable body of hterature has been devoted to the apph-cation of mass spectrometry in clinical studies. In concert with separation techniques such as hquid chromatography, mass spectrometry allows the rapid characterization and quantitative determination of a large array of molecules in complex mixtures. Herein, we present an overview of the above techniques accompanied with several examples of the use of liquid chromatography-tandem mass spectrometry in pharmacokinetics/drug metabohsm assessment during drug development. 

Oxidative cleavage may begin with a loss of an electron from a heteroatom or an anion. Removal of an electron creates a radical cation in the absence of a suitable nucleophile or the possibility of losing a proton, the system may be stabilized by bond cleavage and formation of a double bond in a way similar to that found in mass spectrometry [Eqs. (14) through (17)]. Sulfur is more easily oxidized that nitrogen, which loses an electron more easily than oxygen. 

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