MIKE spectra fragments ions

Certain quinazolines, e,g. l-methylquinazolin-4(l//)-one, fragment by a retro-Diels-Alder process. The MIKE (mass analyzed ion kinetic energy) spectrum of the molecular dication of quinazoline is very similar to that of quinoxaline. MIKE spectra of quinazolinethiones show the loss of NCS radicals from the molecular ionA ... 

The fragmentation which is determinant for the identification of Leu or He corresponds to that leading to the m/z 206 ion, which is 20 times more abundant in the MIKE spectrum of the [Leu-Ile] ion than in that of [Ile-Leu]. This is due to the secondary or tertiary character of the hydrogen atom, which migrates during the elimination of the neutral fragment (Fig. 69). 

The importance of these NICI (negative ion chemical ionization) spectra of modified polypeptides led Howe and co-workers [222] to study the spectra of the free compounds. Thus, with OH as reagent ion with a mixture of three tripeptides, in addition to fragment ions, [M-H] molecular ions are produced in sufficient abundance (Fig. 77) to be analyzed by collisions (MIKE/CAD spectrum). 

The superposition of several pure MIKE/CAD spectra on the same spectrum may indeed cloud the issue . Nevertheless, we believe that by changing either collision gas pressure or type, it is possible to obtain sufficient information to resolve the ambiguities. Those which remain are due (relatively rarely) to the partial isomerization of molecular ions and fragment ions which are most often encountered. In the latter case, the examination of the MIKE/CAD spectra of other fragment ions belonging to the conventional mass spectrum may be useful for determining these structures. 

Mass-analyzed ion kinetic energy spectroscopy (MIKES). This scan is implemented with reverse-geometry (BE-type) instruments, in which the front-end magnetic sector allows exclusive mass selection of a precursor ion [37], Fragmentation occurs in a region between the two analyzers (second FFR). A scan of the electric sector [Eq. (4.12)] yields a product-ion spectrum. MIKES also allows direct measmement of kinetic-energy release values. 

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