Metastable fragmentation ionization process

Several desorphon and spray ionization methods can be used to convert S5mthehc polymers into intact molecular or quasimolecular ions (vide supra), whose exact m/z ratio identifies the composition of the polymer. For structural informahon about the polymer, the dissociation behavior or ion-molecule reactions of the polymer ions must be studied. Such reactions, which rarely take place during the soft ionization processes necessary to generate intact gas phase ions from synthetic macromolecules, are most conveniently assessed by tandem mass spectrometry (MS/MS). With MS/MS, a specific precursor ion is mass-selected, so that its reactivity can be investigated without perturbation from the other ions formed upon ionization. The reaction products of this ion are then mass-analyzed and collected in the MS/MS spectrum. MS/MS studies on polymer ions have so far focused on their spontaneous ("metastable") or collision-induced fragmentation. The fragments arising in these reactions are displayed in metastable ion (MI) or collisionally activated dissociation (CAD) spectra, respectively. Customarily, MI spectra acquired with a TOF mass analyzer have been named "postsource decay (PSD)" spectra similarly, CAD is often referred to as CID (collision-induced dissociation). ... 

Example Secondary kinetic isotope effects on the a-cleavage of tertiary amine molecular ions occurred after deuterium labeling both adjacent to and remote from the bond cleaved (Chap. 6.2.5). They reduced the fragmentation rate relative to the nonlabeled chain by factors of 1.08-1.30 per D in case of metastable ion decompositions (Fig. 2.18), but the isotope effect vanished for ion source processes. [78] With the aid of field ionization kinetic measurements the reversal of these kinetic isotope effects for short-lived ions (lO -lO" s) could be demonstrated, i.e., then the deuterated species decomposed slightly faster than their nonlabeled isoto-pomers (Fig. 2.17). [66,76]... 

Most of these processes are very fast. Ionization happens on the low femtosecond timescale, direct bond cleavages require between some picoseconds to several tens of nanoseconds, and rearrangement fragmentations usually proceed in much less than a microsecond (Fig. 5.3 and Chap. 2.7). Finally, some fragment ions may even be formed after the excited species has left the ion source giving rise to metastable ion dissociation (Chap. 2.7). The ion residence time within an electron ionization ion source is about 1 ps. [9]... 

In addition to those ions formed during or soon after primary ion impact, as in the processes just described, other ions arise through subsequent events. Unimolecular reactions of ions, akin to metastable decompositions in magnetic sector mass spectrometry, occur in the free vacuum. The resulting fragment ions have intensities which contradict the notion that SIMS is a "soft" ionization technique, although some fraction of the ion production events can be so characterized. 

CH3 after rearrangement to the enamine leads to m/z 56 fragment ions and cycloreversion leads to ionized vinylamine 34 and ethene. In fact, these decomposition processes have final states with very close standard enthalpies and they are found to compete in the metastable time frame. However, only the formation of 34 is observed in the experimental conditions of FTICR (long life time and deactivation with Ar) which analyses least-energized parent ions43. This shows the existence of an energy barrier in the formation of the m/z 56 ions attributable to the difficult 1,3 H-transfer required by the rearrangement to enamine. 

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