Metastable Ion Dissociation

The success of any tandem mass spectrometric experiment depends on the occurrence of some kind of reaction between the consecutive steps of precursor ion selection and product ion analysis. This demands that ions entering the zone and/or period for reaction either possess or receive sufficient internal energy for doing so alternatively, a partner for reaction may be presented. 

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During the last decade knowledge of the ion chemistry of nitro compounds in the gas phase has increased significantly, partly due to the more widespread use of specialized techniques. Thus various ionization methods, in particular electron impact ionization and chemical ionization, have been used extensively. In addition, structure investigations as well as studies on fragmentation pathways have involved metastable ion dissociations, collision activation and neutralization/reionization studies, supplementary to studies carried out in order to disclose the associated reaction energetics and reaction dynamics. In general, the application of stable isotopes plays a crucial role in the in-depth elucidation of the reaction mechanisms. 

Methods for the detection of metastable ion dissociations in ReTOF-MS in combination with secondary ion mass spectrometry (SIMS) and Cf plasma desorption ( Cf-PD, Chap. 9.5.2) mass spectrometry were known before the advent of MALDI. [56-59]... 

Note In particular the MALDI-TOF community has coined some sort of an own terminology, e.g., in-source decay (ISD) for all fragmentations occurring within the ion source, post-source decay (PSD) instead of metastable ion dissociation and fragment analysis and structural TOF (FAST) for the specific mode of operation of a ReTOF to detect metastable ions. 

As we have conservation of velocity, i.e., vi = V2 = v, the momentum of a fragment ion mf formed in a FFR preceding the magnetic sector is different from that of such a fragment ion arising from the ion source. The ion formed by metastable ion dissociation thus passes the magnet as if it had the virtual mass m ... 

Note This explains the occurence of diffuse peaks due to metastable ion dissociations at fractional m/z values in the B scan spectra of B and EB instruments (Chap. 2.7.1). [83,84] In turn, the mass spectra obtained from BE instruments do not show any metastable ion peaks in normal operation. 

Table 4.2. Common scan laws for the detection of metastable ion dissociations on magnetic sector mass spectrometers ...

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]... 

The simplest model of an amide bond is found in formamide, and several features of protonated formamide are highly relevant to the cleavage of protonated peptides into b and y ions. Amides are bidentate bases, and it has been demonstrated from correlations between core electron energies and proton affinities [213] and from quantum chemical calculations [214] that the carbonyl oxygen is more basic than the amide nitrogen. As demonstrated by FT-ICR, metastable ion dissociation, and RRKM and quantum chemical model calculations [214], the unimolecular dissociation of a protonated formamide molecule depends on which site the proton is attached to ... 

The values of 5 and g depend upon the structure of the complex. If it is assumed that the cluster is made up of a central core consisting of (COj) [i-e., (C02) J+i = (C02)J(CQ2) i] surrounded by loosely held COj units, then any of the loosely held CO2 units can evaporate. Thus it is assumed that the cluster symmetry number is n — 1 while the transition state has a symmetry of 1, so that g = n-1. Because the intramolecular CO2 modes are assumed too high to contribute to the density of states at the low energies involved in the metastable ion dissociation, the total number of vibrational modes is just 5 = 5 n— 1). (With the loss of each linear CO2 unit, the cluster looses five van der Waals modes.) With these assumptions and the measured (Stace and Shukla, 1982) average kinetic energy releases, the binding energies for CO2 monomers... 

The ions observed as a result of unimolecular dissociations (metastable ions) in the mass spectrometer correspond to reactions in the low- Xs time frame ". Due to the duality of the reaction rate and the available energy, only processes with rather low-energy requirements are observed ", which is nicely reflected in the metastable ion spectrum of nitromethane (Figure 1) . Two dominant processes, i.e. the loss of OH (m/z 44) and CH3O (m/z 30), respectively, are observed leading directly to the conclusion that these reactions have critical energies within a few hundredths of meV. Recent literature reviews offer excellent discussions on the metastable ion dissociations. Nevertheless, it appears reasonable to summarize the more important aspects. 

To detect a random or hidden rearrangement, remember that these will have much higher entropy requirements than a simple ion decomposition, and so should be much more competitive at low electron energies or in metastable ion dissociations. A product ion whose abundance increases at low energy or prolonged reaction time could well be formed by rearrangement, even if a mechanistically plausible simple-cleavage pathway can be drawn to account for its formation. 

Mass spectra formed by metastable ion dissociations. 0.001 atomic mass unit a millidalton. 

A few portions of (metastable) ions dissociate in the analyzer. The resulting lighter daughter ions are ejected from the quadrupole. However, the neutrals thus generated continue more or less along the initial trajectory of the parent ion. Some of them may reach the detector. The impacts then recorded are correlated to the U and V values applied to the electrodes of the quadrupole and converted in a peak in the mass spectrum. In reality, these impacts are not correlated to the arrival of ions. 

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