The scope of metastable peak observations

Abstract

This short review focuses attention upon the present status of metastable ion studies with emphasis upon the relationship between metastable peak shapes, ion structures and fragmentation mechanisms. Some recommendations are made concerning nomenclature and the reporting of observations on Gaussian-type metastable peaks. Experimental methods for recording relative abundances of metastable peaks are critically appraised. The relationship between metastable ion phenomena and isomerization of gaseous ions is reviewed with particular attention drawn to the effect of rate-determining isomerizations. The shapes of Gaussian-type metastable peaks are discussed in some detail and selected examples from recent studies are used to show that such peaks may, by appropriate experiments, be separated into two Gaussian-type components thus revealing new features of the fragmentation reaction. The magnitude and significance of released kinetic energies, T, are considered and it is stated that few conclusions can be drawn from the evaluation of T alone; the importance of accurate thermochemical data as an aid to understanding and interpreting kinetic energy release data is emphasized. Other topics discussed include composite metastable peaks, metastable peaks produced in chemical ionization and field ionization and the partitioning of internal energy of the fragmenting ion into translational degrees of freedom of the products, for reactions with and without a reverse energy barrier.

Intramolecular condensation reactions in protonated dipeptides: Carbon monoxide, water, and ammonia losses in competition

“…In the low-energy regime, CAD spectra were measured as a function of collision energy (energy-resolved MS 2 spectra). For the fragments in the MI spectra of [M ϩ H] ϩ measured at high kinetic energy, the accompanying kinetic energy releases were calculated using fragment peak widths at half height (T 0.5 ) [31]; the quoted T 0.5 values were corrected for the main beam width using established procedures [32,33]. Approximately 100 -200 scans were summed per MI, CAD, or MS 3 experiment, depending on the intensity of the main beam.…”

“…Table 1 lists the MI spectra of the [M ϩ H] ϩ ions from the dipeptides (XxxYyy) studied. Metastable ions have a narrow range of usually low internal energies [31][32][33], which leads to the generation of relatively few fragments. At keV kinetic energies, metastable fragmentations take place within microseconds.…”

“…The kinetic energy releases measured lie in the 0.3-0.6 eV range, which is indicative of dissociation with a substantial reverse barrier, i.e., over a transition state that lies higher in energy than the products. Tight rearrangements are usually associated with such characteristics [31][32][33].…”

Generation and characterization of ionic and neutral P(OH)2+/. in the gas phase by tandem mass spectrometry and computational chemistry

“…Accurate mass measurements of the ion at m/z 65 from I-III were obtained at a resolution of m/⌬m¢7000 (10 % valley definition), using the data system. Kinetic energy release values (eV) in metastable-ion dissociations forming m/z 47 from precursors I-III were measured from peak widths at half maximum and corrected for the main beam peak width [33,34]. The collision induced dissociation mass spectra (CID) were recorded by mass selecting the beam of m/z 65 ions using E 1 B (MS-1), with 7 keV translational energy and allowing collisions with oxygen in the collision cell Cls-3; the resulting ions were analyzed by scanning E 2 (MS-2).…”

Half protons or doubly charged protons? The history of metastable ions

“…The observation that metastable peaks were broader than those of the primary ions formed in the ion source was explained almost from the beginning by the release of internal energy during fragmentation. As the width of the peaks can be correlated with the amount of energy given off [31], information about the reverse activation energy for a fragmentation process or for inter-charge distances in doubly charged ions can be gained-an area of research still of interest [32][33][34].…”