Abundances of Metastable Ions

Metastable ions yield valuable information on fragmentation in mass spectrometry, providing insight into molecular structure. In electron ionization, metastable ions appear naturally along with the much more abundant normal ions. Abundances of metastable ions can be enhanced by collisionally induced decomposition. 

For metastable ions of the same structure, it was suggested that their abundance relative to the precursor ion (C4H4O) could be different depending on their energies the ratio of abundance of metastable ions was therefore measured. The similarity of the relative intensities from 2- and 4-pyrone and the difference of these from those of furan led to the... 

Mass Spectrometry.—Several groups have examined in detail the mass spectra of thiazoles and isothiazoles, of benzothiazoles, and of benzoselenazoles. That thiazole and isothiazole isomerize to a common structure before fragmentation has been shown by an analysis of the abundances of metastable ions and the releases of kinetic energy. ... 

From a study of metastable ion decompositions of [C2(H, D)sS] + ions, the average isotope effect, i, for acetylene loss was reported as 1.6 [137]. Isotope effects on metastable ion decompositions of (C3H7S)+ ions have proved difficult to study, because of hydrogen randomisation and facile isomerisation of ion structures. Nevertheless, the metastable ion abundances for H2S and HDS loss from [CH3(CD3)C = SH]+ have been shown to be in the ratio 2.2 1 [136]. 

In studies of metastable ions [388, 671], H- losses from (CH4)t, (CH3D)t, (CH2D2)t and (CHD3) have been compared and the abundances (corrected for the number of H atoms available) found to be in the respective ratios 1 2.2 5.9 17 [388] and 1 2.2 3.5 11 [671]. The corresponding figures obtained with a tandem magnetic deflection/ICR instrument in which the observation window extended to a few milliseconds (ti ps t2 ms) were 1 2.2 4.4 12 [777]. The isotope effects are... 

Fig. 16. Coraparison of relative abundance of metastable and fragment ions at different electron energies.

This technique also furnishes quantitative possibilities when assaying mixtures. The use of a computer furnishes the values of metastable transitions, as well as the abundance of the ions [117]. 

In studies of metastable ions of acetic acid, the loss of a hydroxyl radical has been used as an internal reference [see Sect. 7.5.3(b)] to justify intermolecular comparisons on loss of water [407, 753]. The ratio of metastable ion abundances, /h,o/ oh. with CH3COOH was... 

Analysis of metastable ion abundancies and shape has demonstrated that indoxazene isomerizes into o-cyanophenol prior to fragmentation in the mass spectrometer.37... 

Lindholm and co-workers [190, 192] obtained some information on the relative abundance of metastable excited states in ion beams during the study of charge-transfer reactions using their perpendicular tandem mass spectrometer. For example, O ion beams produced by electron bombardment of CO, CO2 and N2O react with H2 to produce H2 and H". Since the production of H according to the reaction... 

While their technique is useful in estimating abundances of metastable states of ions, it is obviously subject to several ambiguities and the information obtained must be considered to be rather qualitative. 

It is then satisfying to find that in those few cases where the abundance of metastable states in ion beams has been measured by use of other methods, satisfactory agreement has been obtained. 

Electron impact fragmentation studies on 1,2-benzisoxazoles and benzoxazole indicate that isomerization takes place before degradation. Shape analysis and metastable ion abundances in the mass spectra indicate that isomerization to o-cyanophenols occurred prior to degradation by loss of CO or NCH (75BSB207). 

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