Metastable abundances

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. 

Rather than looking at just the low-abundance metastable ion processes occurring in the second quadrupole, extra fragmentation can be induced by having a neutral collision gas present in this quadmpole. 

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). 

In Table II the state of the ion and the recombination energies in electron volts (computed from (65)) are given. Some very uncertain information is included in the right hand column as to the relative abundances of the metastable states of the ions when produced by electron impact with 100-e.v. electrons from the indicated compounds. 

A second successful prediction is that many so-called metastable species (i.e. isomers) are abundant even if they are quite reactive in the laboratory.66 Perhaps the simplest interstellar molecule in this class is HNC, but large numbers of others can be seen in Table 1. It is assumed that most metastable species are formed in dissociative recombination reactions along with their stable counterparts at approximately equal rates, and that both are destroyed by ion-molecule reactions so that the laboratory reactivity, which is normally determined by reactions with neutral species, is irrelevant. Both HCN and HNC, for example, are thought to derive from the dissociative recombination reaction involving a linear precursor ion ... 

Various instruments allow working in special regimes to detect only metastable ions (MI spectra). The conditions of experiments in this case are the same as for the MS/MS experiments, but without collision activation. Any sort of spectrum (daughter ions, parent ions, constant neutral losses) may be generated this way. These spectra are used to establish the pathways of fragmentation, to resolve structural problems. However, the abundance of the metastable signals and even their presence or absence in the spectrum depends on the energy of the parent ions. Therefore, in contrast to CID (see Chapter 3) spectra the difference in MI spectra of two parent ions does not confirm their different structures. 

Let us consider the dissolution-precipitation process in seawater in the following example. The normal concentrations of calcium and of carbonate in the near-surface oceanic waters are about [Ca2+] = 0.01 and [C032-] 2 x lO"4 M. The CaC03 in solution is metastable and roughly 2U0% saturated (1). Should precipitation occur due to an abundance of nuclei, TC032-] will drop to 10-4 M but [Ca2+] will change by no more than 2%. Therefore, the ionic strength of the ionic medium seawater will remain essentially constant at 0.7 M. The major ion composition will also remain constant. We shall see later what the implications are for equilibrium constants. 

It is generally assumed that similar product ion abundances indicate that the metastable ions exhibit identical structures. It should, however, be emphasized that the abundance ratios can be highly sensitive to variations in internal energies. Hence, based on variation in the product ion abundance ratios only, it cannot unambiguously be concluded that the parent ion structures are different3-5. 

The electron impact mass spectra of series of 2-aryl-3-nitro-2//-chromenes and 2-aryl-3-nitrochromanes with varying functionalities have been reported101. The molecular ion is always of considerable intensity. Loss of NO2 from M+ is responsible for the base peak in the nitrochromenes101. In contrast, the [M — HONO]+ is apparently the more abundant ion in the nitrochromanes. This unusual loss of HONO is always observed in the metastable time-frame101. 

Isotope Abundances Negative Ions Synthetic Models V. High-Resolution Studies VI. Metastable-Ion Techniques VII. Coupled Gas Chromatography and Mass Spectrometry Inorganic and Organometallic Compounds VIII. Conclusion. ... 

A mass spectral study of 2-methyl-, 3-methyl- and 2,3-dimethyl-chromone (136), (137) and (138) has been reported (790MS345). In each case the molecular ion appears as the base peak, together with ions which correspond to [M-CO]-, [M-CHO]t, [RDA]t, [RDA + H]+ and [RDA-CO]t. Metastable peaks confirmed that the formation of [M-CHO]- occurs in two steps from [M]t. The reaction pathway for (136) and (138) is given in equation (2). In compound (137), [M-CHO]t is an abundant fragment ion (60%, cf. 35% for 136). That its generation occurs by more than one route is suggested not only by its high abundance, but also by the appearance of appropriate metastable ion peaks two pathways are operative (Scheme 17). 

---