Stable, Metastable, and Unstable Ions

There is no justification for such a classification of ion stabilities outside the mass spectrometer because almost all ions created under the conditions inside a mass spectrometer would spontaneously react in the atmosphere or in solvents. Nevertheless, this classification is useful as far as ions isolated in the gas phase are concerned and is valid independently of the type of mass analyzer or ionization method employed. 

Example The metastable decay of the o-nitrophenol molecular ion, m/z 139, by loss of NO to yield the [M-NO] ion, m/z 109, has been studied on a single-focusing magnetic sector instrument (Fig. 2.11. [45] The mass spectrum shows a 

---

The rate constant ) for unimolecular dissociation defines the resulting El mass spectra [26]. Ions with dissociation rate constants A (E) < 10 s" can reach the detector as molecular ions since the total time in mass spectrometers 10 s. Those with rate constants )> 10 s are unstable ions and fragment in the ion source, while the intermediate situation (10 s >A(E)> 10 s" ) represents metastable ions (see below) that fragment between the ion source and the detector. Figure 2.2 illustrates the relative portions of stable, metastable, and unstable ions in an internal energy distribution curve. 

---