Metastable ions formation

Field Free Zones and the Formation of Metastable Ions... 

By introducing a collision gas into Q2, collision-induced dissociation (CID) can be used to cause more ions to fragment (Figure 33.4). For example, with a pressure of argon in Q2, normal ions (mj ) collide with gas molecules and dissociate to give mj ions. CID increases the yield of fragments compared with natural formation of metastable ions without induced decomposition. 

On the other hand, a metastable-ion peak at m/e 88.1 (calculated, 88.0) is present in the mass spectrum of 11 (Figure 8) for the formation of m/e 129 from m/e 189, by loss of acetic acid. In the mass spectrum of the D20-exchanged analog, m/e 129 partially shifts to m/e 130 and partially stays at m/e 129. Metastable-ion peaks are also present at m/e 154.8 (calculated, 154.7) and m/e 97.3 (calculated, 97.3) for the loss of water from m/e 189 followed by the loss of ketene, to give an ion at m/e 129. Since m/e 171 from the loss of water remains at m/e 171, the loss of water must involve the hydroxyl hydrogens. Scheme 3 is an attempt to summarize this in terms of structures which are entirely... 

How can metastable ions be registered with a classic magnetic sector mass spectrometer (See Chapter 2, Section 2.2.2) Let ion mj+ leave the ion source and after acceleration with accelerating voltage V fragment, with formation of ion m2+ and a neutral particle m3° between the source and magnetic analyzer (first field-free region, 1 FFR). 

Proximity effects and ortho interactions in 2,2/-disubstituted diaryl amines have been reported104. Thus, phenazine, phenazine-N-oxide and carbazole were formed by loss of small neutral fragments, such as NO, NO2 and NO3, from the molecular ions, as illustrated by the formation of carbazole from 2,2/-dinitrodiphenylamine see Scheme 34. Of particular interest is the loss of NO3, as demonstrated by high-resolution MS data and metastable ion spectra104. 

The dominant formation of m/z 78 in the fragmentation of the metastable ion of 2-cyanonitrobenzene has been rationalized by analogy to the HCN elimination from... 

Beynon, J.H. Saunders, R.A. Williams, A.E. Formation of Metastable Ions in Mass Spectrometers With Release of Internal Energy. Z. Naturforsch. 1965, 20A, 180-183. 

As indicated by metastable ion studies of isopropylbenzoic acids, [202] the formation of the ion at m/z 90 should be described as a two-step process, i.e., the product rather is [M-OH-CHO] than [M-HCOOH]. This would add yet another example to the list of violations of the even-electron rule (Chap. 6.1.3). 

McLafferty, F.W. Bockhoff, F.M. ColU-sional Activation and Metastable Ion Characteristics. 67. Formation and Stability of Gaseous Tolyl Ions. Org. Mass Spectrom. 19H9,14, 181-184. 

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). 

Obviously, the various electronically excited states of an atomic or molecular ion vary in their respective radiative lifetime, t. The probability distribution applicable to formation of such states is thus a function of the time that elapses following ionization. Ions in metastable states, which have no allowed transitions to the ground state, are most likely to contribute to ion-neutral interactions observed under any experimental conditions since these states have the longest lifetimes. In addition, the experimental time scale of a particular experiment may favor some states over others. In single-source experiments, short-lived excited states may be of greater relative importance than in ion-beam experiments, in which there is typically a time interval of a few microseconds between ion formation and the collision of that ion with a neutral species, so that most of the short-lived states will have decayed before collision. There are several recent compilations of lifetimes of excited ionic states.lh,20 ,2,... 

G. K Hubler, in S. T. Picraux and W. J. Choyke, eds., Metastable Materials Formation by Ion Implantation, North Holland, Amsterdam, 1982. 

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