Metastable ion cyclotron resonance

More recently, Audier and McMahon have shown that the unimolecular dissociation spectrum of transient ions can be directly obtained from a simple manipulation of a series of FTICR spectra. The data arising from this approach very closely resemble those obtained from metastable dissociations in conventional sector spectrometers (MIKES), and it has been consequently dubbed metastable ion cyclotron resonance (MICR) spectrometry. Very briefly, the method functions as follows ... 

Figure 16. Metastable ion cyclotron resonance (MICR) spectra for the unimolecular dissociation of the chemically activated adduct ion derived from association of the methoxymethyl cation with pivaldehyde during a 2-s reaction delay at a pressure of pivaldehyde of 1.0 x 10 torr. The three spectra correspond to values of rf amplitude appropriate to eject transient intermediates with lifetimes longer than (a) 60 ps, (b) 80 ps, and (c) 1 70 ps. A partial pressure of CH4 of 1.0 x 10 torr was also present to thermalize ions. The peak at m/z 125 is a secondary reaction product of m/z 85.

In obtaining experimental information about the isomeric forms of ions, a variety of techniques have been used. These include ion cyclotron resonance (ICR),31 flow tube techniques, notably the selected ion flow tube (SIFT),32 and the selected ion flow drift tube (SIFDT)32 (and its simpler variant33), collision induced dissociation (CID),10,11 and the decomposition of metastable ions in mass spectrometers.13 All of these techniques are mentioned in the text of Section in whore they have provided data relevant to the present review. 

The intramolecular isotope effect, IH //D, on metastable ion decomposition of benzene to lose a hydrogen atom has been reported as 1.9 [59]. A tandem magnetic deflection/ion cyclotron resonance (ICR) instrument has been used to study isotope effects on metastable ion decompositions of benzene, toluene and anisole in some detail [779]. 

The kinetic method can be used with mass spectrometers [123, 124] that allow CID electric/magnetic sector (spontaneous unimolecular dissociation of metastable ions in sector instruments may also be exploited), triple quadrupole, quadrupole ion traps, Fourier transform ion cyclotron resonance (FTlCR) and several other mass spectrometers in which different techniques are combined. In early studies, most cation-bound dimers were generated by fast atom bombardment [124], but currently ESI [124] is generally used for their production. The wide accessibility of ESI sources, which may be used for the formation of ions of polar and non-volatile molecules, renders the kinetic method broadly applicable. 

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