Low Abundance of Metastable Ions

Some mild methods of ionization (e.g., chemical ionization. Cl fast-atom bombardment, FAB electrospray, ES) provide molecular or quasi-molecular ions with so little excess of energy that little or no fragmentation takes place. Thus, there are few, if any, normal fragment ions, and metastable ions are virtually nonexistent. Although these mild ionization techniques are ideal for yielding molecular mass information, they are almost useless for providing details of molecular structure, a decided disadvantage. 

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In their paper Derocque and Jochem43 reported the standard 70 eV mass spectra of compounds 57-59, but discussed the main fragmentation routes, summarized in equation 13, on the basis of low-energy mass spectra, metastable ion analysis and exact mass measurements. In the 18eV mass spectra, the dissociation pathways (a) and (b) in equation 13 represented nearly the only decomposition modes of the molecular ions of meta- and para-substituted aryl triflates 57. Their relative abundancies appeared to depend on the Hammett g value of the substituent, the intensity of [ArO] + ions increasing... 

Unfortunately, very often a small fraction of a distinct ion type only is metastable thus, the corresponding product ions are of low abundance. It is quite natural that an enhancement of ion decomposition has been attempted. A possible way has been found in the so-called collisional activation. 

The mass spectrum of Ge(C3H7-i)4 (at 70 eV, depicted as a line diagram) shows the molecular ion in low intensity (1 89) compared with [Ge(C3H7-i)2H]+ as the most abundant species. Most of the major fragments in the fragmentation scheme below are produced by molecular elimination reactions, since the appropriate metastable ions were clearly detected [1] ... 

A line diagram of the mass spectrum (70 eV) is given in [21] in comparison to those of Si(CH2CH=CH2)4 and Sn(CH2CH=CH2)4. The abundance of the molecular ion [M] is extremely low. From the most abundant fragment, [Ge(C3H5)3] +, hexadiene (CeHio) is eliminated (three metastable peaks observed corresponding to the Ge, Ge, and Ge isotopes) [21]. 

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