LT-FAB

More recently, studies concerning cluster ion formation from solid or deeply cooled liquid alcohols [120-122] have gained new interest. [123,124] Low-temperature fast atom bombardment (LT-FAB) of frozen aqueous solutions of metal salts provides a source of abundant hydrated metal ions. [125-127] Organic molecules can also be detected from their frozen solutions. [128] Such LT-FAB applications are particularly interesting when enabling the detection of species that would otherwise not be accessible by mass spectrometry, because they are either extremely air- and/or water-sensitive [129,130] as the phosphaoxetane intermediate of the Wittig reaction [131] or insoluble in standard FAB matrices. [106,132]... 

LT-FAB mass spectra are obtained during thawing of the frozen solution in the ion source of the mass spectrometer, thereby allowing to employ almost any solvent as matrix in LT-FAB-MS. Consequently, neither volatility nor unwanted chemical reactions with the matrix restrict the choice of a matrix. Instead, the solvent matrix may be tailored to the analyte s requirements. 

Fig. 9.17. Partial LT-FAB mass spectmm of the reaction mixture containing the iridium complexes 1 and 2 in toluene. In addition to the changes in mass, the isotopic pattern changes upon exchange of Cl by Br. By courtesy of P. Hofmann, University of Heidelberg.

Example Selective activation of C-H bonds is rarely observed in saturated alkyl groups, but the iridium complex 1 does react by C-H insertion of the metal into a ligand bond upon treatment with LiBr in solution. The reaction can be tracked by LT-FAB-MS (Fig. 9.17). A decreasing intensity of the molecular ion of 1, m/z 812.4, and increasing of 2, m/z 856.4, indicate the progress of this reaction. Furthermore, the halogen exchange is indicated by the isotopic pattern. 

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