Critical re-evaluation of FABMS analysis of ligand–cation interactions

Whilst the use of FABMS in the analysis of ligand-cation interactions has become widespread, there are a number of experimental variables associated with the use of this technique, the effects of which have not previously been addressed. The interactions of 12-crown-4, 15-crown-5 and 18-crown-6 with the alkali metal cations reveal two important observations: (i) that the matrix employed for the FAB studies has a dramatic effect on the observed binding behaviour and (ii) that the observed relative intensities of protonated ions ([M + H]+) and metallated ions ([M + Metal]+) differ enormously. Determination of the ratios of these signals would allow quantitative arguments to be applied to experiments of this type. Accordingly, previous reports of the use of FABMS for the quantitative study of ligand–cation interactions must be viewed with caution.