DETERMINATION OF THE RELATIVE STABILITIES OF METAL-ANTIBIOTIC AND METAL-CRYPTAND COMPLEXES BY ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY

Electrospray ionisation mass spectrometry (ESI-MS) has been used to ch aracterise alkali metal and alkaline earth metal complexes formed in m ethanol solutions containing one of several synthetic cryptand ligands , or the naturally occurring ionophore valinomycin. The spectra are ve ry simple and easily interpreted, reflecting the gentle nature of the electrospray ionisation process. Mass spectra were also obtained for s olutions containing equimolar concentrations of each of the alkali met al ions or alkaline earths, with valinomycin or the cryptands. In each case the intensities of peaks due to metal complexes correlated well with the expected composition of the solution predicted from stability constants obtained by other techniques. These results suggest that ES I-MS will not only prove to be of great benefit for assisting determin ation of the composition of metal complexes formed in systems of these types, but will also be valuable as a tool for rapidly establishing t he metal ion selectivity of new ligands.