The behaviour of caesium iodide cluster ions produced by fast-atom bombardment

Metastable ion and collisionally-activated dissociation spectra of several cluster ions of the type [Cs_n+1I_n]⁺, formed by fast atom bombardment mass spectrometry (FABMS) of CsI, have been studied: It is found that particular cluster ions are more stable than others. Less stable ions undergo more unimolecular dissociation and have larger cross-sections for collisional dissociation than do the more stable ions, and this leads to unexpected anomalies in the FAB mass spectrum of CsI. The collision spectrum of [Cs₃₅I₃₄]⁺ (m/z 8966) has been acquired at an accelerating potential of 10 kV; the extent of both unimolecular and collisionally-activated dissociation of this ion is remarkably high and leads to collision/ transmission efficiencies of 85% for our tandem mass spectrometer. Clusters formed by FAB of an equimolar mixture of CsI and KI have also been studied by their collisionally-activated dissociation (CAD) and metastable ion (MI) spectra.