Generation and collision‐induced dissociation of ammonium tetrafluoroborate cluster ions

Singly and doubly charged cluster ions of ammonium tetrafluoroborate (NH 4 BF 4 ) with general formula [(NH 4 BF 4 ) n NH 4 ] + and [(NH 4 BF 4 ) m (NH 4 ) 2 ] 2+ , respectively, were generated by electrospray ionization (ESI) and their fragmentation examined using collision‐induced dissociation (CID) and ion‐trap tandem mass spectrometry. CID of [(NH 4 BF 4 ) n NH 4 ] + caused the loss of one or more neutral NH 4 BF 4 units. The n = 2 cluster, [(NH 4 BF 4 ) 2 NH 4 ] + , was unique in that it also exhibited a dissociation pathway in which HBF 4 was eliminated to create [(NH 4 BF 4 )(NH 3 )NH 4 ] + . Dissociation of [(NH 4 BF 4 ) m (NH 4 ) 2 ] 2+ occurred through two general pathways: (a) ‘fission’ to produce singly charged cluster ions and (b) elimination of one or more neutral NH 4 BF 4 units to leave doubly charged product ions. CID profiles, and measurements of changing precursor and product ion signal intensity as a function of applied collision voltage, were collected for [(NH 4 BF 4 ) n NH 4 ] + and compared with those for analogous [(NaBF 4 ) n Na] + and [(KBF 4 ) n K] + ions to determine the influence of the cation on the relative stability of cluster ions. In general, the [(NH 4 BF 4 ) n NH 4 ] + clusters were found to be easier to dissociate than both the sodium and potassium clusters of comparable size, with [(KBF 4 ) n K] + ions the most difficult to dissociate. Copyright © 2008 John Wiley & Sons, Ltd.