Abstract:
Electrospray ionization was used to produce distributions of gas-phase cluster ions from solutions of sodium and potassium tetrafluoroborate. The majority of the cluster species followed the trend (MBF(4))(n)M(+), where M=Na and K. The values of n, for both salts, ranged from 1-15. Collision induced dissociation (MS/MS and MS(n)) in an ion trap mass spectrometer was used to determine the dissociation pathways for the cluster ions. The (NaBF(4))(n)Na(+) cluster ions fragmented via two pathways: (a) the loss of one or multiple neutral BF(3) molecules and (b) the loss of one or more NaBF(4) units. Of the two, the product ions corresponding to the loss of BF(3) units were more prominent. Unlike the Na salt, the (KBF(4))(n)K(+) cluster ions decomposed primarily by the loss of one or multiple KBF(4) units. Similar differences in dissociation behavior were observed when the heated transfer capillary, normally used to desolvate ions, was used to investigate cluster ion stability to thermal degradation and dissociation. The dissociation profiles (decrease in ion abundance with increasing activation amplitude) for several (NaF)(n)Na(+) and (KF)(n)K(+) cluster ions were measured and compared to probe the influence of the relative stability of the alkali fluorides (NaF and KF) on the dissociation behavior exhibited by the tetrafluoroborate cluster distributions. We found that the (NaF)(n)Na(+) cluster ions required higher activation amplitudes to induce fragmentation than the corresponding (KF)(n)K(+) species, indicative of stronger ionic bonding and higher gas-phase stability for the former. This in turn indicates that the reaction pathway involving only the loss of one or multiple units of BF(3), favored for the (NaBF(4))(n)Na(+) cluster series, but not for the analogous (KBF(4))(n)K(+) series, may be due to the high gas-phase stability of NaF, and relatively lower stability of KF, towards dissociation.