Secondary ion mass spectrometry of zeolite materials: Observation of abundant aluminosilicate oligomers using an ion trap

Oligomeric oxyanions were observed in the secondary ion mass spectra (SIMS) of zeolite materials. The oxyanions have the general composition AlmSinO2( m+n)H(m-1)- (m + n = 2 to 8) and are termed dehydrates. For a given mass, m ultiple elemental compositions are possible because (Al + H) is an isovalen t and isobaric substitute for Si. Using 18 keV Ga+ as a projectile, oligome r abundances are low relative to the monomers. Oligomer abundance can be in creased by using the polyatomic projectile ReO4- (similar to5 keV). Oligome r abundance can be further increased using an ion trap (IT-) SIMS; in this instrument, long ion lifetimes (tens of ms) and relatively high He pressure result in significant collisional stabilization and increased high-mass ab undance. The dehydrates rapidly react with adventitious H2O present in the IT-SIMS to form mono-, di-, and trihydrates. The rapidity of the reaction a nd comparison to aluminum oxyanion hydration suggest that H2O adds to the a luminosilicate oxyanions in a dissociative fashion, forming covalently boun d product ions. In addition to these findings, it was noted that production of abundant oligomeric aluminosilicates could be significantly increased b y substituting the countercation (NH4+) with the larger alkali ions Rb+ and Cs+. This constitutes a useful tactic for generating large aluminosilicate oligomers for surface characterization and ion-molecule reactivity studies .