title: Secondary ion mass spectrometry of zeolite materials: observation of abundant aluminosilicate oligomers using an ion trap.
abstract: 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, multiple elemental compositions are possible because (Al + H) is an isovalent and isobaric substitute for Si. Using 18 keV Ga+ as a projectile, oligomer abundances are low relative to the monomers. Oligomer abundance can be increased by using the polyatomic projectile ReO4- (approximately 5 keV). Oligomer 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 abundance. The dehydrates rapidly react with adventitious H2O present in the IT-SIMS to form mono-, di-, and trihydrates. The rapidity of the reaction and comparison to aluminum oxyanion hydration suggest that H2O adds to the aluminosilicate oxyanions in a dissociative fashion, forming covalently bound product ions. In addition to these findings, it was noted that production of abundant oligomeric aluminosilicates could be significantly increased by 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.
authors.affiliation: Idaho National Engineering and Environmental Laboratory, Idaho Falls, USA.
authors.forename: G S
authors.initials: GS
authors.lastname: Groenewold
authors.forename: G F
authors.initials: GF
authors.lastname: Kessinger
authors.forename: J R
authors.initials: JR
authors.lastname: Scott
authors.forename: A K
authors.initials: AK
authors.lastname: Gianotto
authors.forename: A D
authors.initials: AD
authors.lastname: Appelhans
authors.forename: J E
authors.initials: JE
authors.lastname: Delmore