Fundamental studies of molecular depth profiling using organic delta layers as model systems

Evidence for the formation of dynamically created pre-formed ions at the interface of isotopically enriched thin films

A novel approach to elucidate the ionization mechanism for the [M + H]+ molecular ion of organic molecules is investigated by molecular depth profiling of isotopically enriched thin films. Using a model bi-layer film of phenylalanine (PHE) and PHE-D8, the results show formation of an [M + D]+ molecular ion for the non-enriched PHE molecule attributed to rearrangements of chemical damage due to successive primary ion impacts. The [M + D]+ ion is observed at the interface for 19.9nm in the enriched-on-top system and 9.9nm for the enriched-on-bottom system. This ion formation is direct evidence for dynamically created pre-formed ions as a result of chemical damage rearrangement induced by previous primary ion bombardment events.

Molecular depth profiling of buried lipid bilayers using C(60)-secondary ion mass spectrometry

An organic delta layer system made of alternating Langmuir-Blodgett multilayers of barium arachidate (AA) and barium dimyristoyl phosphatidate (DMPA) was constructed to elucidate the factors that control depth resolution in molecular depth profile experiments. More specifically, one or several bilayers of DMPA (4.4 nm) were embedded in relatively thick (51 to 105 nm) multilayer stacks of AA, resulting in a well-defined delta layer model system closely resembling a biological membrane. 3-D imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profile analysis was performed on this system using a focused buckminsterfullerene (C(60)) cluster ion beam. The delta layer depth response function measured in these experiments exhibits similar features as those determined in inorganic depth profiling, namely an asymmetric shape with quasi-exponential leading and trailing edges and a central Gaussian peak. The effects of sample temperature, primary ion kinetic energy, and incident angle on the depth resolution were investigated. While the information depth of the acquired SIMS spectra was found to be temperature independent, the depth resolution was found to be significantly improved at low temperature. Ion induced mixing is proposed to be largely responsible for the broadening, rather than topography, as determined by atomic force microscopy (AFM); therefore, depth resolution can be optimized using lower kinetic energy, glancing angle, and liquid nitrogen temperature.