PNNL

Abstract

In situ liquid secondary ion mass spectrometry (SIMS) enabled by System for Analysis at the Liquid Vacuum Interface (SALVI) has proven to be a promising new tool to provide molecular information at liquid interfaces. During in situ liquid SIMS analysis, the primary ion beam was normally scanned on a very small area (e.g., a 2 µm diameter round area) to collect signals with high ion doses (1014-1016 ions/cm2). As a result, beam damage may become a concern when compared with static limit of SIMS analysis, in which the dose is normally less than 1012 ions/cm2. Therefore, a comparison of ion yields in in situ liquid SIMS analysis versus traditional static SIMS analysis of corresponding dry samples is of great interest. In this study, a dipalmitoylphosphatidylcholine (DPPC) liposome solution was used as a model system. Usable ion yields for both positive and negative characteristic signals (including molecular ions and characteristic fragment ions) were achievable based on optimized experimental conditions for in situ liquid SIMS analysis. The ion yield of the key DPPC molecular ion was comparable to that of traditional static SIMS, and unexpected low fragmentation was observed. The flexible structure of liquid plays an important role for these observations. Therefore, beam damage may not be a concern in in situ liquid SIMS analysis if proper experimental conditions are used.