Molecular depth profiling of model biological films using low energy monoatomic ions

As time-of-flight secondary ion mass spectrometry (ToF-SIMS) is increasingly used to characterize biological samples, fundamental studies on model samples are needed to better understand the interactions between primary ions and biological matter, and to help interpreting the complex ToF-SIMS data. In this work, model biological films were prepared by thermal evaporation and were characterized by XPS and ToF-SIMS. Eight biomolecules were carefully chosen to cover important physical and chemical properties such as aromaticity and oxygen content. The model films allowed studying systematically the sputter rates, ion yields and amenability to molecular depth profiling when using low energy Xe and Cs etching ions. The results show that aliphatic molecules such as cysteine and aspartic acid can be easily depth profiled using low energy monoatomic primary ions. However, aromatic samples such as phenylalanine could only be depth profiled using reactive Cs ions. The systematic study also revealed that the presence of oxygen strongly reduces the deprotonated ion signal during cesium ion bombardment. Finally, model multilayer structures were depth profiled using 500 eV Xe and Cs primary ions with depth resolutions lower than 10 nm. Isotopic variations of the same biomolecule were used for the first time to create multilayers structures with constant ion yields and etching rates.