The authors present experimental results showing the effect of an applied bias voltage on backscattered ion spectra acquired from thin films of ruthenium and hafnia in the helium ion microscope. A characteristic peak associated with the presence of a thin layer of material is observed to shift as a function of sample bias voltage. The magnitude of this shift is measured, and the authors qualitatively estimate the composition of their samples as well as investigate the neutralization of ions by the sample in the helium ion micoscope (HeIM). They discuss the phenomenona in terms of thin films of ruthenium and hafnia and show the implications of these results on HeIM spectroscopy. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3502667]