Mass-resolving power was set to 240,000 at m/z 200. Etching removes the particle strata which are sampled sequentially as primary ion bombardment proceeds, with the exception of shadowing and preferential sputter yields discussed in the Yang et al. Calibration of the Orbitrap analyser was performed on the silver sample, using silver clusters following the method described by Passarelli et al. using the Bi1 + liquid metal ion gun as a primary ion beam. We have used information from a published depth profiling organic molecular standard (Irganox) to estimate the depth from the primary ion dose, which is displayed as a secondary x axis (red) in Figure . The sputter yield from 20keV Ar3000 + on Irganox was corrected for the reduced temperature (-170°C) used in this work compared to the literature values using the calculations developed by Seah et al.