SIMS is the most sensitive surface analytical technique, with detection limits of parts per billion in favorable cases. A focused cesium or oxygen ion beam is rastered over an area ranging from 25 to 200 microns of the sample surface. As the surface is ablated, the ejected secondary ions are analyzed by mass, generating a plot of elemental composition vs. depth (typically 50 nm to 5 microns of depth). Results can be quantified using standards. For smooth samples, depth resolution can approach 1 nm.