Secondary ion mass spectrometry (SIMS) is a technique that measures elements at a surface with a detection sensitivity as low as a few parts per billion. A beam of energetic oxygen or cesium ions are directed to a surface to be analyzed. These bombarding ions force atomic and molecular particles to be ejected from the surface, a process called sputtering. Since some of these sputtered particles carry a charge, a mass spectrometer can be used to measure their mass and charge. Continued sputtering measures the elements exposed as material is removed to construct elemental depth profiles. SIMS instruments can produce two-dimensional elemental maps of the original surface or at a particular depth (after sputtering).