ABSTRACT
When a beam of ions with energy of the order of 10 keV strikes the surface of a solid, kinetic energy and momentum are transferred from incident ions to atoms in the solid. With the inter-atomic bonding energy up to 100 kcal mol−1 (approximately 4–5 eV) in rock-forming minerals (e.g. Si–O bonding), the ion beam bombardment sets many atoms in motion and cascades of collisions occur. As a result, some atoms on or near the surface could acquire energy and momentum sufficient to leave the surface. Some of these particles (atoms and clusters of atoms) leave the surface in charged states (secondary ions). This phenomenon is called sputtering, and secondary ion mass spectrometry (SIMS) is a technique for mass spectrometric analysis of secondary ions formed by sputtering. The ion microprobe is an anaytical instrument based on the principles of SIMS, and uses a well-collimated ion beam so that analysis of micrometre-scale areas is possible. Since it is fundamentally mass spectrometry, isotopic compositions of elements can be determined in situ, and quantitative chemical analysis can also be made in situ by converting secondary ion intensities to concentrations.
