The technique of Secondary Ion Mass Spectrometry (SIMS) is the most sensitive of all the commonly-employed surface analytical techniques - capable of detecting impurity elements present in a surface layer at < 1 ppm concentration, and bulk concentrations of impurities of around 1 ppb (part-per-billion) in favourable cases. This is because of the inherent high sensitivity associated with mass spectrometric-based techniques.
There are a number of different variants of the technique :