A method of analysing the surface of a material to detect impurities. An ion beam is used to sputter material in the form of secondary ions from the surface of a semiconductor. The secondary ions are electrostatically accelerated and then analysed using a mass spectrometer. The ion beam may be kept small and scanned across the sample and the term ion microprobe is sometimes used. Over 90% of the secondary ions are emitted from the two top atomic layers. A depth profile can be obtained by sputtering continuously in the vertical direction but the accuracy decreases with increasing depth. SIMS is a destructive analysis technique.