Optimum detection sensitivities in secondary ion mass spectrometry (SIMS) require the use of efficient secondary ion extraction optics placed within a few millimeters of the target. Removal of previously deposited materials form the mechanical structure of the extraction optics are shown to introduce a background which can degrade the limit of detection. The sputtering of GaAs prior to the analysis of As in silicon results in a detectable As background of up to 2.2×1018 atoms/cm3. Similarly, the sputtering of silicon will result in a detectable background of Si in GaAs reaching into the low 1016 atoms/cm3 range. © 1983.