Depth profiles of As‐implanted silicon samples have been determined by secondary ion mass spectrometry in order to establish a practical detection limit for As in Si. By monitoring the AsSi−species, produced in high yield by Cs+ion bombardment, it is shown that As is detectable down to a background count limit of ∼3×1015atom cm−3(50 ppb) during a depth profile of a 250‐&mgr;m square area.