Vacuum[edit]
SIMS requires a high vacuum with pressures below 10−4 Pa (roughly 10−6 mbar or torr). This is needed to ensure that secondary ions do not collide with background gases on their way to the detector (i.e. the mean free path of gas molecules within the detector must be large compared to the size of the instrument), and it also prevents surface contamination by adsorption of background gas particles during measurement.