ToF-SIMS uses a focused, pulsed particle Ga beam to dislodge chemical elements on the sample's surface. Particles closer to the site of impact tend to be dissociated ions and secondary particles generated farther from the impact tend has more of molecular compounds. These particles will accelerate on a flight path to the detector.
This technique is a surface sensitive analytical method that uses a pulsed ion beam to analyse outermost surface of the sample. Hence it is most suitable for monolayer contamination.