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Ion microprobes have been around in various forms for many years. It was not
until the mid 70's that the ion probe was viewed as having the potential to be
the geologist's ultimate weapon. The ion probe uses a focused beam of primary
ions to sputter away the sample surface. A small fraction of the sputtered
material is ionized and can then be accelerated into a mass spectrometer. A
characteristic of secondary ion mass spectrometry (SIMS) is a plethora of
atomic and molecular species which often cause isobaric interferences. The
first ion microprobes relied on low mass resolution mass spectrometers and
tried to strip away interferences by monitoring other peaks containing the
interfering elements. This method is fraught with difficulty because it relies
on the correct identification of all potential interferences. The first SIMS
instrument capable of high mass resolution was the Cameca ims-3f. This
instrument works as an ion microscope, that is, a direct image of the spatial
distribution of the isotopes in the target can be obtained. High mass resolving
power could only be achieved on this instrument at the expense of beam
transmission - entrance and exit slits had to be very narrow thereby reducing
the amount of beam transmission.

[Source: Stanford University.]