SIMS Theory: Sensitivity and Detection Limits

The SIMS detection limits for most trace elements are between 1e12 and 1e16 atoms/cc. In addition to ionization efficiencies (RSF's), two other factors can limit sensitivity. The output of an electron multiplier is called dark counts or dark current if no secondary ions are striking it. This dark current arises from stray ions and electrons in instrument vacuum systems, and from cosmic rays. Count rate limited sensitivity occurs when sputtering produces less secondary ion signal than the detector dark current. If the SIMS instrument introduces the analyte element, then the introduced level constitutes background limited sensitivity. Oxygen, present as residual gas in vacuum systems, is an example of an element with background limited sensitivity. Analyte atoms sputtered from mass spectrometer parts back onto the sample by secondary ions constitute another source of background. Mass interferences also cause background limited sensitivity.

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