SIMS Theory: Isotope Ratio Measurements

Isotope ratio measurements are operationally similar to depth profiles except that precision and accuracy requirements are higher. Since all of the isotopes of an element have the same chemical properties, ionization and detection efficiencies remain nearly constant for the different isotopes. Precisions of 0.1% are commonplace and accuracies approach precisions. Error analysis indicates that the precisions are limited mainly by Poisson counting statistics. To attain these accuracies, SIMS instruments must be carefully tuned, and interferences must be eliminated. Mass spectral peaks should have flat tops and steep sides so that slight magnet instability does not change the ion signal intensity. The figure shows the necessary flat top peaks and the high mass resolution used for elimination of O2 interferences at m/z 32. Both were necessary for measuring accurate 34S/32S isotope ratios.

The ratio of isotope signal intensities must be corrected for slight variations in detection efficiency at different masses, and for slight variations that depend on signal intensity. These corrections are usually larger than the range of expected isotope ratios.

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