Most elements have more than one isotope
E.g. 32S and 34S, or 56Fe and 57Fe
Can use more than one mass for one element for measurements in ICP-MS
IDSM: Isotope dilution mass spectrometry: Use particular isotope of desired analyte as internal standard in ICP-MS
Can buy enriched compounds, e.g. 67ZnO, and use as “tracers”