In
secondary ion mass spectrometry, number of ions of a particular isotope detected divided by the number of atoms of the same element sputtered from the sample.
Source:
PAC, 2020, 92, 1781. 'Glossary of methods and terms used in surface chemical analysis (IUPAC Recommendations 2020)' on page 1846 (https://doi.org/10.1515/pac-2019-0404)