Atomic fraction or concentration of impurity species in a homogeneous matrix below which the
secondary-ion mass spectrometry can safely be assumed to be linear with composition.
Source:
PAC, 2020, 92, 1781. 'Glossary of methods and terms used in surface chemical analysis (IUPAC Recommendations 2020)' on page 1838 (https://doi.org/10.1515/pac-2019-0404)