Calibration of isotope amount ratios by analysis of isotope mixtures

The question as to what constitutes a fully calibrated isotope amount ratio measurement still remains a topic of active research. For years, the definitive calibration approach has been by means of synthetic mixtures of highly enriched isotopes with known chemical purity to give gravimetrically defined ratios. This article outlines the core concepts and assumptions of this method and illustrates the recent developments in the practical metrology of isotope amount ratio measurements.