Abstract. The use of isotope dilution mass spectrometry (IDMS) to make measurements of amount of substance that are traceable to the International System of Units (SI) depends on an analysis of possible sources of uncertainty. A comprehensive application is presented of the law of propagation of uncertainty to the two-way IDMS method, which allows sensitivity coefficients to be calculated and optimum regimes identified for carrying out IDMS measurements. This has allowed the effect of deviations from "exact matching" on the uncertainty of the measurement to be evaluated for the first time. Furthermore, the relationship between traceability and uncertainty has been investigated for the two-way IDMS method, which is shown to have the potential to be a primary ratio method and to provide traceability to the SI. Our analysis shows that proper consideration of correlation in the uncertainty calculation and appropriate experimental design can reduce the requirement for highly characterized spike reference materials.

Print publication: Issue 3 (June 2000)