Uncertainty in SI-traceable measurements of amount of substance by isotopedilution mass spectrometry

The use of isotope dilution mass spectrometry (IDMS) to make measurements o f amount of substance that are traceable to the International System of Uni ts (SI) depends on an analysis of possible sources of uncertainty. A compre hensive application is presented of the law of propagation of uncertainty t o the two-way IDMS method, which allows sensitivity coefficients to be calc ulated and optimum regimes; identified for carrying out IDMS measurements. This has allowed the effect of deviations from "exact matching" on the unce rtainty of the measurement to be evaluated For the first time. Furthermore, the relationship between traceability and uncertainty has been investigate d 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 s hows that proper consideration of correlation in the uncertainty calculatio n and appropriate experimental design can reduce the requirement for highly characterized spike reference materials.