How to Establish Metrological Traceability

Isotope ratio measurements are not only the foundation of new discoveries and models, but also form a basis for decisions in, for example, global trading, environmental monitoring, and food safety issues. In many cases, decisions have financial consequences and therefore need to be based on firm measurement 

Chemical amount of substance or concentration measurements are ideally traceable to the mole as a common reference (see the definition of metrological trace-ability to a measurement unit). Isotope ratios do not have a unit when the amount of the individual isotopes in the ratio is expressed in the same unit (e.g., mol mok ). In an ideal situation, isotope (amount) ratio measurements should be traceable to an isotopic reference material that has been calibrated against a synthetic isotope mixture. In this case, measurements can be made traceable to the SI unit. Since this is a rare case, most isotope amount ratio measurement are only traceable to the corresponding delta 0 reference material (see also Chapter 6). In the case of delta isotope measurements, the traceability chain ends at the delta unit which is and multiples or fractions thereof. Therefore, the reference (the delta 0 reference solution) used for the delta calculations caimot be used for traceability purposes (27). 

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