STABLE-ISOTOPE DILUTION - AN ESSENTIAL TOOL IN METROLOGY

The potential role of isotope dilution (ID) in the future organization of traceability and therefore comparability of chemical measurements (amount measurements in SI terms) is described. Essential is that ID (e.g. in isotope dilution mass spectrometry IDMS), directly measuring in our SI unit for amount of substance (the mole), gives matrix-independent results and reduces a complicated chemical measurement to a simple physical measurement. It is possible to borrow from the ultra-high accuracy isotopic measurement techniques needed in the continuous improvement of the Avogadro constant in order to make high accuracy measurements of the amount of substance: both fields have in common the determination of isotope abundance ratios with small but well known total uncertainties (conditions for so-called ''absolute'' measurements). In addition, the use of such ratio measurements in an isotope dilution procedure for amount measurements seems to constitute a form of direct traceability of amount measurements to the ''Avogadro measurement procedure'' and therefore to the closest realisation of the mole so far. All of this will have far-reaching consequences: Will enriched isotopes be available in a systematic, continuous, affordable supply to ensure the possibility of isotope dilution in the future? Will simpler and, above all, cheaper isotope mass spectrometers be available for the key laboratories of future measurement networks needed in the organization of the traceability of chemical measurements? Will the difference between ''chemical'' and ''physical'' measurements not gradually fade away in the organization of traceability of amount measurements? Is further development and application of IDMS - but also of ID using other isotope-specific measurement techniques - not needed for all elements?