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The planned re-definition of the SI system of units requires a larger number of experiments. Among them the re-determination of the Avogadro constant NA with a drastically reduced measurement uncertainty (urel(NA) ≤ 5 × 10-8). The, within the scientific community simply called, Avogadro Project tries to do just this by counting the atoms within a silicon single crystal, extremely enriched with respect to the isotope 28Si (x(28Si) > 0.9999 mol/mol). One of the input quantities of NA is the molar mass M of the highly enriched silicon material. For several years now, we determine this molar mass almost “routinely” with measurement uncertainties associated to the molar mass of urel(M) ≤ 5 × 10-9. For this purpose, a highly specialized isotope dilution technique (VE-IDMS) was developed and brought to life. The implementation of the VE-IDMS technique is founded in parts on the creation of several novel basic principles for the determination of isotope ratios, which were developed in PTB. In cooperation with a hand-full of renowned NMIs, an extremely low measurement uncertainty was achieved, unparalleled in analytical chemistry. At the moment,  the work is focused on checking for any inhomogeneities of the isotopic pattern within the silicon crystal and also on the determination of its chemical purity.

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