Determination of the Avogadro constant by counting the atoms in a 28Si crystal.

The Avogadro constant links the atomic and the macroscopic properties of matter. Since the molar Planck constant is well known via the measurement of the Rydberg constant, it is also closely related to the Planck constant. In addition, its accurate determination is of paramount importance for a definition of the kilogram in terms of a fundamental constant. We describe a new approach for its determination by counting the atoms in 1 kg single-crystal spheres, which are highly enriched with the 28Si isotope. It enabled isotope dilution mass spectroscopy to determine the molar mass of the silicon crystal with unprecedented accuracy. The value obtained, NA = 6.022,140,78(18) × 10(23) mol(-1), is the most accurate input datum for a new definition of the kilogram.

The International Measurement Evaluation Programme, IMEP-8: carbon and oxygen isotope ratios in CO(2).

In IMEP-8, two samples of high purity CO(2)(g), with different carbon and oxygen isotope ratios were distributed to 27 participants, originating from 14 countries and from various isotopic measurement domains (geochemistry, atmospheric and food chemistry), but particularly set up for food laboratories. In total 19 laboratories reported results. The outcome of this comparison exercise shows that the laboratories reported carbon and oxygen isotope results in good agreement with the reference values across the domains. The reported results for delta(13)C(VPDB) (carbon) for both materials are within 1 per thousand. However, for the reported results of delta(18)O(VPDB) (oxygen) for both materials the overall spread of the reported results is about 11 per thousand. Within this spread two distinct groups of participants can be identified, where the results within each group vary about 2 per thousand. The latter seems to be caused by calculation errors by participants of the reporting delta(18)O(VPDB) values. As requested, participants also reported the isotope amount ratio for carbon and oxygen in the CO(2) samples. For carbon, all reported results for both materials agree with the isotope ratio value, which can be traced back to the value reported by Craig. For oxygen, all results are in good agreement and deviate by a maximum of 0.5% from the reference values measured at IRMM. Work carried out indicates the carbon isotope ratio, for both samples IMEP-8A and IMEP-8B, differ from those reported by Craig by as much as 1.2%. In the case of oxygen, this deviation is far smaller. Both data sets, i.e. the one realised by Craig and the one realised at IRMM, demonstrate traceability to SI. It is clear that both values significantly disagree.

Microscale Synthesis of Fe(PF(3))(5) for Gas Source Mass Spectrometry.

Down-scaling of a cocondensation method to produce volatile pentakis(trifluorophosphine)iron(0) for measuring isotope ratios by gas source mass spectrometry is described. Starting from 10 mg of iron, this method allows synthesis of a sufficient amount of Fe(PF(3))(5) that can be used to measure iron content and/or isotope ratios in real-life samples. The advantage over thermal ionisation mass spectrometry is that more precise determinations of ratios/amounts are possible. Reproducibilities for n((54)Fe)/n((56)Fe) and n((57)Fe)/n((56)Fe) are (1-3) × 10(-4), while those obtained by thermal ionization mass spectrometry are at the 10(-3) level.

Letter to the Editor: New Values for Silicon Reference Materials, Certified for Isotope Abundance Ratios.

New isotope abundance and relative atomic mass (atomic weight) values - with low, hitherto unattained uncertainty - are reported for two previously described silicon reference materials using a well-known method with an improved isotope-ratio mass spectrometer. These new values are directly traceable to the SI, more specifically to the unit for amount of substance, the mole, and independent of the SI unit of mass and of the Avogadro constant. Besides the residual mass-spectrometric uncertainties, these new values depend in effect only on a recently published direct comparison of the cyclotron frequency in a Penning trap of 28Si+ with that of 12C+.

Letter to the Editor: Silicon Reference Materials Certified for Isotope Abundances.

In a series of gas mass-spectrometric measurements performed near the highest attainable accuracy, samples from two highly homogeneous batches of silicon crystals and silica powder were compared directly with a synthetic mixture of the three stable isotopes of silicon. Thereby, this work not only established the "absolute" atomic weight of these batches, but also makes portions of these batches available as an Isotopie Reference Material for accurate isotopic abundance measurements in geochemical and other isotope-abundance studies of silicon.