P. De Bièvre

Determination of the Avogadro constant via the silicon route

A value for the Avogadro constant, NA, was derived from new measurements of the lattice parameter, the density and the molar mass of a silicon single crystal. The result NA = 6.022 135 3 × 1023 mol−1 has a relative measurement uncertainty and is in excellent agreement with other published data based on the x-ray crystal density molar mass method, indicating the high repeatability of these experiments. The value differs significantly from the Committee on Data for Science and Technologys most recent recommended value of 6.022 141 99 × 1023 mol−1 by more than 1 × 10−6 NA.

Considerations on future redefinitions of the kilogram, the mole and of other units

The definitions of some units of the Systeme International are likely to be revised as early as 2011 by basing them on fixed values of fundamental constants of nature, provided experimental realizations are demonstrated with sufficiently small uncertainties. As regards the kilogram, experiments aiming at linking it to the Planck constant and the atomic mass constant are under way in several laboratories. The other units likely to be redefined are the ampere, the kelvin and the mole. We discuss the advantages and disadvantages of different alternatives for revised definitions of the kilogram and the mole. From physical considerations, metrological consequences and ease of understanding, a definition of the kilogram based on the mass of a particle, such as an atom or the electron, is favoured. One of the proposed definitions fixes the value of the Planck constant through the Compton frequency of a material, though unphysical, particle. Finally, a redefinition of the mole, the counting unit of the amount-of-substance, is proposed which fixes the Avogadro constant as a dimensionless number.

A new determination of N/sub A/

The Avogadro constant was determined by measurements of the (220) lattice spacing, density, and molar mass of silicon crystals. The measured value is N/sub A/=(6.0221379/spl plusmn/0.0000025)/spl times/10/sup 23/ mol/sup -1/. >

Toward the Avogadro constant-preliminary results on the molar volume of silicon

The silicon molar volume has been measured through atomic weight and density measurements. Mass spectrometric determination of the isotopic composition was performed on two differently grown samples by comparison with the US National Bureau of Standards (NBS) reference material SRM 990. The density of specimens from the same crystals was obtained through direct mass and dimension measurements on one single-crystal sphere and through hydrostatic comparison for another sphere. Compatibility with previous measurements was checked through hydrostatic comparison with IMGC Zerodur density standards. The two molar volume determinations differ by 0.14 p.p.m. The preliminary value obtained is 12.058, 843+or-0.000, 029 cm/sup 3//mol at 22.5 degrees C and in vacuo. >

A more accurate value for the Avogadro constant

Significant improvements in the method for determining the absolute molar mass of silicon have been achieved and are reported. They have led to a reduction in the uncertainty of the molar mass and hence in its contribution to the uncertainty of a previously published Avogadro constant N/sub A/. The new value for N/sub A/ is almost unchanged at 6.022 136 5/spl times/10/sup 23/ mol/sup -1/ but the uncertainty is reduced to 0.000005 1/spl times/N/sub A/. >

The silicon-28 path to the Avogadro constant-first experiments and outlook

For the first time an attempt has been made to determine the lattice parameter, density, and molar mass of an isotopically highly enriched silicon-28 crystal. The goal was to achieve an independent value of Avogadros constant of accuracy equal or superior to current best values obtained from similar measurements of very pure and highly perfect silicon single crystals of natural isotopic composition. That aim has been foiled by unexplained contaminations of the /sup 28/Si crystal and perhaps other unidentified causes. Under these circumstances this paper restricts itself to a brief record of the method used and the results obtained. The value of the Avogadro constant so derived is considered of no further significance and differs from the CODATA and other credible values by about 1/spl middot/10-/sup 5/N/sub A/. >

Molar Mass Determinations for NA (Avogadro) Projects: Present Status and Potential for the Future

The present status of molar mass measurements of silicon is summarized because of their relevance to modern determinations of the Avogadro constant: principles of the measurement, measurement procedures and preparation of the synthetic isotope mixtures used to calibrate the measurements are described. Typical results are shown and the potential for improvements achievable in the near future is outlined.

SI-traceable values for cadmium and lead concentration in the candidate reference material, MURST-ISS A1 Antarctic sediment, by combination of ICP-MS with isotope dilution

Abstract The meaning of traceability to SI and how this can be achieved by using primary methods is demonstrated in this paper, using the example of the certification for Cd and Pb concentration in the candidate reference material, MURST ISS A1 Antarctic sediment. The analytical method used was inductively coupled plasma mass spectrometry (ICP-MS) in combination with isotope dilution (ID). ID is recently recognised as a primary method, and its advantages are presented. Microwave pressure digestion and separation (anion exchange) were applied. The uncertainty budget calculation is performed according to the EURACHEM and ISO guidelines, and some useful considerations about other approaches are given.

Certified reference materials for the determination of cadmium in polyethylene

Four polyethylene reference materials with cadmium concentrations in the range of 40 to 400 mg/kg have been certified by IRMM on behalf of Verband der Automobilindustrie e.V. (VDA), Frankfurt, Germany. The homogeneity of the materials was verified using Solid Sampling Zeeman Atomic Absorption Spectrometry (SS-ZAAS). Their cadmium content was determined by Isotope Dilution Mass Spectrometry (IDMS) using a quadrupole thermal ionization mass spectrometer. The certified uncertainties were about 3% relative and correspond to confidence levels of 95%; they take into account all contributions of statistical and systematic nature. The material was used within the scope of IRMMs International Measurement Evaluation Programme (IMEP).

Preparation of two synthetic isotope mixtures for the calibration of isotope amount ratio measurements of sulfur

Two synthetic isotope mixtures for the calibration of sulfur isotope amount ratio measurements were gravimetrically prepared from high purity Ag2S materials enriched in 32S, 33S, and 34S. The mixtures were made so as to closely resemble the (natural) isotopic composition of the materials to be “calibrated”. This allowed a totally independent evaluation, on the same samples, of the relative combined uncertainty of: (a) the procedure to perform direct measurements of the amount of substance ratios of gas isotopes in the redetermination of the Avogadro constant and (b) the gravimetric preparation procedure. The result of both procedures, mass spectrometry and gravimetry, agree to a relative uncertainty of 3 × 10−4 for sulfur amount ratio measurements of the major abundant isotopes. Thus it seems that a direct measurement of isotopic gas mixtures (e.g. of natural isotopic composition) is now possible for sulfur—and probably also for other gaseous isotopes—without necessarily having to rely on “calibration” by means of values provided by measurements of gravimetrically prepared isotope mixtures. However, synthetic mixtures may be needed for validation and verification purposes, in particular for quality assurance.