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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 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 Technology's most recent recommended value of 6.022 141 99 × 1023 mol−1 by more than 1 × 10−6 NA.

The half-life of 241Pu for β decay has been determined on an initially 93 % isotopically enriched material by measuring the decreasing amount ratio n(241Pu)/n(240Pu) as a function of time over more than twenty years. Over the same period the decrease of this ratio was measured relative to the amount ratio n(240Pu)/n(239Pu) in a "ratio-of-ratio" measurement which allowed for correction of isotopic effects in the ion source, taking into account the decay of 240Pu (half-life = 6553 a) and 239Pu (half-life = 24110 a). A regression procedure of the ratio-of-ratio measurements as a function of time resulted in a value for the 241Pu half-life of (14.290 ± 0.006) a (expanded uncertainty with coverage factor k = 2). The slight difference found in the half-life value as measured by the authors on the same material in the period 1976 to 1981 is discussed.

The first interlaboratory study of the Consultative Committee for Amount of Substance (CCQM) focused on the determination of lead concentration in a synthetic water sample. In the framework of the CCQM activities, only leading metrology institutes participated in the study. The prescribed analytical method was double isotope dilution mass spectrometry (IDMS) as a primary method of measurement (PMM). At the Institute for Reference Materials and Measurements (IRMM) the material was additionally analysed by direct IDMS as a primary method of measurement using, completely independently, a certified spike reference material. The work described in this paper concerns direct IDMS. Conclusions are drawn on the use of IDMS as a PMM, on the comparison of the results with those of double IDMS, on the use of a certified spike reference material and on the calculation of uncertainty.

Procedures developed for accurate measurement of amount of substance ratios for gas isotopes, as used to improve our knowledge of the Avogadro constant and checked for quantitative conformity against kinetic gas theory, were used to measure synthetic mixtures of enriched xenon gas isotopes, prepared by (micro)-gravimetry as is usual in the preparation of synthetic gas compound mixtures. This allowed a totally independent evaluation, on the same samples, of the relative combined standard uncertainty of (a) the procedure to perform direct measurements of amount of substance ratios of gas isotopes in the redetermination of the Avogadro constant and (b) the gravimetric preparation procedure. The results of both procedures, mass spectrometry and gravimetry, agree to a relative uncertainty of 2 × 10-3 for amount 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 xenon isotopes- 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 (QA).

Basic principles for traceability of measurements to the Systeme International d'Unites (SI) in general, and for measurements of amount of substance in particular, are described. Analogies with long-standing traceability schemes in physical measurements are given and an attempt is made to show the probable development of traceability for amount-of-substance measurements, a process now in its early stages.

Progress in science is often achieved by modifications of hypotheses imposed by reliable results of measurements with small uncertainty. The application of the discipline of metrology to chemistry, based on the SI and the estimation of uncertainties, is illustrated by suggested refinements of the presentation of molar-mass values of the elements as recommended, and biennially revised, by the International Union of Pure and Applied Chemistry (IUPAC). All these values from 1969 to 1995 are tabulated here with attributed uncertainties, and the changes achieved in all these uncertainties are presented pictorially. Molar mass is the factor that converts, with its entity-specific uncertainty, the mass of that specified entity in a given sample from a value expressed as a mass quantity to one in the amount-of-substance SI base quantity. An element with two or more isotopes may exhibit differences in isotope abundances. A small range of molar-mass values may exist even in normal terrestrial materials. The IUPAC combines, with only one-digit precision, such ranges with experimental uncertainties. Nevertheless, the IUPAC values in the past have proved highly reliable for use in chemistry. For anticipated improvements in the accuracy of some chemical measurements, two-digit uncertainties, evaluated and published separately from normal terrestrial ranges of molar mass, are proposed.

The isotopic composition of atomic nitrogen and its molar mass (numerically equal to its atomic weight) M(N) were re-measured using recently improved accurate gas isotope-ratio mass spectrometry. We found in one sample of pure nitrogen, M(N) = 14,006 721 4 g mol-1 with a combined uncertainty uc = 0,000 001 8 g mol-1 (an average of four partially independent values). Another sample, atmospheric nitrogen, similarly from four values, yielded M(N) = 14,006 727 8 g mol-1 with uc = 0,000 001 5 g mol-1 corresponding to an isotope substance ratio n(14N) /n(15N) = f(14N)/f(15N) = 271,87 with uc = 0,11. The latter is in good agreement with the currently accepted IUPAC value of 272,0 with uc = 0,3. The measurements were carried out by means of the same rigid measurement procedures and instrumentation which are used in the isotopic measurements for ongoing redeterminations of the Avogadro constant. Although the measurements were not calibrated against synthetic N-isotope mixtures, the results compare favorably, within quoted uncertainties, with those selected by the International Union of Pure and Applied Chemistry (IUPAC) as the hitherto "best measurements from a specific source" and those representative of terrestrial compositions. They suggest a new standard of reliability for molar-mass determinations. They also establish the best realization so far of traceability of nitrogen isotopic measurements to the mole in the Systeme International d'Unites (SI), as well as to the Avogadro constant.

The chemical steps needed to prepare specimens for "absolute" atomic-weight [(mean) relative atomic mass] determinations are outlined and discussed in detail for the atomic weight of silicon, Ar (Si), as recently measured for a redetermination of the Avogadro constant. The estimated combined uncertainties arising from the chemical procedures in that Ar (Si) measurement are shown to be almost equal to those from the associated mass-spectrometric measurements.

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.