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International traceability to the SI is increasingly demanded for measurements in chemistry. These concern measurements made not only for industrial chemical manufacturing but also for checking the safety of a wide range of food and agricultural products, for environmental protection, for medicine, as well as for a multitude of regulatory purposes. This article outlines the role that the Comité Consultatif pour la Quantité de Matière (CCQM) can play in establishing traceability at the highest level for such measurements. A brief account is given of the actions taken so far by the Committee. These include the precise description of primary methods, the meaning of traceability for measurements in chemistry and the organization of some key international comparisons designed to test the procedures drawn up to guide them. Plans for future work are described.

Entitic quantities are quantities, such as entitic number (or number of entities) N_e(B), , entitic mass m_e(B) or entitic volume V_e(B), relating to a specified ''generalized molecule'' or entity B, such as H (meaning a single atom of hydrogen) or H₂O or {(1 - x)H₂0 + xCH₃OH}. Molar quantities are the quotients of an extensive quantity and amount of substance n_B, such as molar number N_m {or Avogadro constant L = N_e(B)/n(B)}, molar mass m_m(B) {or M(B)}, or molar volume V_m(B), also of an entity B. It is shown that there is no imperative need to retain quantities like amount of substance n, or the Avogadro constant L, or the molar gas constant R, or the SI unit: mole. All of these could be discarded from the equations of physical science, though whether it would be useful to discard them is of course open to question.

Apart from the measurement standards available in general metrology in the form of the realizations of units of measurement, measurement standards for chemical composition are needed in the vast field of measurements of chemical composition, because the main aim of such measurements is to quantify non-isolated substances, often in complicated matrices, to which the ''classical'' measurement standards and their lower-level derivatives are not directly applicable. At present, most chemical measurement standards are material artefacts. But not all of these artefacts are measurement standards according to the metrological definition, i.e. firmly linked to the SI unit in which the chemical composition represented by the standard is expressed. In fact, only a very restricted number of really reliable chemical measurement standards is available at present. Since it is very difficult and time-consuming to substantially increase this number and, on the other hand, reliable reference points are increasingly needed for all kinds of chemical measurement, primary methods of measurement and high-level reference measurements as well as primary measurement devices and procedures will play an increasingly important role for ensuring world-wide comparability and hence mutual acceptance of chemical measurement results.

Among the analytical methods used to determine chemical composition, there are two special methods based on reaction chemistry. These are gravimetry and titrimetry, and taken together they are called classical analysis. They are special because they lead directly to independent values of chemical quantities expressed in SI units. Gravimetry and titrimetry can be performed in such a way that their operation is completely understood, and all significant sources of error in the measurement process can be evaluated and expressed in SI units together with a complete uncertainty budget. Such chemical measurements are metrologically valid and stand alone. They have no need to be compared with a reference material of the quantity being measured and therefore are called definitive or primary methods. In this paper the basic equations and terms of gravimetry and titrimetry are discussed, together with detailed examples of each. This is followed by a discussion of the errors associated with gravimetry and titrimetry, and instrumental techniques that can be used for evaluating these errors. There is also a discussion of the uncertainties associated with both systematic and random errors in gravimetry and titrimetry. The paper is underpinned by historical sketches of several important issues and scientists connected with the development of classical analysis. A case is made for a revival of the use of classical analysis in chemical metrology, together with some suggestions as to how such a revival might be implemented.

The Russian base of standards in the field of physico-chemical measurements includes ten hierarchical systems of standards as well as many certified reference materials. The structure and peculiarities of the hierarchical systems of standards are discussed.

Reference materials have been widely used as measurement standards in the fields of chemistry, biology, engineering and physics, and have played an important role in ensuring the comparability of measurement results. In recent years, an increasing number of countries and international organizations have made efforts to develop reference materials and have discussed such basic underlying issues as terminology, hierarchy, traceability and uncertainty. This paper presents a personal view on some of these matters and proposes the development of a hierarchy of reference materials certified for chemical composition.

Metrology in general, and issues such as traceability and measurement uncertainty in particular, are new to most analytical chemists, many of whom are unconvinced of their value. There is a danger of a widening of the cultural gap between metrologists and analytical chemists and it is important to encourage greater collaboration and cross-fertilization. This paper discusses some of the similarities and differences in the approaches adopted by metrologists and analytical chemists and indicates how these approaches could be combined to establish a unique metrology of chemical measurement acceptable to both cultures.

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 definition and concept of primary method are explained, taking examples from the fields of thermometry and measurements of amount of substance. It is shown that although few methods can be considered strictly primary, small departures from the strict definition can be quantified and useful practical primary methods are thus available. The links between primary methods of measurement, primary standards of measurement and primary reference materials are discussed.

Basic principles for traceability of measurements to the Système International d'Unités (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.

Titrimetry has wide-ranging applications for amount-of-substance measurements and can be used for both metrological and working-level measurements. This paper is a review of the potential of titrimetry as a primary method. Many parts of a traceability system are in place but these are not formally recognized at either a national or international level. Further work is required to assess existing expertise and to establish internationally recognized primary reference materials (standards) and measurement procedures.

A new method is presented for amount-of-substance measurements in organic chemistry using isotope dilution mass spectrometry in combination with gas chromatography. Its main feature is the conversion of analyte and spike, after the chromatographic separation, into carbon dioxide which is subjected to the isotope ratio measurement. This modification is used because the techniques of n(¹³C)/n(¹²C) ratio measurements in CO₂ are very well developed so that high accuracy in results is achieved. As carbon dioxide is used for the isotope ratio measurement, it is necessary to use ¹³C-labelled organic compounds as spikes. From the results of a model analysis it is reasonable to expect that the new GC/C/IDMS method is capable of providing measurement results with a higher accuracy than has been possible up to now.

The Comité Consultatif pour la Quantité de Matière (CCQM) endeavours to identify and carry out key activities with the objective of facilitating world-wide comparability and traceability of chemical measurements. Towards this goal, the CCQM has identified comparisons to be carried out using candidate primary methods of chemical analysis. One such method is isotope dilution mass spectrometry. In the first comparison carried out by the CCQM (Study I), the concentrations of various inorganic elements were determined in water solutions using IDMS. The results did not meet the target level of 1% maximum relative deviation from the reference values of the unknowns. It was concluded that more guidance was necessary on the execution of the IDMS method and that a detailed protocol should be developed. Participants would use inductively coupled plasma (ICP) as the ionization source. Here we present the protocol developed for use by CCQM participants in the next comparison (Study III) on the determination of the concentration of lead in water using the IDMS method with an inductively coupled plasma-mass spectrometer.

The hierarchy of references used in Russia to transfer a unit value for ozone concentration measurements is described. The equipment and methods used in its practical implementation are listed.

Some aspects of the use and misuse of scientific language are discussed, particularly in relation to quantity calculus, the names and symbols for quantities and units, and the choice of units - including the possible use of non-SI units. The discussion is intended to be constructive, and to suggest ways in which common usage can be improved.