Editorial

Metrology in chemistry: a different approach to comparable analytical results

The term metrology is defined in many dictionaries as the scientific study of measurement, but it is most widely used with regard to the definition and standardization of the units of measurement, such as the metre, kilogram or mole. It is well known that accurate and comparable physical measurements are achieved by ensuring that each measurement result for a particular unit of measurement is traceable to a unique reference which is accepted throughout the world. This reference may be an artefact, such as the international standard kilogram or, more likely today, the relevant SI unit, most of which are based on a quantum phenomenon. In either case, the concept of traceability depends on a chain of measurements linked back to the appropriate international primary standard through a series of calibrations (i.e., comparisons between two standards in the chain). Provided the uncertainties of the comparisons are known, a measurement result obtained through calibration against one of these standards will itself be traceable to the agreed reference.

This concept has been applied universally for the past 130 years and ensures that physical measurements, including those which form part of every chemical analysis, are comparable no matter where in the world they are made. The success of the concept depends not merely on the definition of the SI units but on the formal, worldwide co-operation of national measurement institutes (NMIs) to maintain and inter-compare their measurement standards or calibration services. This extensive and long-standing collaboration is commonly referred to as the international measurement system. It is maintained by the International Committee for Weights and Measures (CIPM), through the auspices of the Treaty of the Metre (1875), and has as its focus the International Bureau of Weights and Measures (BIPM), located at Sèvres near Paris. Participating NMIs around the world have recently formalised their collaboration in a Mutual Recognition Arrangement (MRA) which is co-ordinated by the BIPM.

Chemists have long depended on the international measurement system, as already mentioned, for calibration of measurements such as mass, volume or wavelength, but complete calibration of an analytical method in this way using traceable chemical standards is complicated by the dependence of the chemical measurement process on the sample matrix. The measurement does not usually take place directly on the original sample and an instrumental determination, such as spectrometry, is often the final step of a complex analytical method involving extensive pre-treatment of the sample. Hence, calibration of the instrument alone is insufficient to achieve reliable and comparable analytical results. Thus, there are relatively few traceable chemical measurement standards in the sense used for physical measurement standards and the concept of traceable analytical measurements is not widely used by analysts. Instead, they have traditionally used two different approaches to achieving reliable and comparable chemical measurements: matrix reference materials and inter-laboratory comparisons.

The matrix-matched, certified reference material (CRM) is a unique type of chemical standard commonly used to validate complete measurement methods and sometimes for instrumental calibration (e.g., in XRF). Such standards are prepared to correspond to each required analyte/matrix combination. Similarly, inter-laboratory comparisons are undertaken for each relevant analyte/matrix combination in order to establish comparability of their measurement data. These comparisons range from ‘round robin’ studies, which collaboratively test a new method, to formal proficiency testing (PT) or external quality assurance (EQA) schemes which assess agreement between laboratories on an ongoing basis. CRMs and PT schemes have been used with reasonable success over many years but they both have a number of technical, practical and economic limitations. The need for a wide variety of application-specific CRMs has led to fragmented production without any formal relationship between the certified values of CRMs produced for different applications or by different organisations. Inter-laboratory comparisons also have a number of limitations, particularly that they are time-consuming and expensive. Comparability usually extends only to the immediate participants in a single comparison because comparability between different comparisons is rarely established, even when they are co-ordinated by the same organisation. It is impracticable to organise comparisons for every routine application or to organise a world-wide comparison involving all the laboratories requiring comparability for each measurement application.

The problems of achieving global comparability of analytical results have existed for many years but today expansion of trade means more countries and more laboratories need to be brought into each inter-laboratory comparison. In addition, increasing numbers of chemical measurements are used in support of regulations, which creates an expanding requirement for rigorously proven reliability and comparability. The application of metrology to the entire chemical measurement method offers a potential means of improving the situation but organisational and scientific problems need to be overcome. In order to address these, the CIPM decided in 1993 to establish an international, collaborative programme of metrology in chemistry. This programme is organised through the CIPM’s Consultative Committee on Amount of Substance (CCQM). The CCQM aims to resolve the practical difficulties of achieving comparable chemical measurements through traceability and to provide an international structure based on a chain of national and regional laboratories. These laboratories demonstrate the equivalence of their measurement data through measurement comparisons as well as implementing a quality management system for their calibration or measurement certificates. The CCQM, and corresponding regional metrology organisations, have organised a series of key comparisons which reflect applications relevant to industry, trade, health, environment, etc., and not just measurements on single substances or calibration solutions.

The key comparisons of the CCQM together with related activities of the CIPM MRA form the basis for a system of metrology in chemistry. Links to the system for field laboratories are provided by the NMIs and associated reference laboratories through the availability of traceable CRMs, calibration standards, and other calibration services such as reference values for PT or EQA schemes. Examples of the activities of the CCQM Inorganic Analysis Working Group and the work of some of the participating NMIs are included in this special metrology edition of JAAS. General information about metrology, the activities of the CCQM, and details of key comparisons are available on the BIPM web site (http://www.bipm.org). Information about the UK contribution, which is part of the DTI VAM programme, can be found on the VAM web site (http://www.vam.org.uk).

Mike Sargent

Chairman, CCQM Inorganic Analysis Working Group

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