Metrological traceability of chemical analyses

The standard ISO 17025:2017 The "General Requirements for the Competence of Testing and Calibration Laboratories" specifies that the traceability of measurement results to the units of the International System (SI) must be established either by calibration, by certified reference materials themselves traceable to the SI, or by direct realizations of SI units guaranteed by means of a comparison with national or international standards.

In cases where it is technically impossible to establish metrological traceability to SI units, standard ISO 17025:2017 requires the laboratory to demonstrate traceability of measurement results to appropriate references such as certified reference materials (not traceable to the SI) or approved specified methods (standards or reference methods).

This requirement is designed to ensure that the references used by testing and calibration laboratories are common and, consequently, that the measurements they produce are comparable.

While traceability schemes for most physical measurements have long been established, the same cannot be said for chemical measurements. Indeed, chemical metrology is a young metrology: the definition of the mole came into being in 1971, almost a century after the signing of the Metre Convention in 1875, and the corresponding Consultative Committee was only set up in 1993.

The multiplicity of parameters measured and the complexity of the measurement processes implemented by laboratories make it extremely difficult to establish traceability schemes for chemical analyses.

The steps involved in preparing samples for analysis, and the differences in the response of measurement systems depending on the nature of the sample matrix, are often the cause of breaks in the traceability chain.

However, laboratories have long since developed a number of tools to ensure the comparability of their analytical results. These tools include standardized methods, reference materials and interlaboratory comparisons.

Interlaboratory comparisons, while essential for ensuring the comparability of measurement results, do not generally guarantee the traceability of measurement results. Only the role of certified reference materials and standardized methods in establishing the traceability of chemical analysis results is described in this article.