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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.3, #8)
Meeting of JCGM (Joint Committee for Guides in Metrology), Working Group 2 (Successor of ISO/TAG 4/WG 1) April 30, 1998, Geneva, Switzerland
by Paul De Bièvre (pp. A340-A340).
Traceability and analytical chemistry by F. C. Adams (pp. 308-316).
The basic concepts of traceability as they are defined by the Comité Consultatif pour la Cluantité de Matière are contrasted with the practical exploitation in chemical analysis. The applicability of traceability concepts are tested for their practical applicability on four different analytical methodologies, neutron activation analysis, plasma mass spectrometry, beam microscopical analysis and speciation analysis of organometallic compounds.
Keywords: Key words Traceability; Certified reference materials; Primary methods; Analytical chemistry
Qualification and validation of software and computer systems in laboratories by L. Huber (pp. 317-321).
Existing software and computer systems in laboratories require retrospective evaluation and validation if their initial validation was not formally documented. The key steps in this process are similar to those for the validation of new software and systems: user requirements and system specification, formal qualification, and procedures to ensure ongoing performance during routine operation. The main difference is that frequently qualification of an existing system is based primarily on reliable operation and proof of performance in the past rather than on qualification during development and installation.
Keywords: Key words Validation; Qualification; Computers; Analytical laboratories; Existing systems
The influence of different evaluation techniques on the results of interlaboratory comparisons by Thomas Peter Josef Linsinger; Wolfgang Kandler; R. Krska; Manfred Grasserbauer (pp. 322-327).
The influence of different evaluation techniques on the results of an interlaboratory comparison for the determination of nutrients in ground- and surface water was investigated. The outlier-test procedure was found to influence the interlaboratory standard deviations (SDs), but not the averages. It was shown that even small differences in the numbers of outliers detected can change the SD severely. Comparing the outlier-test procedures of Hampel, Grubbs and Graf-Henning, it was found that Hampel's test detected the most outliers, thus generally resulting in smaller SDs between interlaboratory comparisons. The Graf-Henning test detected the fewest outliers and its application resulted in the highest SDs of the three test procedures investigated. The comparison of different summarising indices, namely the rescaled sum of z-scores, average of absolute z-scores and average deviation showed no comparability. Possibilities to improve the comparability of interlaboratory comparisons and to minimise misunderstandings are suggested.
Keywords: Key words Interlaboratory comparisons; Outliers; Z-scores
Measurement uncertainty and its meaning in legal metrology of environmental chemistry and public health by M. Buzoianu (pp. 328-334).
The need for reliability of measurements supporting legal decisions in environmental policy or medical diagnosis and treatment is well known and widely accepted. This prerequisite can be met only by ensuring that legal measurements are accurate and traceable to national or international standards. Consequently, an outline of the organizational structure of the Romanian National Institute of Metrology (INM) for ensuring uniformity, consistency and accuracy of all measurements including legal measurements performed in chemical laboratories is presented. Since reliable measurements can only be accomplished within an appropriate traceability chain, the experience of the INM in identification and evaluation of measurement uncertainty in legal activities concerning the environment and health is reviewed. Practical examples of measurement uncertainty evaluation in spectrophotometric determination of five analytes, commonly determined in environmental and clinical chemistry are described. The implications of measurement uncertainty for interpretation of regulatory compliance are discussed.
Keywords: Key words Measurement uncertainty; Analytical chemistry; Environment; Clinical chemistry
Uncertainty of measurement and legislative limits by E. Desimoni; Saverio Mannino (pp. 335-337).
The uncertainty affecting analytical measurements has to be taken into account when evaluating compliance of suspect matrices to legislative limits. To this aim Type 1 and 2 errors must be considered. This necessarily leads to the evaluation of the minimum detectable inadmissible signal, from which the minimum detectable inadmissible concentration can be obtained. The signal of suspect matrices thus has to be compared with the minimum detectable inadmissible signal. This paper aims to discuss practical problems involved in the comparison.
Keywords: Key words Measurement uncertainty; Legislative limits; α and β errors; Compliance
Derived measurement standards of reduced uncertainty – A contradiction? by W. Hässelbarth; Wolfram Bremser (pp. 337-339).
If the value of a derived measurement standard is assigned by comparison with a reference standard of the same quantity, the uncertainty is increased by the additional uncertainty on the difference measurement. This basic fact has lead to the general belief that the uncertainty of derived standards is always larger than that of the reference standards. However, if the value of a derived standard is assigned by comparison with several independent reference standards using an appropriate average, the increase of uncertainty due to the uncertainty on difference measurement may be counterbalanced by the the well-known decrease of uncertainty through averaging. The gain of accuracy made possible by this mechanism is restricted to second-generation standards. Further gain through iteration is prevented by correlation between standards derived from the same set of reference standards. As a consequence, the concept of metrological hierarchy levels, relating to traceability chains, becomes questionable for traceability networks.
Keywords: Key words Uncertainty; Measurement standards; Reference materials; Correlation; Traceability networks