On practical metrology and chemical analysis: etalons and traceability systems

 National measurement systems are infrastructures to ensure, for each nation, a consistent and internationally recognised basis for measurement. Such complex systems have historical, technical, legal, organisational and institutional aspects to connect scientific metrology with practical measurements. Underlying any valid measurement is a chain of comparisons linking the measurement to an accepted standard. The ways the links are forged and the etalons (measurement standards) to which they connect are defining characteristics of all measurement systems. This is often referred to as traceability which aims at basing measurements in common measurement units – a key issue for the integration of quantitative chemical analysis with the evolving physical and engineering measurement systems. Adequate traceability and metrological control make possible new technical capabilities and new levels of quality assurance and confidence by users in the accuracy and integrity of quantitative analytical results. Traceability for chemical measurements is difficult to achieve and harder to demonstrate. The supply of appropriate etalons is critical to the development of metrology systems for chemical analysis. An approach is suggested that involves the development of networks of specialised reference laboratories able to make matrix-independent reference measurements on submitted samples, which may then be used as reference materials by an originating laboratory using its practical measurement procedures. Received: 31 July 1995 Accepted: 19 August 1995     

Metrological role of neutron activation analysis. IB. Inherent characteristics of relative INAA as a primary ratio method of measurement

This is the second part of a paper which deals with the advantages and disadvantages of relative instrumental neutron activation analysis concerning traceability and uncertainty, and the current scope and possible future extensions of neutron activation analysis as a primary ratio method. The first part of this paper has been published in this Journal [1]. Received: 19 March 2001 Accepted: 2 October 2001     

Comparability of analytical results obtained in proficiency testing based on a metrological approach

A “yes–no” type of criterion is proposed for the assessment of comparability of proficiency testing (PT) results when the PT scheme is based on a metrological approach, i.e. on the use of a reference material as the test sample, etc. The criterion tests a null hypothesis concerning the insignificance of a bias of the mean of the results from a traceable value certified in the reference material used for the PT. Reliability of such assessment is determined by the probabilities of not rejecting the null hypothesis when it is true, and rejecting it when it is false (the alternative hypothesis is true). It is shown that a number of chemical, metrological and statistical reasons should be taken into account for careful formulation of the hypotheses, enabling the avoidance of an erroneous assessment of the comparability. The criterion can be helpful for PT providers and laboratory accreditation bodies in analysis of PT results.     

Traceable measurements in clinical laboratories

 Reliable, traceable and comparable measurements provide the rational basis for evaluation of the quality of a result and the starting point for recognized laboratory accreditation in any national area. Modern medical diagnostics and treatment involve rapidly rising numbers and types of clinical laboratory measurements, that are reliable. Therefore, the basic principles to be followed to assure the traceability of clinical measurements as required by the Romanian Laws of Metrology are reviewed. Main sources affecting the quality of the unbroken chain of calibrations that relate the measurements back to appropriate measurement standards are discussed. Examples of how to achieve traceable measurements in clinical laboratories are presented. Details of specific uses of reference materials, measuring instruments and standard measurement methods are also discussed. Received: 8 January 1998 · Accepted: 21 April 1998     

Traceability and analytical chemistry

 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. Received: 31 March 1998 · Accepted: 6 June 1998     

A way of establishing quality of measurements in gas analysis—national and international aspects

Standardising quality of measurements at both the national and international level results in a unity in measurement that may be regarded a synonym of traceability. In gas analysis, the key issue in achieving this aim is production of gaseous reference materials. Establishing a network of secondary measurement standards at gaseous CRM producers’ sites in Ukraine might be interesting for those involved in gas analysis activities. Inevitably, problems concerning the recognition of measuring capabilities of both national metrological institutes and local CRM producers are now an international issue. Thus, the problems of national adoption of international standards, as well as harmonisation between different international documents are currently relevant, and Ukrainian experience may be useful to others in this field. Presented at the 3rd International Conference on Metrology, November 2006, Tel Aviv, Israel.     

Evaluation of the limit of performance of an analytical method based on a statistical calculation of its critical concentrations according to ISO standard 11843: Application to routine control of banned veterinary drug residues in food according to European Decision 657/2002/EC

Traceability of the measurement of analytical parameters capable of evaluating the performance of methods is an important concept for the assessment of quality for the routine control, especially for residue monitoring of non-authorized medicinal substances in food from animal origin. The European Decision no. 657/2002/EC recommends to calculate two statistical limits, CCα and CCβ, which allow to evaluate the critical concentrations above which the method reliably distinguish and quantify a substance taking into account the variability of the method and the statistical risk to take a wrong decision. The calculation, which can be derived from the ISO standard no. 11843 is applied on a routine basis. An example displays a very simple way for evaluating the performance of an LC-MSMS method which has been validated a few years ago and is qualified onto a Micromass Quattro LCZ tandem mass spectrometer to monitor and confirm the nitrofuran metabolite residues in food from animal origin. Community Reference Laboratory for Antimicrobial Veterinary Drug Residue Control in Food from Animal Origin     

Primary methods of measurement in chemical analysis

 Primary methods of measurement have a central function in metrology. They are an essential component in the realisation of the SI units and therefore are indispensable for establishing traceability of measurements of all kinds of physical quantities to the corresponding SI units. This is also true for chemical analysis. Gravimetry, titrimetry, coulometry, and isotope dilution mass spectrometry (IDMS) are evaluated with regard to their potential to be primary methods according to a general definition of primary methods recently given by the Comité Consultatif pour la Quantité de Matière (CCQM). Optical absorption spectrometry and methods based on colligative properties are also considered. A general scheme for establishing traceability of chemical measurements to the SI units using primary methods is discussed. Received: 17 April 1997 · Accepted: 9 August 1997     

Validation steps for traceability of linear calibrated chemical measurements

Based on the new draft of the EURACHEM/CITAC Guide Traceability in Chemical Measurement, this publication describes how traceability can be achieved for chemical measurements using a linear calibration function. Traceability can be accomplished without larger expenditure, if the measurement is calibrated on the basis of appropriate reference standards and the linear regression employed is selected and validated statistically in a suitable form. The determination of nickel in aqua regia eluates of sediment samples, employed for an ICP-OES measurement, is used as a practical illustration of this approach.     

Implementation of traceability – needs and perspective of the in-vitro-diagnosticum industry

Manufacturers support the concept of traceability. However, only a small number of the medically relevant measurands can be traced to the highest metrological order. In many cases, the measured substances are heterogeneous mixtures where traceability can be established only to either an international conventional reference measurement procedure or to a manufacturers own in-house reference system. The traceability concept needs to be seen in the context that the results of medical laboratories are not an aim per se, but are meant to provide useful medical information to clinicians, and that pre- and post-analytical steps may also contribute significantly to errors. There is a need for the further development of suitable reference measurement systems, but in view of the multitude of tasks and limited resources, priorities need to be set.Presented at BERM-9 – 9^th International Symposium on Biological and Environmental Reference Materials, 15–19 June 2003, Berlin, Germany