Comparability and recognition of chemical measurement results – an international goal

As a consequence of the globalisation of trade and industry and other human activities, reliability of and confidence in measurement results is increasingly required, also in the field of chemical analysis, so that measurements made in one country will be accepted in other countries without the necessity to repeat them. The prerequisite for confidence is comparability on the basis of known uncertainties which in turn are based on traceability to recognised references. Traceability structures for chemical measurements are required which, by providing calibration means traceable to national standards, allow uncertainty statements to be made at field level, thus establishing comparability. Such traceability structures are now being developed in all industrialised countries. To ensure international comparability, mutual recognition of the national activities in metrology in chemistry is required in addition. The Mutual Recognition Agreement (MRA) for national measurement standards and calibration certificates issued by national metrology institutes, which is currently under way within the framework of the Metre Convention, aimes at providing the necessary international confidence for all kinds of measurements. The field of chemical analysis is included in the international metrological infrastructure through the new Consultative Committee for Amount of Substance (CCQM). Carefully selected key comparison measurements, which cover the most important areas where traceability is required, and which are carried out by national metrology institutes in cooperation with other national institutes entrusted with the provision of part of the national references for chemical measurements, form the basis for declarations of equivalence under the MRA. The results of the first key comparisons and studies carried out so far clearly show that the group of laboratories involved in the key comparisons is capable of establishing the international references (key comparison reference values) for chemical measurements with sufficient accuracy, also in complicated matrices.     

A national traceability system for chemical measurements

Current developments in Germany for establishing a traceability system for chemical measurements are reported. The focus is on a dissemination mechanism which employs chemical calibration laboratories accredited within the framework of the German Calibration Service (DKD) and acting as "multipliers" between the national standards level and the user level by providing the user with calibration means which are traceable to the SI via national standards. At the national standards level, a network of high-level chemistry institutes coordinated by the national metrology institute, PTB, provides the primary references for chemical measurements.The use of the metrological dissemination system provided by the DKD also for chemical measurements is a logical extension of a traceability mechanism, successful for more than two decades in general metrology, to metrology in chemistry. In detail, traceability structures in clinical chemistry, electrochemistry, elemental analysis and gas analysis are described. This system has become an important part of the efforts made in Germany to support chemical laboratories in meeting the traceability requirements of the market and of legal regulations.     

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     

Achieving traceability in chemical measurement—a metrological approach to proficiency testing

ISO/IEC 17025 requires that testing laboratories establish the traceability of their measurements, preferably to the SI units of measurement. The responsibility for establishing traceability lies with each individual laboratory and must be achieved by following a metrological approach.The results of measurements made in such a way are traceable to the standards used in method validation and to the calibration standards used during the measurement process. If these standards are traceable to SI then the measurements will also be traceable to SI.Participation in appropriate proficiency studies (an ISO/IEC 17025 requirement) enables laboratories to demonstrate the comparability of their measurements. If the materials used for the studies have traceable assigned values, then proficiency testing also provides information about measurement accuracy and confirms, or otherwise, that appropriate traceability has been established. This paper will report on a new approach for the establishment of traceable assigned values for chemical testing proficiency studies. The work is conducted at a "fit for purpose" level of measurement uncertainty, with costs contained at a level similar to previous "consensus" based proficiency studies. By establishing traceable assigned values in a cost effective way, NARL aims to demonstrate the added value of the metrological approach to participant laboratories.     

Traceable measurements of pH

 The primary method for pH is based on the measurement of the potential difference of an electrochemical cell containing a platinum hydrogen electrode and a silver/silver chloride reference electrode, often called a Harned cell. Assumptions must be made to relate the operation of this cell to the thermodynamic definition of pH. National metrology institutes use the primary method to assign pH values to a limited number of primary standards (PS). The required comparability of pH can be ensured only if the buffers used for the calibration of pH meter-electrode assemblies are traceable to these primary pH standards. To assess the degree of equivalence, comparisons of primary measurement procedures for pH were organized in co-operation with EUROMET. Typical results will be presented. In 1998, the Consultative Committee for Amount of Substance (CCQM) decided to include the field of pH in its working programme. The first key comparison for this quantity was recently carried out on two phosphate buffer solutions. Received: 14 June 2000 / Accepted: 22 August 2000     

Metrological concept for comparable measurement results under the European water framework directive: demonstration of its applicability in elemental analysis

Within the scope of a project of the “European Association of National Metrology Institutes??(EURAMET), a European metrological dissemination system (network) providing traceable reference values assigned to matrix materials for validation purposes is described and put to the test. It enables testing laboratories (TL) to obtain comparable results for measurements under the “EU Water Framework Directive 2000/60/EC??(WFD) and thus, to comply with a core requirement of this very directive. The dissemination system is characterized by the fact that it is available to all laboratories throughout Europe which intend to perform measurements in the context of the WFD and that it can ensure sustainable metrological traceability to the International System of Units (SI) as a reference point for the measurement results. This dissemination system is set up in a hierarchical manner and links up the level of the national metrology institutes (NMI) with that of the TLs via an intermediate level of calibration laboratories (CL) by comparison measurements. The CLs are expert laboratories with respect to the measurement of the analytes considered here (within the project, the CLs are called potential calibration laboratories (PCL)) and are additionally involved in the organization of comparison measurements within the scope of regional quality assurance (QA) systems. Three comparison measurements have been performed to support the approach. A total of about 130 laboratories participated in this exercise with the focus on the measurement of the priority substances Pb, Cd, Hg, and Ni defined in the WFD. The elemental concentrations in the water samples roughly corresponded to one of the established environmental quality standards (EQS), the annual average concentration (AA-EQS), which is defined in the daughter Directive 2008/105/EC of the WFD. It turned out that a significant number of TLs still need to improve their measurement methods in order to be able to fulfill the minimum requirements of the WFD, in particular, with regard to the elements Cd and Hg probably due to their low EQS values. Furthermore, it became obvious that the hierarchical dissemination system suggested here actually corresponds to the measuring capabilities of the three participating groups (NMIs, PCLs, and TLs).     

Interlaboratory system to ensure and improve the quality of glassware calibration and use in a large laboratory

This paper describes the implementation and methodology of an interlaboratory system that ensures the quality of glassware calibration and use in a large laboratory. The interlaboratory system involves periodic comparisons between laboratories with evaluations and improvements made over time. Two similar items are calibrated in each exercise according to a detailed calibration procedure. The reference value is traceable to the international system supplied by a metrology laboratory. The results are evaluated as normalized errors and analyzed by Youden graphs. The calibration procedure is presented. An interlaboratory experiment is described in which 7 participating laboratories performed calibrations of 2 volumetric flasks. The reported results, the interlaboratory evaluation, and the actions taken are presented.     

How to achieve international comparability for chemical measurements

It is the central aim of the current activities of metrology in chemistry to build confidence in the reliability of chemical measurement results so that they are accepted without costly duplication being necessary. An important prerequisite for such confidence is comparability based on traceability to recognised common references, ideally the SI units. Since metrology is organised within a national framework according to the national laws and regulations, a two-step procedure is to be followed to achieve international comparability for chemical measurements which is increasingly required as a result of the globalization of trade and economy: (1) establishment of national traceability structures for chemical measurements and (2) mutual recognition of the national traceability structures on the basis of equivalence criteria. The first step is at present being taken in many countries. Examples are presented for Germany. The second step has been initiated by the Mutual Recognition Arrangement (MRA) of the Meter Convention for national measurement standards and measurements and calibrations provided by national metrology institutes, which is based on international comparison measurements (key comparisons) carried out on the national standards level. Chemical analysis is included in this process through the Consultative Committee for Amount of Substance (CCQM).     

How to achieve international comparability for chemical measurements

 It is the central aim of the current activities of metrology in chemistry to build confidence in the reliability of chemical measurement results so that they are accepted without costly duplication being necessary. An important prerequisite for such confidence is comparability based on traceability to recognised common references, ideally the SI units. Since metrology is organised within a national framework according to the national laws and regulations, a two-step procedure is to be followed to achieve international comparability for chemical measurements which is increasingly required as a result of the globalization of trade and economy: (1) establishment of national traceability structures for chemical measurements and (2) mutual recognition of the national traceability structures on the basis of equivalence criteria. The first step is at present being taken in many countries. Examples are presented for Germany. The second step has been initiated by the Mutual Recognition Arrangement (MRA) of the Meter Convention for national measurement standards and measurements and calibrations provided by national metrology institutes, which is based on international comparison measurements (key comparisons) carried out on the national standards level. Chemical analysis is included in this process through the Consultative Committee for Amount of Substance (CCQM).

     

Traceability issues in measurement

This paper reviews the current state of play of the Mutual Recognition Arrangement created by the International Committee for Weights and Measures in 1999. The aim of the MRA is to provide a framework within which National Metrology Institutes can demonstrate the equivalence of their realisations of the units and quantities of the SI system to which accredited laboratories are traceable. The article offers some views on the need for traceable measurements, their relevance to technical barriers to trade, and the use that is being made of the MRA framework by national and international bodies.     

A convenient and economic approach to achieve SI-traceable reference values to be used in drinking-water interlaboratory comparisons

Metrologically traceable reference values add an essential benefit to interlaboratory comparisons: unlike consensus values, they can be used to establish national and international comparability. Furthermore, the participating laboratories obtain a reliable and unbiased benchmark to check their results for accuracy. Usually, metrologically traceable reference values are obtained by so-called primary methods which demand excessive efforts at great expense. Within the framework of two national drinking-water interlaboratory comparisons (proficiency testing rounds), a new approach to provide metrologically traceable reference values was applied. It is solely based on existing data which were collected during the comparison itself. Lead (Pb) measurements serve as an example to show how metrologically traceable reference values were derived from the lead amount added during sample preparation and the amount of lead already present in the drinking-water matrix used to prepare these samples. Within this approach, the matrix content is calculated in a way similar to a standard addition experiment. An uncertainty budget for the reference value was set up which describes the link to the corresponding SI units. Isotope dilution mass spectrometry (IDMS) as a primary method was used to validate this approach in the case of cadmium, chromium, copper, lead, and nickel.     

Development and supply system of reference materials based on the Measurement Law in Japan

 Analytical instruments used for measurements of air and water pollution are calibrated by using reference materials such as standard gases and standard solutions. In Japan, since the middle of the 1970s, those reference materials which are traceable to the national standards maintained at national research institutes have been supplied to users by reference material producers. In order to establish the primary standards and to secure the traceability from the working standards to the national ones, various analytical methods such as coulometric, titrimetric and gravimetric analyses for purity determination and highly sensitive atomic spectrometry for trace analysis have been developed as the primary methods and reference methods. The Japanese Measurement Law, revised in 1992, has introduced a new traceability system in which a public organization, a "designated calibration body", can also prepare and maintain the national standards under the advice and instruction of national research institutes. The designated calibration body can provide calibration services to reference material producers (accredited calibration bodies) by using the national standards. The reference materials supplied in conformity with the traceability system include standard gases, pH standard solutions, metal standard solutions and non-metal ion standard solutions. Received: 4 October 1996 Accepted: 2 December 1996     

International comparability of chemical measurement results

The international system of units (SI) is an internationally recognized system based on standards of long-term stability; by the use traceable measurements it provides an international infrastructure for realizing comparable measurements. The work of the Consultative Committee for Amount of Substance (CCQM) and the implementation of the Mutual Recognition Arrangement (MRA) are facilitating an international programme for metrology in chemistry to extend this infrastructure to the field of chemical measurements. The major points of this programme, which include the execution of international comparisons and the construction of a key comparison and calibration database at the BIPM, are described.     

Metrological traceability in gas analysis at I.N.Ri.M: gravimetric primary gas mixtures

Metrologically traceable measurements are needed, in environmental monitoring, to provide meaningful information on the pollution level and the possibility to have suitable reference standards is crucial to this purpose to calibrate instruments. The Italian National Institute of Metrological Research (I.N.Ri.M.), as a primary metrological institute, realizes and maintains primary standards for the basic and derived units of the International System of Units (SI). In this framework and regarding gas analysis, in the last few years I.N.Ri.M. has started a research line on the preparation of primary reference mixtures (PRMs) by the gravimetric method that led to plan and build a facility for evacuating and filling cylinders and a device for high precision weighing. In the present work the developments in such field at I.N.Ri.M. are reported to underline the relevance, in the atmospheric pollutants monitoring, of performing accurate and reliable measurements and having reference standards fit for the calibration of the analytical instrumentation.     

Proficiency evaluation as a traceability link in chemical metrology

Traceability implies comparison of the results of measurements, or comparison to national or international measurement standards. One of several approaches that have been used in chemistry to provide for such comparisons is distribution of proficiency evaluation materials which have been measured by a reference laboratory. A newer approach is based on receipt and measurement at a reference laboratory of materials that have been produced and analyzed by other laboratories. Traceability concepts and approaches to realization will be described together with discussion of the relative merits of various approaches. Extension into metrological fields other than chemistry will also be explored. Received: 14 November 2000 Accepted: 11 December 2000     

Measurement uncertainty and its meaning in legal metrology of environmental chemistry and public health

 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. Received: 3 January 1998 · Accepted: 9 June 1998     

The analysis of process gases: a review

 A general review of key issues involved in the analysis of process gases is presented. The reasons for such measurements – which include safety, quality, environmental and economic factors are considered. The technical issues arising from these measurements are dependent upon a variety of factors, including the overall sampling system, the type of analytical instrumentation, methods of data collection and the specified calibration protocols. The use of gas calibration cylinders as transfer standards is detailed and issues of stability and traceability to reference material discussed. Received: 1 March 2000 / Accepted: 31 March 2000     

UK delivery of traceable chemical measurements in the 21st century: building on the foundation of the VAM programme

The paper discusses the requirements for achieving traceable chemical measurements in the UK. It is emphasised that success will depend on establishing an appropriate UK chemical measurement infrastructure and encouraging reference and field laboratories to make use of it. The demanding requirements of the BIPM Mutual Recognition Arrangement (MRA) also require a point of focus to link UK reference laboratories into international metrology. Two key factors are described which have provided the UK with the means to meet these requirements and which have established a sound basis on which to build a system of traceable chemical measurements in the 21st century. These two factors are LGC's long-standing role as the UK's national centre for analytical chemistry and the development and delivery over many years of the UK's Valid Analytical Measurement (VAM) Programme.     

How to achieve world-wide mutual recognition of calibration and test results – or: Who accredits the accreditor?

 The ongoing globalization of the economy demands cost- and time-effective conformity assessment procedures through mutual cross-border recognition of calibration and test results. Accreditation is an important tool to ensure the technical competence of laboratories performing calibrations and tests. The question, however, arises as to which mechanisms should be applied to achieve technical equivalence of accreditations throughout the world as a basis for mutual recognition of agreements. To avoid third party "accreditation of accreditors", accreditation bodies have established a specific procedure of regional technical cooperation in order to create a climate of confidence resulting in formal mutual recognition agreements. Key elements of this cooperation are the harmonization of operation procedures in addition to existing international standards, a continuos programme of interlaboratory comparisons, and an extensive procedure for mutual peer assessments. The present article provides detailed information on such cooperation within the European region as an example, describes the present situation with respect to mutual recognition agreements within Europe, and gives a brief look into the future with respect to inter-regional cooperation. Received: 28 September 1996 Accepted: 2 November 1996