The practical realization of the traceability of chemical measurements standards

 Metrology is based on the concept of traceability. Traceability provides a means of relating measurement results to common standards thereby helping to ensure that measurements made in different laboratories are comparable. Good progress has been made in the application of metrological principles to chemical measurement, but there remains confusion about how you actually achieve traceability in a practical way.

This paper elaborates on the meaning and application of much used phrases such as 'the value of a standard', 'stated references', 'unbroken chain of comparisons', and 'stated uncertainties'. It also explains how traceability can be established in a practical way for different types of stated references, namely pure substance reference materials, matrix reference materials, and primary and reference methods. Finally, traceability chains for some typical examples of chemical measurement are described.

     

Stated references for ensuring traceability of chemical measurements for long-term environmental monitoring

Traceability is now considered to be a key concept in chemical and biological measurement sciences. While this concept is increasingly used in many areas, it is still prone to misunderstandings with respect to more ‘classical' terms such as e.g. accuracy. Traceability implies that measurement data are linked to stated references through an unbroken chain of comparisons, all with stated uncertainties. What does the word ‘stated references' mean when it is applied to complex environmental analysis? This paper discusses how traceability can be conceived in the context of environmental monitoring, in particular the various stated references (documented standards, reference materials, environmental specimens) to which chemical environmental data may be linked to.     

The role of reference materials

In this article the role of reference materials is confined to chemical measurements only. Recognized reference materials are one of the tools to obtain comparability of analytical results. Recognition demands confidence in the reference materials and in the reference material producers. A reference material producer is a technical competent body that is fully responsible for the certified or other property values of the reference material. The "analyte" has to be specified in relation to the selectivity of analytical procedure. The full range of reference materials can be presented as a three-dimensional space of the coordinates: analyte, matrix and application. If reference materials are used for calibration or correction of calibrations they establish the traceability of results of chemical measurements. The traceability is only valid within a stated range of uncertainty. Pure substances can represent the unit of amount of substance. A precondition is the microscale specification of the analyte and the accurate determination of the main component and/or the impurities.     

The role of reference materials

 In this article the role of reference materials is confined to chemical measurements only. Recognized reference materials are one of the tools to obtain comparability of analytical results. Recognition demands confidence in the reference materials and in the reference material producers. A reference material producer is a technical competent body that is fully responsible for the certified or other property values of the reference material. The "analyte" has to be specified in relation to the selectivity of analytical procedure. The full range of reference materials can be presented as a three-dimensional space of the coordinates: analyte, matrix and application. If reference materials are used for calibration or correction of calibrations they establish the traceability of results of chemical measurements. The traceability is only valid within a stated range of uncertainty. Pure substances can represent the unit of amount of substance. A precondition is the microscale specification of the analyte and the accurate determination of the main component and/or the impurities.

     

Some aspects of the evaluation of measurement uncertainty using reference materials

 In practice there are three aspects that need to be considered in order to achieve the required traceability according to its definition: the 'stated reference', the 'unbroken chain of calibrations' and the "stated uncertainty". For a certain chemical result, each of these aspects highly depends on the measurement uncertainty, both on its magnitude and how it was estimated. Therefore, the paper describes the experience of the Romanian National Institute of Metrology in estimating measurement uncertainty during the certification of reference materials (RMs), in metrological activities (calibration, pattern approval, periodical verification, etc.), as well as during the analytical measurement process. Practical examples of estimation of measurement uncertainty using RMs or certified reference materials are discussed for their applicability in spectrophotometric and turbidimetric analysis. Use of the analysis of variance to obtain some additional information on the components of measurement uncertainty and to identify the magnitude of individual random effects is described. Received: 12 November 1999 / Accepted: 25 February 2000     

On practical metrology and chemical analysis: legal,institutional and technical evolutions in the Australian national measurement system

 Economic and technological change, regional and international trade and the globalisation of industry have led to intense pressures for improvements to analytical quality, reliability and comparability. Of central importance are national traceability structures connecting chemical measurements in the field with internationally accepted measurement units and their practical realisations. Australia has a developed physical and engineering measurement system, a legislative framework for analytical traceability and, in the National Association of Testing Authorities, a recognised laboratory accreditation system. The need has been identified to develop the technical capability to perform matrix-independent reference measurements for the certification of traceable reference materials, useable as practical analytical etalons to establish metrological control systems in field measurements for amounts of substance. Recently, a unique collaborative consortium has proposed a National Analytical Reference Laboratory (NARL). The NARL is designed to be a metrological mass spectrometry facility for the transference of measurement units to more widely useable chemical measurement standards and reference materials. Received: 10 October 1995 Accepted: 26 October 1995     

Some aspects of the evaluation of measurement uncertainty using reference materials

To ensure and to confirm the required traceability according to the definition given in the International Vocabulary of Basic and Standard Terms in Metrology, three main aspects need to be considered in practice: “stated reference”, “unbroken chain of calibration” and “stated uncertainty”. For a certain spectrochemical result, each of the aspects above mentioned is highly dependent on measurement uncertainty, both on its magnitude and how it was estimated. The paper describes the experience of the Romanian National Institute of Metrology (INM) in estimating measurement uncertainty during certification of reference materials, in metrological calibration and during specific analytical processes. Practical examples of the use of reference materials or certified reference materials issued by the INM to estimate measurement uncertainty are discussed for their applicability in spectrochemical and turbidity analysis. Some aspects of the use of analysis of variance (ANOVA) to obtain additional information on the components of measurement uncertainty and to identify the magnitude of individual random effects are presented.     

The key elements of traceability in chemical measurement: agreed or still under debate?

 Talking about "traceability" means talking about a "property of the result of a measurement", about "the value of a standard", about "stated references" and about an "unbroken chain of comparisons". It describes by which comparison, and to which other value, the result of a measurement has been obtained, i.e. is "traceable to". It is about the underlying structure of the measurement process of the result of a measurement and therefore about the authority of the result. Since values carried by (certified) reference materials have also been obtained by measurement, the definition of traceability equally applies. Traceability in the context of reference materials is also about the authority of the values carried by the (certified) reference materials and is, therefore, of key importance for the authority of the reference materials themselves. Hence, values of results of measurements constitute part of the traceability chain and their uncertainties are an intrinsic accompanying phenomenon. Uncertainties need a traceability chain against which they can be evaluated, and a traceability chain is an a priori requirement for evaluating the uncertainty budget of a measurement result. An attempt has been made to exemplify "traceability" chains in some types of chemical measurement and to identify the degree of international agreement on the key elements of "traceability". It is concluded that there is less than universal agreement on this issue. The debate should continue in order to arrive at the international understanding and agreement needed, as "traceability" is now being incorporated in the International Organization for Standardization (ISO), the International Laboratory Accreditation Co-operation (ILAC) and in other "guiding" or regulatory documents. It is also the reason why the Institute for Reference Materials and Measurements (IRMM) has taken up the study of the concept in its core programme on Metrology in Chemistry, and why it sponsored the Workshop in Bratislava.

     

Some aspects of the evaluation of measurement uncertainty using reference materials

To ensure and to confirm the required traceability according to the definition given in the International Vocabulary of Basic and Standard Terms in Metrology, three main aspects need to be considered in practice: "stated reference", "unbroken chain of calibration" and "stated uncertainty". For a certain spectrochemical result, each of the aspects above mentioned is highly dependent on measurement uncertainty, both on its magnitude and how it was estimated. The paper describes the experience of the Romanian National Institute of Metrology (INM) in estimating measurement uncertainty during certification of reference materials, in metrological calibration and during specific analytical processes. Practical examples of the use of reference materials or certified reference materials issued by the INM to estimate measurement uncertainty are discussed for their applicability in spectrochemical and turbidity analysis. Some aspects of the use of analysis of variance (ANOVA) to obtain additional information on the components of measurement uncertainty and to identify the magnitude of individual random effects are presented.     

Quality and the development of reference materials, including the role of traceability and comparability

The global recognition that quality is an economic issue is requiring analytical chemists to look at the chemical measurement process in a way that has not been done before. Much work has been done in certifying reference materials, writing measurement protocols, creating measurement networks, developing analytical measurement techniques and other efforts to make good measurements. This article explores the meaning of quality in chemical measurements and discusses quality in terms of credibility, reliability, traceability and comparability. The importance of understanding the contribution of comparability and traceability to quality in chemical measurements and chemical metrology is emphasized.     

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     

A framework for the application and materials in analytical chemistry

 Working group 5 of EuraChem Nederland has developed a framework for the implementation of reference materials in analytical chemistry. In this discussed paper, the framework is proposed as a tool for the development of standard operation procedures (SOPs) in laboratories. The implementation of (certified) reference materials in these SOPs is of major importance in establishing comparability and traceability in measurement results, which in turn play a crucial role in measurement in support of trade, environmental issues, and characterisation of materials. Recent developments in the field of uncertainty analysis require the application of reference materials. It is recognised that the calculation of the combined measurement uncertainty becomes almost impossible without the use of certified reference materials with a stated uncertainty. Received: 1 December 1995 Accepted: 20 December 1995     

The key elements of traceability in chemical measurement: agreed or still under debate?

Talking about "traceability" means talking about a "property of the result of a measurement", about "the value of a standard", about "stated references" and about an "unbroken chain of comparisons". It describes by which comparison, and to which other value, the result of a measurement has been obtained, i.e. is "traceable to". It is about the underlying structure of the measurement process of the result of a measurement and therefore about the authority of the result. Since values carried by (certified) reference materials have also been obtained by measurement, the definition of traceability equally applies. Traceability in the context of reference materials is also about the authority of the values carried by the (certified) reference materials and is, therefore, of key importance for the authority of the reference materials themselves. Hence, values of results of measurements constitute part of the traceability chain and their uncertainties are an intrinsic accompanying phenomenon. Uncertainties need a traceability chain against which they can be evaluated, and a traceability chain is an a priori requirement for evaluating the uncertainty budget of a measurement result. An attempt has been made to exemplify "traceability" chains in some types of chemical measurement and to identify the degree of international agreement on the key elements of "traceability". It is concluded that there is less than universal agreement on this issue. The debate should continue in order to arrive at the international understanding and agreement needed, as "traceability" is now being incorporated in the International Organization for Standardization (ISO), the International Laboratory Accreditation Co-operation (ILAC) and in other "guiding" or regulatory documents. It is also the reason why the Institute for Reference Materials and Measurements (IRMM) has taken up the study of the concept in its core programme on Metrology in Chemistry, and why it sponsored the Workshop in Bratislava.     

Traceability of (values carried by) reference materials

 Traceability is a property of the result of a measurement. Since values carried by (reference) materials must also have been obtained, of necessity, by measurement, the definition of traceability also applies to reference materials. It is extremely helpful to give the traceability (of the origin) of a reference material a separate name, i.e. 'trackability'. An analysis of the function of values carried by reference materials, shows that they can fulfill different functions, depending on the intended use. One of the functions located outside the traceability chain – and hence not very relevant for establishing traceability – is evaluating the approximate size of the uncertainty of the measurement of an unknown sample by performing a similar measurement on a reference material, used as a 'simulated sample'. Another function is located inside the traceability chain, where the reference material is used as an added 'internal standard'. Then, the value carried by the reference material is essential for establishing the traceability of the measured value of an unknown sample. In the latter application, the reference material acts as an 'amount standard' (the certified value for amount is used). Received: 11 November 1999 / Accepted: 24 February 2000     

The history and development of a rigorous metrological basis for pH measurements

This paper discusses the basis and historical development of the traceability chain for pH. The quantity pH, first introduced in 1909, is among the most frequently measured analytical quantities. The practical measurement of the pH value of a sample is inexpensive, easy to perform, and yields a rapid result. However, the problems posed by the traceability of pH are not easy to solve. Most pH measurements are performed by potentiometry, using a glass electrode as the pH sensor. Such pH electrodes must be calibrated at regular intervals. Confidence in the reliability of pH measurements requires establishment of a metrological hierarchy including an uncertainty budget for calibration that links the pH measured in the sample to an internationally agreed and stated reference. For pH, this reference is the primary measurement of pH. A traceability chain can be established that links field measurements of pH to primary buffer solutions that are certified using this primary method. This allows the user in the field to estimate the measurement uncertainty of the measured pH data. As the realization of the primary measurement is sophisticated and time-consuming, primary standards are generally realized at national metrology institutes. A number of potentiometric methods are suitable for the determination of the pH of reference buffer solutions by comparison with the primary standard buffers. The choice between the methods should be made according to the uncertainty required for the application. For reference buffer solutions that have the same nominal composition as the primary standard, the differential potentiometric cell, often called the Baucke cell, is recommended.     

Traceability of environmental chemical analyses: can theory match practice?

According to the ISO definition, the traceability concept basically implies that measurement data are linked to stated references through an unbroken chain of comparisons, all with stated uncertainties. This concept may be quite clear in theory, but we may wonder how it may be applicable to complex chemical measurements such as environmental chemical analyses in practice. This paper discusses this issue, giving some examples of drawbacks that are being faced in different environmental sectors (water, sediment, soil, biota and particulate atmospheric samples).     

Using high-performance quantitative NMR (HP-qNMR®) for certifying traceable and highly accurate purity values of organic reference materials with uncertainties <0.1 %

Quantitative nuclear magnetic resonance (qNMR) in combination with metrological weighing is optimised to demonstrate the power of the qNMR measurement method. It is shown that with ¹H-qNMR it is possible to certify the purity of organic reference materials (expressed as mass fraction) with relative expanded uncertainties of <0.1 % for a 95 % confidence interval (k = 2). Following well-defined selection criteria, a set of twelve different chemical compounds is evaluated and certified to serve as internal references for ¹H-qNMR measurements. A series of comparison measurements is made amongst a subset of the selected compounds. The purity of maleic acid is determined by six different ¹H-qNMR measurement series, and all results show full consistency. All the six mean values are covered within the range of ±0.05 %. In two more measurement series, four different nuclei are analysed within the same sample against one calibrator. Even with non-optimised signal intensity ratios and varying signal pattern, a high consistency was obtained. Therefore, the validity and robustness of ¹H-qNMR measurement results are demonstrated. ¹H-qNMR measurement results are directly traceable to a variety of internationally accepted primary reference materials, and therefore, traceability to SI units is obtained. All experiments are performed under ISO/IEC 17025 and ISO Guide 34 accreditation.     

The traceability scheme in chemical measurement

 Traceability is an essential property of a measurement result. However, it is recognized that the results of chemical measurements can be lacking in this property. In this paper we try to show how to understand and establish traceability in chemical measurement. The traceability connotation and the necessity of tracing back to SI units are described by means of comparability well-known. The roles and interrelationships of quality assurance, accreditation, calibration, reference material, analytical method, comparison and uncertainty in establishing traceability are explained with the aid of a block diagram. The paper also includes diagrams illustrating the Chinese situation and experience of establishing traceability for chemical measurement in China.     

What is new in certified reference materials and measurements at IRMM?

IRMM is developing more and more isotopic spike reference materials since these are increasingly needed in trace analysis. In addition, they appear to play an increasingly important role in ensuring traceability to the SI system of isotope-specific measurements made by e.g. isotope dilution mass spectometry (IDMS). Their present availability and future development is summarised. IDMS is also applied in a definitive way to establish reference measurements for IRMM's international measurement evaluation programme (IMEP), which aims at the realisation of traceability for field laboratories. IRMM uses its advanced technology for the preparation and contamination-free handling of large quantities of biological and environmental reference materials. These facilities are used for the preparation of both BCR CRM's (orange juice, sheep milk curd, sediments, etc.) and reference materials for private customers (catalytic converter materials, apple powder, flour, animal innards, tomato powder, etc.).     

Traceability to units

On the grounds that clear and direct communication is required of us today, it is proposed that traceability be regarded as the ability to demonstrate that measurements are what they are purported to be and that traceability is thus to measurement units rather than reference values per se. It is suggested that such an approach may give greater flexibility in the establishment, maintenance and propagation of traceability, and that accreditation practices are becoming central to the practical establishment of traceability for chemical and biological measurement.