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.     

The traceability of analytical measurements

 Traceability to the System International (SI) is an important prerequisite for international comparability and uniformity of chemical measurements to ensure mutual recognition of the results. In theory, all measurements can be traced back to the seven base units of the SI. Although the traceability system works well for most physical measurements, in many analytical and in some spectrophotometric measurements this system is not satisfactory. This paper describes the particular and practical problems and the contribution of the Romanian National Institute of Metrology in this field. The paper discusses the following concepts: clearly defined targets in the form of requirement specification, knowledge of trueness and/or measurement uncertainty, and traceability through an unbroken chain of calibration to primary standards. Traceability and uncertainty being two concepts inherently coupled, two examples of assessment of the uncertainty of measurement results are given for two spectrophotometric methods currently used in chemical laboratories. Received: 17 July 1996 Accepted: 2 September 1996     

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.     

Protocols for traceability in chemical analysis

 In continuing their attempt to bring general issues concerned with trustworthy chemical measurements to review and international discussion, the authors propose basic aims and requirements for protocols of chemical-measurement procedures with traceability to the SI or, where this is not possible, to units of internationally recognized measurement scales. Documents describing such protocols could be useful in science, technology, law, or trade. Concepts and definitions for protocols have been introduced in Part I of this contribution. Part II here deals with the development and application of protocols for intended in-laboratory, commercial, national, or international recognition. Protocols deal with measurement methods, instrumentation, and the estimation of uncertainties from all possible sources of measurement errors. Uncertainties define the quality of all links in a traceability chain starting from the value of a measurand in a sample, often through a certified value in a reference material, either to the SI, or – if this is not possible – to a value on a suitable, internationally agreed measurement scale. A protocol may concern itself with the complex interplay between uncertainties, tolerances, and any limit values introduced by the set aims of specific measurements. Received: 23 April 1997 Accepted: 27 April 1997     

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.     

Neutron activation analysis: A primary method of measurement

Neutron activation analysis (NAA), based on the comparator method, has the potential to fulfill the requirements of a primary ratio method as defined in 1998 by the Comité Consultatif pour la Quantité de Matière — Métrologie en Chimie (CCQM, Consultative Committee on Amount of Substance — Metrology in Chemistry). This thesis is evidenced in this paper in three chapters by: demonstration that the method is fully physically and chemically understood; that a measurement equation can be written down in which the values of all parameters have dimensions in SI units and thus having the potential for metrological traceability to these units; that all contributions to uncertainty of measurement can be quantitatively evaluated, underpinning the metrological traceability; and that the performance of NAA in CCQM key-comparisons of trace elements in complex matrices between 2000 and 2007 is similar to the performance of Isotope Dilution Mass Spectrometry (IDMS), which had been formerly designated by the CCQM as a primary ratio method.     

Practical considerations on the traceability to conventional scales

 The basic concepts of traceability as they are defined by the Comité Consultatif pour la Quantité de Matière (CCQM) are difficult to apply to some chemical results. For instance, for some environments or chemical analyses measurement results are expressed in conventional units. Such units are realized on conventional scales relying on two fundamental pillars: reference materials and standard specification. The octane number of fuel or water turbidity measurements are typical examples of such units. Traceability concepts are discussed in terms of their practical applicability for turbidimetric analysis. Some outcomes on the validation of the metrological performance of turbidimeters and the comparability of turbidity measurement results are also presented. Received: 8 June 1999 / Accepted: 13 December 1999     

Link to the SI via primary direct methods

 The possible approaches to realising a link to the SI system and the status of primary direct methods in the traceability chain of chemical measurements are discussed. Some results obtained with the new coulometric standard system are presented.

     

Link to the SI via primary direct methods

The possible approaches to realising a link to the SI system and the status of primary direct methods in the traceability chain of chemical measurements are discussed. Some results obtained with the new coulometric standard system are presented.     

The role of the CCQM OAWG in providing SI traceable calibrators for organic chemical measurements

Metrological traceability for organic chemical measurements is a documented unbroken chain of calibrations with stated uncertainties that ideally link the measurement result for a sample to a primary calibrator in appropriate SI units (e.g., mass fraction). A comprehensive chemical purity determination of the organic calibrator is required to ensure a true assessment of this result. We explore the evolution of chemical purity capabilities across metrology institute members of the Consultative Committee for Amount of Substance: Metrology in Chemistry and Biology’s Organic Analysis Working Group (OAWG). The OAWG work program has promoted the development of robust measurement capabilities, using indirect “mass balance” determinations via rigorous assessment of impurities and direct determination using quantitative nuclear magnetic resonance spectroscopy methods. A combination of mass balance and qNMR has been shown to provide a best practice approach. Awareness of the importance of the traceability of organic calibrators continues to grow across stakeholder groups, particularly in key areas such as clinical chemistry where activities related to the Joint Committee for Traceability in Laboratory Medicine have raised the profile of traceable calibrators.

     

Traceability in perspective

Results that reference SI units rarely pose problems in chemical measurement because traceable standards, with uncertainties derived from a chain of calibrations from the SI, are readily available at the analyst??s bench. These uncertainties are nearly always far smaller than that required for fitness for purpose in the analytical result. Moreover, the greater part of the uncertainty in a typical result is not derived from primary measurements traceable to the SI but from recovery problems and matrix effects. Even so, the incidence of wildly inaccurate results stems not from this uncertainty but from ??blunders??, deviations from the correct procedure. Attention to traceability beyond that employed by any competent analyst therefore cannot reduce the uncertainty. Furthermore, there is no rational reason to reduce the uncertainty if the result is already fit for purpose. The current focus on traceability is distracting analysts from the more pressing task of eliminating blunders.     

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).

     

Measurement principles for traceability in chemical analysis

 By the definition of the mole as a base unit for amount-of-substance measures within the International System of Units (SI), chemists can make chemical measurements in full compliance with established metrological principles. Since the mole requires exact knowledge of the chemical entity, which is often neither available nor of practical relevance to the purpose of the measurement, the SI units of mass or length (for volume) are unavoidable in the expression of results of many chemical measurements. Science, technology, and trade depend upon a huge and ever increasing number and variety of chemical determinations to quantify material composition and quality. Thus, international harmonization in the assessments of processes, procedures, and results is highly desirable and clearly cost effective. The authors, with relevant experience and responsibilities in Europe and America, have found some consensus in the interpretation of the metrological principles for chemical measurements, but believe open discussion should precede wide implementation by chemical communities. In fostering this dialogue, this paper shows, for instance, that more precise interpretation of the definitions for "traceability," "calibration," and "validation" is needed for present-day chemical measurements. Problems that face scientists in making measurements do not all vanish just by adherence to the SI. However, such compliance can improve communication among chemists and metrologists. Received: 30 June 1995 Accepted: 30 June 1995     

Preparation and certification of creatinine and urea reference materials with certified purity as a traceability source in clinical chemical measurements

Purity certified reference materials (CRMs) are playing a key role in metrological traceability, because they form the basis for many traceability chains in chemistry. Recently, the National Metrology Institute of Japan (NMIJ) has developed two purity CRMs for creatinine (NMIJ CRM 6005-a) and urea (NMIJ CRM 6006-a), because the concentrations of these two compounds are frequently measured in clinical laboratories for monitoring the renal functions. In the certification of purity CRMs, it is essential that the materials have been thoroughly characterized for purity, and the purity should preferably be determined directly by a primary method of measurements. In the development of these two CRMs, we used the purified materials as candidates. The certified values were assigned based on the results of two different methods; acidimetric titration and nitrogen determination by the Kjeldahl method. Since both methods cannot distinguish some impurities from the target compounds, major impurities in the candidate materials were also identified, quantified, and subtracted. These CRMs can provide a traceability link between routine clinical methods and SI units. Presented at BERM-11, October 2007, Tsukuba, Japan.     

Interlaboratory comparison for the determination of pesticide residues in Chinese cabbage

The results of and findings from an interlaboratory comparison among laboratories carrying out food testing of pesticide residues in the APEC (Asia–Pacific Economic Cooperation) region are presented and discussed to show critical roles of chemical metrology infrastructure in establishing traceability of measurements and in supporting existing measurement capability in safety and quality of food trade. The study material, which was prepared and certified by Korea Research Institute of Standards and Science (KRISS), was freeze-dried Chinese cabbage powder fortified with two organophosphorous pesticides (diazinon and chlorpyrifos). Among 14 participants, 12 laboratories were accredited based on ISO/IEC17025 and one laboratory was under assessment for the accreditation at the time of this study. Though all participants demonstrated very good intra-day repeatability and inter-day intermediate precision, many of them showed a large bias from the certified values. It is suggested that in addition to the accreditation system, economies are encouraged to develop appropriate chemical metrology infrastructure, which could effectively support laboratories to assure measurement traceability to SI, for which NMIs could play significant roles through their metrological services recognized in Mutual Recognition Arrangement (MRA) of the International Committee for Weights and Measures (Comité International des Poids et Mesures, CIPM).     

Metrological traceability is not always a straight line

The two most important concepts in metrology are certainly “traceability to standards” and “measurement uncertainty evaluation”. So far the questions related to these concepts have been reasonably solved in the metrology of “classical quantities”, but for the introduction of metrological concepts in new fields, such as chemistry and biology, a lot of problems remain and must be solved in order to support international arrangements. In this presentation, the authors want to develop the strategy implemented at Laboratoire national de métrologie et d’essais (LNE) in metrology in chemistry and biology. The strategy is based on: (1) pure solutions for calibration of analytical instruments, (2) use of certified reference materials (matrix reference materials), and (3) participation to proficiency testing schemes. Examples will be presented in organic and inorganic chemistry. For laboratory medicine, proficiency testing providers play an important role in the organization of External Quality Assessment Schemes. For the time being, the reference value or the assigned value of the comparison is calculated with the results obtained by the participants. This assigned value is not often traceable to SI units. One of the methods suggested by LNE is to ensure the metrological traceability to SI units of the assigned value for the more critical quantities carried on analytes by implementing the Joint committee for traceability in laboratory medicine reference methods.     

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).     

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