Meeting future needs for metrological traceability—a physicist’s view

Abstract Metrological traceability of measurement results provides the comparability needed for reliable measurement in almost every aspect of our daily life, ranging from fundamental science, through health and safety to global trade. From the authors (a physicist) perspective and understanding of recent literature, measurements in chemistry compare the amount of analyte, are often made indirectly, are sometimes not fully traceable to the SI, and metrological traceability is perhaps not always the major concern of the practising analytical chemist. The present article examines which of these views are typically chemical, and which are common to both the physicist and the chemist. Much can be gained by formulating a common understanding of basic concepts of traceability of measurement results in meeting ever-increasing demands for comparability of measurement results in both traditional as well as new areas of technology and societal concern.Based on a lecture given by the author at a mini-symposium Emerging issues in metrology in chemistry organised by the IUPAC Analytical Chemistry Division and IUPAC Working Party for Harmonisation of Quality Assurance at IAEA Headquarters, Vienna, 17th February 2004.     

Key metrological issues in proficiency testing––response to “Metrological compatibility-a key issue in further accreditation” by K. Heydorn

A discussion of proficiency testing (PT) topics started by Heydorn (Accred Qual Assur 15:643–645, 2010) is continued in the present paper. The role of PT in the accreditation of testing/analytical laboratories, the use of consensus values (average or weighted average, median, observed standard deviation, etc.) and a metrological background of PT schemes are discussed. It is shown that metrological traceability, comparability, and compatibility, as well as commutability of a reference material, are the key issues of any PT scheme that applies certified reference material as test items. Metrological compatibility of PT results in such schemes is a property demonstrating the closeness of the PT results to the certified value in comparison with the measurement uncertainty of their difference. The metrological background is especially important for the selection and use of PT schemes for a limited number of participants (fewer than 30) as detailed in IUPAC/CITAC Guide on the topic published in 2010 in Pure Appl Chem 82(5):1099–1135.     

Recent developments in Australia — reference materials and proficiency testing

The Australian Government Analytical Laboratories (AGAL) has historically been the major provider of chemical and micro biological analytical services to the Australian Government. An equally important role has been the provision of specialist functions associated with:

?development and implementation of new and improved testing methods;

?evaluation and commissioning of new instrumentation;

?advice and assistance to government with the design, implementation and conduct of proficiency testing programs, particularly in areas of chemical testing related to international trade.

In July 1993, in response to increasing international interest in chemical metrology, AGAL established a Valid Analytical Measurement program focused on improving the acceptability of laboratory data and supporting the development of appropriate standards through the National Standards Commission. AGAL has recently surveyed Australian residue testing laboratories to determine their priority requirements for reference materials and inter laboratory check sample programs. Survey results have clearly demonstrated a need for reasonably priced, certified reference materials for pesticide and antibiotic residue testing in raw agricultural produce. There are few reference materials available in this field of testing, particularly for residues of concern in Australia. AGAL has therefore initiated action as follows:

?Preparation and certification of two priority reference materials

1. organochlorines (dieldrin and heptachlor epoxide) in animal fat,
2. organophosphates (diazinon, chlorpyrifos and ethion) in animal fat.

?Investigation of storage stability of animal liver samples containing avermectin, sulphonamide and other antibiotic residues to ascertain suitability for reference material production.

In the area of proficiency testing, AGAL provides ongoing services (in close consultation with NATA, the National Association of Testing Authorities) to the Australian export meat industry. AGAL is seeking to expand this role to involve a wider range of laboratories engaged in residue testing of both export and domestic produce. The Australian Government believes it has an important role to play, particularly in the South East Asian and Pacific Rim regions, in co-operation, collaboration, assistance with training, and provision of specialist technical services.     

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.

     

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.     

Do we need to accredit proficiency testing schemes?

 Proficiency testing (PT) is being increasingly used as an important quality assurance tool for laboratories. The subject of quality of the providers of PT schemes has been discussed increasingly in recent years. Some countries have implemented systems for the accreditation of PT schemes. This paper looks at the background to the accreditation of PT schemes, the likely mechanisms which could be employed for accreditation, and some of the practical aspects.     

Do we need to consider metrological meanings of different measurement uncertainty estimations?

Along the years, several approaches for measurement uncertainty estimation have been suggested. Emphasis has been put on the general metrological interpretation of measurement uncertainty, but not on its different meanings when it is associated to given conditions of measurement where analytical work is performed and errors are originated. Three different definitions for uncertainty are proposed for reproducibility and intermediate precision conditions of measurement. These definitions inherit features from the VIM 3 definition of measurement uncertainty. It is argued that if a high performance laboratory keeps errors under control with proper validation and quality assurance programs, measurement uncertainty from intermediate precision condition of measurement is justified as a suitable estimation of its capability to attribute values to a measurand. Alternatively, a laboratory that does not keep errors under control should use uncertainty from reproducibility condition of measurement as the cost of its imperfections. Selection of information sources for measurement uncertainty estimation should be in harmony with its metrological meaning.     

Examples of the ‘characteristic’ function applied to instrumental precision in chemical measurement

The ‘characteristic function’ with two empirically determined parameters α and β, is proposed as a general purpose function to describe the variation of precision (in terms of standard deviation σ), or uncertainty, with analyte concentration c (here denoting any compositional quantity), for specific analytical methods applied to a defined type of test material. In this study it is applied to examples of analytical data collected under ‘instrumental’ conditions for estimating precision. The function fitted the data well, with no systematic lack of fit. The study therefore extends the range of applications of this function.     

A stepwise simulation approach to the chemical potential of fluids

A new approach to the computation of the chemical potential of fluids is presented. In this method the particle-insertion operation in the conventional test particle method is replaced by the growth of a specific particle. Application of the new technique to hard sphere and Lennard-Jones fluids shows that it is capable of providing reliable estimates of the chemical potential, even at high density where the conventional test particle methods are difficult to apply.     

A mathematical approach to chemical equilibrium theory for gaseous systems—I: theory

Equilibrium theory occupies an important position in chemistry and it is traditionally based on thermodynamics. A novel mathematical approach to chemical equilibrium theory for gaseous systems at constant temperature and pressure is developed. Six theorems are presented logically which illustrate the power of mathematics to explain chemical observations and these are combined logically to create a coherent system. This mathematical treatment provides more insight into chemical equilibrium and creates more tools that can be used to investigate complex situations. Although some of the issues covered have previously been given in the literature, new mathematical representations are provided. Compared to traditional treatments, the new approach relies on straightforward mathematics and less on thermodynamics, thus, giving a new and complementary perspective on equilibrium theory. It provides a new theoretical basis for a thorough and deep presentation of traditional chemical equilibrium. This work demonstrates that new research in a traditional field such as equilibrium theory, generally thought to have been completed many years ago, can still offer new insights and that more efficient ways to present the contents can be established. The work presented here can be considered appropriate as part of a mathematical chemistry course at University level.     

The interlaboratory comparison "IMEP-19 trace elements in rice"—a new approach for measurement performance evaluation

The aim of IMEP is to present objectively the quality of chemical measurements. Participants in IMEP compare their reported measurement results with independent external certified reference values with demonstrated traceability and uncertainty, as evaluated according to international guidelines. IMEP-19 focused on measurements of trace elements in rice aiming to support the Commission Regulation (EC) No. 466/2001 on maximum levels for certain contaminants in foodstuff. Measurement results for the elements Cd, Cu, Pb and Zn were reported by 267 field laboratories involved in food analysis from 43 countries. Performance criteria for the evaluation of the reported measurement results in IMEP-19 are suggested. The chosen performance indicators not only take into account the deviation of the reported measurement value from the certified reference value, but also set criteria for maximum and minimum acceptable uncertainty. The IMEP-19 participants' performance is reviewed by means of using new simple graphical tools, called "Naji plots".     

From a single analyzer to multiple instruments—The GUM approach

Assessment and expression of analytical quality have become novel spotlights in medical laboratories since accreditation began in the early 1990s, in Europe. Evaluation of uncertainty of measurement by definition was launched in Finland when the Finnish Accreditation Service (FINAS) accredited the first medical laboratories in the mid 1990s. In spite of all the analytical and statistical knowledge which has been available in medical laboratories for years, evaluation of total uncertainty of measurement has not yet caught on. The concept is still unfamiliar to experts and, indeed, little guidance has been available. National and international activities, with good results, can be shown when the educational aspect is considered. The Guide to the Expression of Uncertainty in Measurement (GUM) remains the main document for uncertainty evaluation. Uncertainty of measurement together with target value of uncertainty can be used as a good measure for analytical quality in large or smaller laboratories over time, because it is a quantitative indication and the evaluation is easy to repeat as running practical tools are available.Presented at the 8th Conference on Quality in the Spotlight, 17–18 March 2003, Antwerp, Belgium     

Capabilities of laboratories to determine melamine in food—results of an international proficiency test

A proficiency test to assess the capabilities of laboratories to determine melamine in a milk powder and a baking mix, representing starch-containing foods like bread and biscuits, was carried out in January 2009. The need for such an interlaboratory comparison arose from a health scare in China about melamine-tainted powdered milk in the second half of 2008. Laboratories in 31 countries, including Australia, China, India, Japan, New Zealand and the USA, and 21 of the 27 Member States of the European Union participated and reported back 114 results for the milk powder and 112 for the baking mix test materials. The reported results were compared to reference values determined by exact-matching double isotope dilution mass spectrometry. The so-determined assigned values were 10.0 ± 0.6 mg/kg melamine in the milk powder and 3.18 ± 0.17 mg/kg melamine in the baking mix. A coverage factor k of 2 was applied to calculate the expanded uncertainties. Three quarters of all reported results for both materials had associated z scores which were satisfactory (z ≤ |2|). Of the reported results, 90% was accompanied by a measurement uncertainty statement, and the majority of the measurement uncertainty ranges were reasonable. A number of laboratories were found to underestimate their measurement uncertainties. Methods that involved the use of stable-isotope-labelled melamine were shown to be clearly advantageous with regard to the accuracy of the results. However, no significant influence by other method parameters could be identified.     

A mathematical approach to chemical equilibrium theory for gaseous systems—II: extensions and applications

Straightforward mathematical techniques are used innovatively to form a coherent theoretical system to deal with chemical equilibrium problems. For a systematic theory it is necessary to establish a system to connect different concepts. This paper shows the usefulness and consistence of the system by applications of the theorems introduced previously. Some theorems are shown somewhat unexpectedly to be mathematically correlated and relationships are obtained in a coherent manner. It has been shown that theorem 1 plays an important part in interconnecting most of the theorems. The usefulness of theorem 2 is illustrated by proving it to be consistent with theorem 3. A set of uniform mathematical expressions are associated with theorem 3. A variety of mathematical techniques based on theorems 1–3 are shown to establish the direction of equilibrium shift. The equilibrium properties expressed in initial and equilibrium conditions are shown to be connected via theorem 5. Theorem 6 is connected with theorem 4 through the mathematical representation of theorem 1.     

Molecular cartography in acute Chlamydia pneumoniae infections—a non-targeted metabolomics approach

Infections with Chlamydia pneumoniae cause several respiratory diseases, such as community-acquired pneumonia, bronchitis or sinusitis. Here, we present an integrated non-targeted metabolomics analysis applying ultra-high-resolution mass spectrometry and ultra-performance liquid chromatography mass spectrometry to determine metabolite alterations in C. pneumoniae-infected HEp-2 cells. Most important permutations are elaborated using uni- and multivariate statistical analysis, logD retention time regression and mass defect-based network analysis. Classes of metabolites showing high variations upon infection are lipids, carbohydrates and amino acids. Moreover, we observed several non-annotated compounds as predominantly abundant after infection, which are promising biomarker candidates for drug-target and diagnostic research.     

The definition of primary method of measurement (PMM) of the ’highest metrological quality’: a challenge in understanding and communication

Problems with understanding, explaining and communication of the present definition of primary method of measurement are described and amendments put forward for discussion. The conclusion is drawn that in many cases more attention should be given to the measurement result and its uncertainty statement, rather than to a method. Some cases are discussed where methods might have a fundamental characteristic that other methods do not have, a condition for the epitheton ’primary’. Received: 23 December 2000 Accepted: 13 November 2000