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.     

Systematic errors in analytical measurement results

Definitions of the concepts of bias and recovery are discussed and approaches to dealing with them described. The Guide To Uncertainty in Measurement (GUM) recommends correction for all significant systematic effects, but it is also possible to expand measurement uncertainty to take account of uncorrected bias. Run, laboratory and method bias can be defined as components of the bias of a particular measurement result, and can be useful as concepts used in method validation. Estimation of run bias allows a simplification of the estimation of measurement uncertainty. Multivariate calibration brings its own biases that must be quantified and minimised.     

Quantifying the measurement uncertainty of results from environmental analytical methods

The Eurachem–CITAC Guide Quantifying Uncertainty in Analytical Measurement was put into practice in a public laboratory devoted to environmental analytical measurements. In doing so due regard was given to the provisions of ISO 17025 and an attempt was made to base the entire estimation of measurement uncertainty on available data from the literature or from previously performed validation studies. Most environmental analytical procedures laid down in national or international standards are the result of cooperative efforts and put into effect as part of a compromise between all parties involved, public and private, that also encompasses environmental standards and statutory limits. Central to many procedures is the focus on the measurement of environmental effects rather than on individual chemical species. In this situation it is particularly important to understand the measurement process well enough to produce a realistic uncertainty statement. Environmental analytical methods will be examined as far as necessary, but reference will also be made to analytical methods in general and to physical measurement methods where appropriate. This paper describes ways and means of quantifying uncertainty for frequently practised methods of environmental analysis. It will be shown that operationally defined measurands are no obstacle to the estimation process as described in the Eurachem/CITAC Guide if it is accepted that the dominating component of uncertainty comes from the actual practice of the method as a reproducibility standard deviation.     

Target measurement uncertainties for results of chemical measurements in Slovenia

Target measurement uncertainties (TMUs), which should be achieved in chemical measurements, are proposed. Such TMUs are intended to set goals for measurement laboratories and could provide independent and objective criteria for laboratory assessors verifying the measurement capability of laboratories. A TMU is specified relative to a stated metrological reference for the measurand concerned. TMUs are common and objective, but are external criteria for both the measurement laboratories and their assessors. They are decided on the basis of the intended use of the measurement result, not evaluated as is the case for measurement uncertainty. The concept of TMU is illustrated by means of desirable applications in food measurements in Slovenia.P. De Bièvre is an Independent Consultant on Metrology in ChemistryPresented at the session Metrology in Chemistry of the XVII IMEKO World Congress, 26 June 2003, Dubrovnik-Cavtat, Croatia     

Quantifying the measurement uncertainty of results from environmental analytical methods

The Eurachem-CITAC Guide Quantifying Uncertainty in Analytical Measurement was put into practice in a public laboratory devoted to environmental analytical measurements. In doing so due regard was given to the provisions of ISO 17025 and an attempt was made to base the entire estimation of measurement uncertainty on available data from the literature or from previously performed validation studies. Most environmental analytical procedures laid down in national or international standards are the result of cooperative efforts and put into effect as part of a compromise between all parties involved, public and private, that also encompasses environmental standards and statutory limits. Central to many procedures is the focus on the measurement of environmental effects rather than on individual chemical species. In this situation it is particularly important to understand the measurement process well enough to produce a realistic uncertainty statement. Environmental analytical methods will be examined as far as necessary, but reference will also be made to analytical methods in general and to physical measurement methods where appropriate. This paper describes ways and means of quantifying uncertainty for frequently practised methods of environmental analysis. It will be shown that operationally defined measurands are no obstacle to the estimation process as described in the Eurachem/CITAC Guide if it is accepted that the dominating component of uncertainty comes from the actual practice of the method as a reproducibility standard deviation.     

Human being as a part of measuring system influencing measurement results

The role of human being as a part of a measuring system in a chemical analytical laboratory is discussed. It is argued that a measuring system in chemical analysis includes not only measuring instruments and other devices, reagents and supplies, but also a sampling inspector and/or analyst performing a number of important operations. Without this human contribution, a measurement cannot be carried out. Human errors, therefore, influence the measurement result, i.e., the measurand estimate and the associated uncertainty. Consequently, chemical analytical and metrological communities should devote more attention to the topic of human errors, in particular at the design and development of a chemical analytical/test method and measurement procedure. Also, mapping human errors ought to be included in the program of validation of the measurement procedure (method). Teaching specialists in analytical chemistry and students how to reduce human errors in a chemical analytical laboratory and how to take into account the error residual risk, is important. Human errors and their metrological implications are suggested for consideration in future editions of the relevant documents, such as the International Vocabulary of Metrology (VIM) and the Guide to the Expression of Uncertainty in Measurement (GUM).     

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.     

Use of prediction equations for reviewing measurement results in the clinical laboratory

In the clinical laboratory, one of the most objective ways to perform the final review of measurement results is the use of the so-called plausibility control (i.e., set of procedures used to decide if a measurement result is valid or not according to established clinical and biological criteria). The present study is focused on the estimation of several prediction equations derived from pairs of biological quantities having a pathophysiological relationship and statistically correlated to detect objectively doubtful results in the context of plausibility control. These prediction intervals, that may be used alone or combined with other procedures involved in the plausibility control, are a very useful tool for the improvement of the final review of the laboratory results.     

Guidelines and recommended terms for expression of stable-isotope-ratio and gas-ratio measurement results

To minimize confusion in the expression of measurement results of stable isotope and gas-ratio measurements, recommendations based on publications of the Commission on Isotopic Abundances and Atomic Weights of the International Union of Pure and Applied Chemistry (IUPAC) are presented. Whenever feasible, entries are consistent with the Système International d'Unités, the SI (known in English as the International System of Units), and the third edition of the International Vocabulary of Basic and General Terms in Metrology (VIM, 3rd edition). The recommendations presented herein are approved by the Commission on Isotopic Abundances and Atomic Weights and are designed to clarify expression of quantities related to measurement of isotope and gas ratios to ensure that quantity equations instead of numerical value equations are used for quantity definitions. Examples of column headings consistent with quantity calculus (also called the algebra of quantities) and examples of various deprecated usages connected with the terms recommended are presented.     

Proposed guidelines for the final review of measurement results in the clinical laboratory

The standard ISO 15189 requires a systematic review of results of clinical laboratory examinations, but it does not provide details on how to carry out such a review. In this article, the Catalan Association of Clinical Laboratory Sciences proposes a guide for this review of patients’ clinical laboratory results pertaining to rational or difference scales (‘quantitative values’). The review process is based on the so-called plausibility control, which may be defined as the set of procedures used to decide if a measurement result is valid or not according to established clinical and biological criteria, considering four variables: (1) alert limits; (2) consistency with the previous result, if any; (3) consistency with other results obtained from the same sample, if any; and (4) consistency with the diagnosis (presumed or confirmed) or, when it is not known, the origin of the request, though these last criteria are generally very weak and the derived decisions may be scantly reliable. This guide has been prepared on the behalf of the Technical Committee of the Catalan Association for Clinical Laboratory Sciences. Papers published in this section do not necessarily reflect the opinion of the Editors, the Editorial Board and the Publisher.     

Monte Carlo simulation for the evaluation of measurement uncertainty of spent fuel analytical results

The present paper describes an approach based on Monte Carlo simulation for the evaluation of uncertainty of nuclear spent fuel analysis. The mathematical model of measurement was established by examining the dissolution process step by step. The results are consistent with those obtained by the classical propagation of variance approach. This paper shows the importance of taking the process into account in order to give a more reliable uncertainty assessment to the result of a concentration ratio of two isotopes in spent fuel. Indeed, for some radionuclides, the uncertainty associated with the upstream steps of the analysis (“process” uncertainty) can represent up to 95 % of the overall uncertainty.     

Microelectronic sensors for measurement of electromagnetic fields of living cells and experimental results.

Microelectronic sensors are used for measurements of electromagnetic fields generated by synchronized cultures of yeast cells. Cold sensitive mutant tub2-401 of Saccharomyces cerevisiae is used. The measured electromagnetic signals in the frequency range from 8 to 9 MHz are compared with evolution of the reassembled microtubules. The detected signals peak in the time interval 25-30 min and 45-60 min after the release of the cells from the restrictive to the permissive temperature. The first maximum corresponds to the stage when the mitotic spindle is formed and binds chromatids. The second maximum is measured when the processes of anaphase A and of anaphase B take place.