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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.15, #3)
Ethanol/water solutions as certified reference materials for breath alcohol analyzer calibration by Rosemarie Philipp; Olaf Hanebeck; Sebastian Hein; Wolfram Bremser; Tin Win; Irene Nehls (pp. 141-146).
The Federal Institute for Materials Research and Testing (BAM), Germany, has issued a series of large volume ethanol in water certified reference materials (CRMs), primarily developed for the calibration of evidential breath alcohol analyzers in Germany. The certified parameter is the ethanol mass concentration at 20 °C. When used in a wet bath simulator, the solutions deliver gas samples that meet the requirements set by the Organization of Legal Metrology for calibration of breathalyzers. The materials were prepared gravimetrically by spiking of ethanol into water in single 5 L units. A complete uncertainty budget for the preparation process has been established. The purity of the commercial ethanol stock solution was identified to be the main source of uncertainty. For stability and homogeneity measurements and for the verification of the gravimetric mass concentration of the CRMs, a robust high-precision gas chromatography, with flame-ionization detection method for ethanol determination in aqueous samples was developed and validated. The good performance of this method has been demonstrated in several international comparisons organized by the Consultative Committee for Amount of Substance—Metrology in Chemistry at the International Bureau of Weights and Measures.
Keywords: Certified reference material; Ethanol in water; Breath alcohol; GC-FID
Optimising calibration and measurement capabilities in terms of economics in conformity assessment by Gaber Beges; Janko Drnovsek; Leslie R. Pendrill (pp. 147-154).
Calibration measurement capabilities (CMC) are key factors in declaring the metrological performance of national metrology institutes (NMIs). Different countries have different CMC capabilities, reflecting both the existing measurement science competence as well as the perceived national needs for traceable calibration. This paper deals with increasing interest in decision-making in conformity assessment in terms of effective costs associated with measurement, testing and incorrect decision-making. The work examines the CMCs of calibration laboratories and NMIs with economic decision theory, in particular, in terms of customer satisfaction and with respect to conformity assessment issues. Optimal strategies for calibration costs, maintenance of national measurement standards, testing and production costs are illustrated in practical examples. CMCs are an essential instrument to enable conformity assessment both for product safety, legal metrology, quality requirements as well as scientific research. The newly defined term “target measurement uncertainty”, introduced in the latest international metrology vocabulary (VIM), should be therefore always related to appropriate CMCs and related dissemination paths in the whole conformity assessment procedure. These requirements are clear and transparent justification for the development of required national metrological infrastructures, in order to fulfil the requirements of target measurement uncertainty for intended use or application in the particular conformity assessment procedure.
Keywords: Calibration measurement capabilities; Metrological infrastructure; Optimisation; Economic decision theory; Cost characteristics; Technological development; Conformity assessment; Target uncertainty
Application of the FAPAS protocol to a proficiency testing on Pb and Cd extracted from glazed ceramic by Lin Tao; Haibo Xiang; Hongmin Xu; Jizan Sun; Miao Zhang; Kegang Wang; Zuxing Mao (pp. 155-162).
The proficiency-testing scheme (CNCA-07-06) concerning lead and cadmium determination extracted from glazed ceramic surfaces involved 43 Chinese laboratories as participants, and has been well implemented. All the test materials passed the statistical homogeneity tests, and were deemed suitable for distribution. Reference values were determined from the obtained results and the performance of participants was based on z-scores. As a result, the majority of participants obtained satisfactory z-scores and laboratories that need to revise their procedures were singled out. Overall, about 8.5% of all results submitted in this PT round were unsatisfactory in terms of z-scores (22 out of 258 total data sets submitted), while the percentage of doubtful z-scores was 4.6% (12 out of 258 total data sets). Besides, the factors that might be associated with unsatisfactory results were also evaluated and suggestions were made to help improve the performance of laboratories in this PT.
Keywords: Interlaboratory exercise; Lead; Cadmium; Proficiency testing; Ceramic surfaces; Flatware
A comparison of two nuclear analytical techniques for determination of 210Pb-specific activity in solid environmental samples by M. S. Al-Masri; M. Hassan; Y. Amin (pp. 163-170).
Two nuclear analytical techniques for determination of 210Pb-specific activity in solid environmental samples have been validated and compared. The first technique depends on determination of 210Pb via its alpha emitting daughter 210Po using alpha-particle spectrometry, while the second technique is based on direct determination of 210Pb by measuring its activity at the 46-keV gamma line by low-energy gamma-ray spectrometry. Detection limits, repeatability, reproducibility, and surrogate recovery were the main validation parameters. Measurement uncertainties were estimated and compared for both techniques. Results of this study have shown that the expected activity of 210Pb in the environmental samples and the required measurement uncertainty are the main factors influencing a selection of the appropriate method for the application.
Keywords: Method validation; 210Pb; Alpha-particle spectroscopy; Gamma-ray spectroscopy; Environmental samples; Soil; Uncertainty of measurement
Evaluation of the source of bias caused by losses of solvent vapour during sample preparation by Peter J. Apps; Marcellé Archer (pp. 171-180).
Evaporation losses of organic solvents used to dissolve reference solutions and samples lead directly to errors in the measurement of amount of substance. This is due to changes in the mass fraction composition and the mass:volume concentration of solutions. We have found that measuring the amount of solute added to a solution by weighing by difference is vulnerable to negative bias in the measured amount of solute added, because solvent vapour is lost from opened vials and from syringe needles. Relative negative bias became larger with higher solvent volatility and with smaller volumes, reaching −28% for 10 µL of dichloromethane. Straightforward precautions that reduce the impact of evaporation on gravimetric operations with volatile solvents are presented.
Keywords: Bias; Solvent vapour; Weighing; Evaporation
Uncertainty in repeated measurement of a small non-negative quantity: explanation and discussion of Bayesian methodology by Robin Willink (pp. 181-188).
This article considers the problem of uncertainty evaluation when there are repeated measurements of a small quantity known to be non-negative. A solution to this problem has recently been put forward by the Analytical Methods Committee of the Royal Society of Chemistry (Accred Qual Assur 13:29–32). The Bayesian statistical basis of this solution is explained and discussed. It is shown that the performance of this procedure can be poor in an important subset of measurement situations.
Keywords: Bayesian statistics; Confidence interval; Credible interval; Frequentist statistics; Interval estimation; Prior information
Response from the Analytical Methods Committee (AMC) to the paper “Uncertainty in repeated measurement of a small non-negative quantity: explanation and discussion of Bayesian methodology” (pp. 189-191).
Willink’s paper, “Uncertainty in repeated measurement of a small non-negative quantity: explanation and discussion of Bayesian methodology” provides a clear exposition of the Bayesian paradigm but unfortunately reads like a rebuttal of a previous paper from the AMC: the reader is left with the impression that the AMC is recommending a procedure that performs poorly. The AMC here refutes any such impression by presenting a balanced assessment of the strengths and weaknesses of the Bayesian and frequentist approaches to formulating the uncertainty of small non-negative quantities.
Keywords: Bayesian statistics; Frequentist statistics; Confidence interval; Uncertainty; Non-negative quantity; Maximum density interval
PT-WFD: the network of PT providers to support the implementation of the European water framework directive by Frank Baumeister; Ulrich Borchers; Michael Koch (pp. 193-198).
This paper introduces a newly formed network of proficiency testing (PT) providers, who are active and experienced in the field of PTs with relevance to the water framework directive (WFD). The objective of this new network is to support the implementation of the WFD in Europe, by providing PT schemes that meet the specific requirements of the WFD and which are run and evaluated in a harmonised way. The main value of creating such a network lies in the potential to foster the harmonised performance evaluation of European laboratories involved in WFD monitoring analyses, thus enhancing the equivalence of WFD monitoring data obtained throughout Europe. In its first year the network organised two joint PT rounds, one for volatile organic substances (VOC) and another for selected pesticides from the list of Priority Substances, with greater than 100 water analysis labs from a number of European countries participating.
Keywords: PT schemes; Water framework directive; Harmonisation; Surface water; Priority substances; Networking