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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.15, #7)
Using Bayes methods and mixture models in inter-laboratory studies with outliers by Garritt L. Page; Stephen B. Vardeman (pp. 379-389).
Inter-laboratory studies (especially so-called key comparisons) are conducted to evaluate both national and international equivalence of measurement. In these studies, a reference value of some measurand (the quantity intended to be measured) is developed and results for all laboratories are compared to this single value. How to determine the reference value is not completely obvious if there are observations and/or laboratories that could be considered outliers. Since ignoring results from one or more participating laboratories is untenable in practical terms, developing methods that are robust to the possibility that a small fraction of the laboratories produces observations unlike those from the others is critical. This paper outlines two Bayesian methods of analyzing inter-laboratory data that have been proposed in the literature and suggests three modifications of one that are more robust to outliers. A simulation study is conducted to compare the five methods.
Keywords: Hierarchical models; Inter-laboratory studies; Mixtures; Outlying laboratories
Investigation on drying conditions and assays of amidosulfuric acid and sodium carbonate by acidimetric coulometric titration and gravimetric titration by Toshiaki Asakai; Akiharu Hioki (pp. 391-399).
Amidosulfuric acid and sodium carbonate as standards for acid–base titrimetry were assayed by coulometric titration and gravimetric titration. Amidosulfuric acid was directly assayed by coulometric titration with electrogenerated hydroxide ions, and sodium carbonate was assayed by gravimetric back-titration. For sodium carbonate, excess amount of sulfuric acid, whose concentration was determined by coulometric titration, was added to sodium carbonate, and then gravimetrically back-titrated using a sodium hydroxide solution whose concentration was determined by gravimetric titration using the sulfuric acid. The accuracy of the coulometric titration for amidosulfuric acid and sulfuric acid was evaluated by examining the current efficiency of pulse electrolysis, the amount of the electrolysis current used, and the time spent for a titration. In addition, the drying conditions for high purity primary standards have a significant effect on the titration results due to changes in the acid–base assay. The suitable drying conditions for amidosulfuric acid and sodium carbonate were evaluated by mass-change measurements, coulometric titration and gravimetric titration. The measurement uncertainties were estimated from the uncertainties on the titration processes. Finally, the assays of amidosulfuric acid and sodium carbonate were 99.986% ± 0.010% (k = 2) after drying at 50 °C for 2 h, and 99.970% ± 0.016% (k = 2) after drying at 280 °C for 4 h, respectively. In addition, the international consistency was confirmed by measuring certified reference materials (CRMs) available from different National Metrology Institutes, and the compatibility of values among CRMs was experimentally ascertained.
Keywords: Coulometric titration; Gravimetric titration; Certified reference material; Acid–base titration; High purity amidosulfuric acid; High purity sodium carbonate
International comparability of determination of mass fractions of chromium, copper, iron, manganese and zinc in aluminium alloy: Comité Consultatif pour la Quantité de Matière key comparison K42 by Sebastian Recknagel; Siegfried Noack; Thomas Steiger; Heinrich Kipphardt; Ralf Matschat (pp. 401-407).
The degree of equivalence within the participating national metrology institutes for the measurement results of the mass fractions of the analytes Cr, Cu, Fe, Mn and Zn in an aluminium alloy was assessed. This interlaboratory comité consultatif pour la quantité de matière key comparison (CCQM-K42) was organised as an activity of the Inorganic Analytical Working Group of CCQM. In total seven laboratories participated, six of them for all analytes. Measurands were the mass fractions of the analytes in a range of 0.05 and 0.2%. As an outcome the consistency of the results for all elements investigated was acceptable, hence satisfactory comparability was established. An aluminium based certified reference material—undisclosed to the analysts which one it was—was used as test sample. For the purpose of this study homogeneity was tested at BAM. Each laboratory was free to choose any analytical method they wanted to use for the analysis. Consequently various methods of measurement were employed: instrumental neutron activation analysis, X-ray fluorescence spectrometry (XRF) using fused cast-bead method combined with reconstitution technique, inductively coupled plasma optical emission spectrometry (ICP OES) and inductively coupled plasma mass spectrometry. Metrological traceability of the measurement results to the SI unit had to be demonstrated. Therefore, methods such as spark OES or XRF (without fused cast-bead technique)—both of them being most important methods for the analysis of metals and alloys in industrial laboratories—could not be used in the frame of the key comparison.
Keywords: Aluminium alloy; International comparison; CCQM-K42; Key comparison
Elemental mercury vapour in air: the origins and validation of the ‘Dumarey equation’ describing the mass concentration at saturation by Ronny Dumarey; Richard J. C. Brown; Warren T. Corns; Andrew S. Brown; Peter B. Stockwell (pp. 409-414).
The Dumarey equation has been the dominant relationship used to calculate the mass concentration of saturated elemental mercury vapour in air for the calibration of mercury vapour measurement equipment for over 25 years. However, the origin of the equation, and the validation data supporting its accuracy have never been published. This paper addresses that deficiency, compares the Dumarey equation with other data sets to which it has been wrongly attributed in the past, and describes why it remains superior to the use of mercury vapour pressure data in combination with the ideal gas law, for the purposes of calibrating mercury vapour measurement equipment.
Keywords: Dumarey equation; Mercury vapour; Vapour pressure; Atomic spectrometry; Saturated mass concentration
Reacting to spontaneous feedback from external customers in a university hospital laboratory by Paula I. Oja; Timo T. Kouri; Arto J. Pakarinen (pp. 415-420).
Modern quality standards for laboratories recommend the use of customer feedback in quality improvement. Spontaneous customer feedback is one method to obtain information from customers. In the present study, we analysed the feedback data from external customers (health centres and other hospitals) to our university hospital laboratory during 2001–2006. The most common subject matters of the feedback, covering 87% of a total of 95 reports, were lacking test results, suspicion of the validity of test results, return of samples in transportation boxes to customers and delays in service. On investigation, errors or defects were revealed in 78 cases. The most common errors were lacking (45 cases; 58%), erroneous (20 cases; 26%) or delayed test results (7 cases; 9%). As much as 68% of these errors took place in the pre-analytical phase of the laboratory process, occurring most commonly during specimen transfer, when entering orders into the laboratory information system at the university hospital or during sample processing. The most common underlying causes for errors were unintended errors and non-compliance with operating instructions. Latent errors were identified in 14% of the cases. Corrective actions were performed in 79% of the reported cases. It is important that the feedback reports are investigated to find out possible errors and their underlying causes so that appropriate corrective actions can be taken. When processed properly, spontaneous customer feedback is a method that can be used supplementarily to other methods to detect errors and defects in clinical laboratories and to aid in quality improvement.
Keywords: Complaints; Customer feedback; Laboratory errors; Quality improvement
Failures of the global measurement system. Part 1: the case of chemistry by Gary Price (pp. 421-427).
This discussion puts a case of advocatus diaboli: that the Treaty of the Metre, its associated administrative apparatus and the International System of measurement units (SI) has basically failed for chemical measurement and is largely irrelevant to modern analysis, much of practical measurement in modern economies and much of recent technology. The practical use of the chemical unit termed the mole, the introduction to the SI units of the thermodynamic mole and the invention of a new physical quantity called “amount of substance” are each reviewed with the conclusion that the current means of expressing the results of chemical measurements are unsatisfactory in both practice and theory and are imposing large and readily avoidable costs on all advanced economies.
Keywords: Treaty of the metre; Mole; Amount of substance; Metrology
Stephen L. R. Ellison, Vicki J. Barwick, Trevor J. Duguid Farrant: Practical statistics for the analytical scientist. A bench guide
by D. Brynn Hibbert (pp. 429-431).
Erratum to: International comparison of the determination of cadmium and lead in herb: the Comité Consultatif pour la Quantité de Matière (CCQM) pilot study CCQM-P97
by Y. C. Wong; D. W. M. Sin; Y. C. Yip; L. Valiente; A. Toervenyi; J. Wang; G. Labarraque; P. Gupta; D. Soni; Surmadi; E. Hwang; C. Yafa; O. Cankur; E. Uysal; G. Turk; R. Huertas; K. E. Murphy (pp. 433-433).