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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.16, #11)

From competition back to cooperation? by Paul De Bièvre (pp. 531-532).

Can the usual validation standard series for quantitative methods, ISO 5725, be also applied for qualitative methods? by Steffen Uhlig; Ludwig Niewöhner; Petra Gowik (pp. 533-537).
Repeatability standard deviation, laboratory standard deviation, and reproducibility standard deviation for quantitative methods according to ISO 5725 series were recently proposed to estimate the precision of qualitative measurements, giving a presence/absence response. In this paper, it is shown that for qualitative methods, the reproducibility standard deviation across laboratories does not reflect the performance of the method as suggested. It is demonstrated that the benefit of the respective laboratory standard deviation is very limited. Alternative performance measures are introduced which are based on another approach also directly linked to ISO 5725. Thereby, meaningful information about the precision of qualitative test methods can be achieved.

Keywords: Qualitative methods; Quantitative methods; ISO 5725; Reproducibility standard deviation; Repeatability standard deviation; Laboratory standard deviation; Probability of detection response curve


Performance evaluation for proficiency testing with a limited number of participants by Siu-kay Wong (pp. 539-544).
Proficiency testing (PT) is an essential tool for laboratories to assess their competency. Also, participation in PT has become one of the mandatory requirements for laboratory to seek accreditation according to ISO/IEC 17025. For this reason, the effectiveness of performance evaluation by PT scheme is of great concern for the participants and for accreditation bodies as well. In practice, owing to unavailability of other appropriate alternatives, PT scheme providers may have to choose using consensus values to evaluate the performance of participants. However, such consensus values approach was not recommended by relevant international guidelines for PT schemes with limited number of participants. With the use of Monte Carlo simulation technique, this study attempted to investigate the effectiveness of using consensus values for performance evaluation in PT schemes with limited number of participants. The simulation process was schemed according to the statistical model provided by ISO 5725-1 for laboratory measurement results, which covered components like method bias, laboratory bias, and measurement precision. The effectiveness of the consensus value approach was expressed as the percentage of participants in a simulation run could get the same evaluation result, either satisfactory or unsatisfactory, against the “true value.” The findings indicated that the number of participants, choice of consensus values, mass fraction of analyte, method bias, laboratory bias, and measurement repeatability of participating laboratories would all affect the effectiveness of the consensus value approach but at different extent. However, under certain circumstances, use of consensus value could still be considered as an acceptable approach for performance evaluation even the number of participants was limited. Some of the findings were further verified using real data from PT schemes where appropriate certified reference materials or reliable reference values were available.

Keywords: Proficiency testing; Monte Carlo simulation; Consensus values; Consistency rate


A metrological approach for the assessment of selenium status in human serum by Petru Jitaru; Caroline Oster; Guillaume Labarraque; Sophie Vaslin-Reimann; Paola Fisicaro (pp. 545-552).
This work addresses a metrological approach for the assessment of Se status in humans in terms of serum selenomethionine (SeMet). The quantification of SeMet was carried out using a primary method of chemical analysis, namely species-specific isotope dilution (SSID) in combination with HPLC coupled to collision/reaction cell inductively coupled plasma-mass spectrometry. SeMet was released from the serum selenoalbumin (a seleno-containing protein where SeMet is randomly incorporated) by enzymatic hydrolysis of the whole serum. This study is a follow-up of the analytical method development reported previously, and it focuses primarily on the evaluation of the uncertainty budget and the main uncertainty sources for SeMet determination in three commercial serums, namely BCR-637 (certified for total Se) and two serum standards, SERONORM level 1 (SERO-L1) and 2 (SERO-L2) (with indicative concentrations of total Se). The metrological approach reported here could be considered as a pilot study in terms of metrological determination of SeMet in human serum, hence being suitable for method validation and inter-laboratory comparison.

Keywords: Human serum; Selenomethionine; Species-specific isotope dilution; HPLC-ICP-MS Expanded uncertainty


Performance of commercial soil laboratories in a proficiency test program in Brazil by John O. Agbenin; Heitor Cantarella (pp. 553-559).
A soil proficiency test (PT) was administered to 50 participant laboratories in which two sets of samples, consisting of 20 yearly PT samples and 5 ‘blind’ samples in clients’ names were analyzed for pH, organic matter, total acidity, extractable calcium, magnesium, potassium and phosphorus by the laboratories. Our objective was to determine whether laboratories take extra care to analyze clients’ samples as they do with regular PT samples. The analytical data were evaluated essentially by the procedure described in the international harmonized protocol for proficiency testing of analytical chemistry laboratories. Performance of participant laboratories was assessed by z-scores and summary z-scores statistics involving sum of squared z-scores interpreted as chi-square $$ left( {chi_{n}^{2} } ight) $$ distribution for zero-centered z-scores with unit variance. From 8 750 determinations, outliers and stragglers accounted for less than 2% of the entire data. Over 93% of the data were satisfactory, whereas between 2 and 4% were either unsatisfactory or questionable in both the PT and ‘blind’ tests. On the basis of sum of squared z-scores interpreted from $$ chi_{n}^{2} $$ distribution table, between 30 and 40% of the laboratories had more than 90% probability of having their measurement data within the robust mean and standard deviation for each soil parameter, while another 30–42% of the laboratories had less than 50% probability of having measurement data within the robust mean and standard deviation. Overall, 21 laboratories (42%) were ranked in Class A either in the PT or ‘blind’ tests out of which 12 of them (57%) retained this ranking in both tests. Fourteen laboratories (28%) were ranked in Class C in either the PT or ‘blind’ tests with only 5 of them (36%) consistently ranking in this class in both tests.

Keywords: Proficiency tests; z-score; Robust mean; Standard deviation; Laboratory ranking


Standard Method Performance Requirements by Arlene Rose Fox (pp. 561-566).
AOAC INTERNATIONAL has developed a process to set Standard Method Performance Requirements (SMPRs). The fitness for intended use of the result of the measurement determines the criteria for these requirements. These criteria are specified by the stakeholder panels made up of key experts from global government, industry, academia, and contract research organizations. This process has been implemented for assays aiming at the detection of bio-threat agents, endocrine disruptors, and quantifying nutrients in infant formula. The process begins with the formation of a balanced stakeholder panel. The stakeholder panel specifies analytes, matrices, current technologies, analytical challenges, and regulatory information at the first meeting. Working groups are formed to provide specific technical expertise to the stakeholder panels. These groups perform the task of translating the specified needs of the stakeholder panel into concise measurement requirements. The draft SMPRs go back to the stakeholder panel for review and approval and then are posted for comment on the AOAC web site. These requirements become consensus requirements that have been developed with openness, balance of interests, an appeals procedure, consensus, and transparency.

Keywords: Standard Method Performance Requirements; Infant formula


Practical estimation of the uncertainty of analytical measurement standards by Ruud J. B. Peters; Ingrid J. W. Elbers; Mirjam D. Klijnstra; Alida A. M. Stolker (pp. 567-574).
Nowadays, a lot of time and resources are used to determine the quality of goods and services. As a consequence, the quality of measurements themselves, e.g., the metrological traceability of the measured quantity values is essential to allow a proper evaluation of the results with regard to specifications and regulatory limits. This requires knowledge of the measurement uncertainties of all quantity values involved in the measurement procedure, including measurement standards. This study shows how the uncertainties due to the preparation, as well as the chemical and compositional stability of a chemical measurement standard, or calibrator, can be estimated. The results show that the relative standard uncertainty of the concentration value of a typical analytical measurement standard runs up to 2.8% after 1 year. Of this, 1.9% originates from the preparation of the measurement standard, while 2.0 and 0.53% originate from the chemical and compositional stability during storage at −20 °C. The monthly preparation of working calibrators stored at 4 °C and used on a weekly basis, results in an additional standard uncertainty of the analyte concentration value of 0.35% per month due to compositional stability. While the preparation procedure is the major contributor to the total measurement uncertainty, the uncertainties introduced by the stability measurements are another important contributor, and therefore, the measurement procedure to evaluate stability is important to minimize the total measurement uncertainty.

Keywords: Metrological traceability; Measurement and testing; Quality assurance; Measurement uncertainty; Analytical measurement standards; Chemical stability


Reasonable scope for change by M. J. T. Milton (pp. 575-576).
Recent proposals to redefine four of the base units of the SI, including the mole, have triggered a wide-ranging debate about the terminology as well as the definitions that form the SI. Since the SI is a practical system that is used worldwide, the reasonable scope for changing it must avoid leaving large numbers of users with obsolete implementations or significantly changed values. Changes that go beyond these limits would require consultation processes that might take decades and whose cost and complexity would be difficult to justify in view of the likely benefits.

Keywords: SI; Traceability; Mole


Alternative interpretations of the mole and the ideal gas equation by B. P. Leonard (pp. 577-581).
The fundamental concept of the mole stems from the idea of the gram-atom (or gram-molecule): that an amount of one mole of a given substance has an associated mass, in grams, numerically exactly equal to the relative atomic (or molecular) mass of the substance. This means that the particular number of entities comprising one mole, Avogadro’s number, must be exactly equal to the gram-to-dalton mass ratio. Various interpretations of what a mole actually means and how this affects the ideal gas equation depend on how the amount of a given substance, n, is related to the corresponding number of entities, N. We look at three alternatives: (1) the conventional interpretation in which n is directly proportional to N, where the proportionality factor is the reciprocal of the (evidently poorly understood) Avogadro constant; (2) a more easily comprehended alternative in which n is equal to N entities; and (3) an interpretation in which n and N both represent the number of entities, but expressed in different ways. Regardless of these different interpretations, the form of the stoichiometric equations (relating N, n and the total sample mass) and the form of the relationship between the universal gas constant and the Boltzmann constant remain the same.

Keywords: Mole; Ideal gas; Entity; Avogadro’s number; Avogadro constant
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