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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.12, #6)
Defining and measuring biological activity: applying the principles of metrology by Craig M. Jackson; M. Peter Esnouf; Donald J. Winzor; David L. Duewer (pp. 283-294).
A definition of biological activity is proposed that is superficially analogous to the equation relating the thermodynamic activity of a solute to its concentration via an activity coefficient. The biological activity of a molecular entity is defined as A=cf, where A is the activity, c the amount-of-substance concentration, and f is a parameter designated as “inherent activity.” Units and dimensions are determined by the type of activity, catalytic (katal) or binding (mol−1 L). The measurand is described by a chemical equation that identifies the entity for which an activity is being monitored. This definition of biological activity has the advantage of separating the chemical characterization of the entity in terms of structure and amount from the assessment of biological activity. Ideally, a homogeneous entity is used for the measurement of f. In instances where impure materials are used or the chemical equation defining the activity is unknown, the evaluated parameter should be designated as f′ to denote its empirical nature. Any measurement of f or f′ should be qualified with an appropriate estimate of measurement uncertainty.
Keywords: Biological activity; Activity definition; Inherent activity
Dry mass determination: what role does it play in combined measurement uncertainty? by E. Zeiller; E. Benetka; M. Koller; R. Schorn (pp. 295-302).
Elemental mass fractions are normally reported on a dry-mass basis. In addition, producers of reference material state their property values for dry masses and relate them to a defined dry mass determination method. This paper describes how biases and uncertainties in dry mass correction factors may affect values reported as ‘concentration per dry mass’. The influence of the use of different drying methods, accuracy of drying temperature, length of drying, environmental humidity, and the sample mass used for dry mass determination have been evaluated using two candidate International Atomic Energy Agency reference materials. Deviation from proposed drying methods was observed to lead to differences of up to 5% in dry mass correction factors. In addition, the heterogeneity of sample moisture as well as hygroscopic behaviour during weighing play an important role in the uncertainty estimation. Proposals are made regarding which sources of uncertainty arising from dry mass determination should be considered and included in the combined uncertainty of analytical results.
Keywords: Reference material; Uncertainty dry mass; Standard uncertainty; Dry mass determination
Evaluation of measurement uncertainty in the determination of jasmonic acid in Lemna minor L. by liquid chromatography with fluorescence detection by Janja Kristl; Božidar Krajnčič; Darinka Brodnjak-Vončina; Marjan Veber (pp. 303-310).
The uncertainty was estimated for the determination of jasmonic acid (JA) content in Lemna minor L. plant extracts using a reverse-phase liquid chromatography with fluorescence detection. JA was extracted from plant material, followed by solid-phase extraction procedures, derivatisation and quantification. In the estimation of uncertainty, the sampling, sample processing and chromatographic determination that may significantly influence the uncertainty of analytical data were considered. The results show that the method recovery and sample homogeneity are the two main contributors to uncertainty. The method has a relative expanded uncertainty (coverage factor k = 2) of about 17% at the JA content of approximately 100 ng/g.
Keywords: Measurement uncertainty; Jasmonic acid; Liquid chromatography
Proficiency test exercise on the determination of natural levels of radionuclides in mushroom reference material by Shahida Waheed; Asma Rahman; Naila Siddique; Shujaat Ahmad; Matthias Rossbach (pp. 311-316).
The results from a proficiency test exercise for the determination of natural levels of radionuclides in mushroom reference material carried out by laboratories in six different countries are discussed. These laboratories submitted data on seven radionuclides, namely 134Cs, 137Cs, 40K, 90Sr, 241Am, 226Ra, and 210Po. However, only the data received for three of these radionuclides could be statistically evaluated to obtain consensus mean values and confidence intervals (at a significance level of 0.05). The contents of 134Cs, 137Cs and 40K were quantified as 4.4, 2,899 and 1,136 Bq/kg, respectively.
Keywords: Accuracy; Mushrooms; Precision; Proficiency test; Radionuclides; Reference materials (RMs)
Experience of implementing ISO 17025 for the accreditation of a university testing laboratory by D. Zapata-García; M. Llauradó; G. Rauret (pp. 317-322).
The experience of implementing a quality system on ISO 17025, and the accreditation of some tests for a university laboratory, is presented in the example of the Environmental Radiology Laboratory. Such implementation in a university institution is a difficult task, since the setting and environment are both unfavourable. The particularities are the collaboration of many independent units of the university, and the involvement of teachers and young researchers who have to coordinate all these different tasks, which makes the number of people working quite variable. Some specific aspects of the system are discussed. The conclusion is that implementation of a quality system on ISO 17025 and accreditation are completely achievable, and are helpful activities to put university members in touch with the real world and broaden their minds, which in the end has a positive impact on research studies and academic content.
Keywords: Quality system; Environmental radioactivity; Accreditation; ISO 17025
Handling undetected and low-level components in purity determination by Simon Cowen; Stephen L. R. Ellison (pp. 323-328).
Impurities expected to be present at low levels in a high-purity material may go undetected or be measured with near-zero values. This complicates assessment and reporting of uncertainty, and in particular can lead to underestimation of uncertainty, anomalies in estimating degrees of freedom, and coverage intervals, which extend outside the feasible range for the measurand. These issues are considered and appropriate remedies are suggested.
Keywords: Uncertainty; Purity; CRM certification; Detection limits