Accreditation and Quality Assurance (v.16, #1)
Does accreditation ensure competence in measurement? by Paul De Bièvre (1-2).
Hidden contributors to uncertainty and accuracy of results of residue analysis by Árpád Ambrus; Andrea Zentai; Judit Sali; István Ficzere (3-11).
Accurate analytical results with known uncertainty are required for the safety assessment of pesticides and testing the conformity of marketed food and feed with the maximum residue limits. The available information on various sources of errors was examined with special emphasis to those which may remain unaccounted for based on the current practice of many laboratories. The method validation typically covers the steps of the pesticide residue determination from the extraction of spiked samples to the instrumental determination, which contribute to only 10–40% of total variance of results. Though the variability of sampling, sample size reduction and sample processing may amount to the 60–90% of total variance, it generally remains unnoticed leading to wrong decisions. Another important source of gross error is the mismatch of the residues analysed and those included in the relevant residue definition. Procedures which may be applied for eliminating or reducing the errors are discussed.
Keywords: Pesticide residues; Sources of uncertainty; Systematic and gross errors; Sampling; Sample processing; Chromatographic analysis
Use of the ‘characteristic function’ for modelling repeatability precision by Michael Thompson; Barry J. Coles (13-19).
The ‘characteristic function’ is a two-parameter function relating precision or uncertainty in analytical results to the concentration of the analyte. In previous papers, in this series, it has been shown to provide a good model of precision measured: (a) under reproducibility conditions and (b) under ‘instrumental’ conditions. The present study shows that it is also a valuable model for precision estimated under repeatability conditions. The study data were large sets of duplicated results obtained for the purposes of quality control on typical test materials in routine analysis. As the analytes exhibited concentration ranges encompassing between one and three orders of magnitude, there was ample scope to demonstrate goodness of fit to the function under different circumstances.
Keywords: Chemical analysis; Functional relationship between precision and concentration; Repeatability conditions; Characteristic function
Measurement uncertainty evaluation and in-house method validation of the herbicide iodosulfuron-methyl-sodium in water samples by using HPLC analysis by Andreja Drolc; Albin Pintar (21-29).
A method for separation and quantitative determination of the iodosulfuron-methyl-sodium in water samples by high-performance liquid chromatography (HPLC) was developed and in-house validated in order to demonstrate its performance for monitoring of heterogeneous photocatalytic elimination of the herbicide iodosulfuron-methyl-sodium from water. Surface and ground water samples were used to demonstrate its selectivity, detection and quantification limits, linearity, trueness and precision. In addition, stability of iodosulfuron-methyl-sodium was studied in function of temperature and time. Method accuracy was quantified through measurement uncertainty estimate based on method validation data. The paper gives practical and easy to follow guidance on how uncertainty estimates can be obtained from method validation experiments. It shows that, if properly planned and executed, key precision and trueness studies undertaken for validation purposes can also provide much of the data needed to produce an estimate of measurement uncertainty. Our analytical protocol allowed us to quantify iodosulfuron-methyl-sodium in ground water and surface water in concentration level between 2.50–50.0 μmol L−1 with satisfactory recoveries (99–104%) and repeatability lower or equal than 0.3% for all the matrices. We also estimated within-laboratory reproducibility over 3-month period, which was 0.7%. We proved that the method was selective for determination of iodosulfuron-methyl-sodium in the relevant matrices. Measurement uncertainty of results was evaluated to be 4.0% with 95% confidence level. After validation and measurement uncertainty evaluation steps, results obtained showed that the method can be applied to efficiently monitor heterogenous photocatalytic degradation of the herbicide iodosulfuron-methyl-sodium.
Keywords: Photocatalytic degradation; HPLC; Iodosulfuron-methyl-sodium; Method validation; Measurement uncertainty
The meaning of measurement in metrology by Rod White (31-41).
The latest version of the International Vocabulary of Metrology gives a meaning of measurement restricted to quantities that can be represented by numerical values and placed in an ordinal sequence. This restrictive definition fits poorly with both the colloquial and the wider scientific understanding of measurement. This paper suggests an extension to the metrological definition of measurement, based on the measurement classification scheme of Stevens, to incorporate non-numerical and nominal measurements. The more inclusive definition and the classification scheme offers insights into the utility, metrological traceability, and limitations of measurements and uncertainty treatments, and enables clarification of other measurement-related definitions.
Keywords: Measurement; Metrology; Definitions; Scales; Nominal; Ordinal
The US Pharmacopeia: interfacing chemical metrology with pharmaceutical and compendial science by William F. Koch; Bei Ma (43-51).
The United States Pharmacopeial Convention (USP) is an official public standards-setting authority for prescription and over-the-counter medicines and other healthcare products manufactured or sold in the United States. USP’s standards are recognized and used in more than 130 countries around the globe. Over the last decade, USP has worked to advance measurement science for food and drug articles of commerce. Adoption of modern metrologic principles helps ensure that a measurement of one or more property values (attributes) of a food or drug article is acceptable without regard to when (time), where (space), or how (technology) the measurement was taken. The vision of USP is to achieve the international recognition, harmonization, and official acceptance for all USP reference standards based on sound scientific metrological principles. Current hot topics at USP, such as elemental impurities, adulteration, and contamination, as well as counterfeit drugs, are also discussed in this article.
Keywords: Metrology; Traceability; Uncertainty; Impurities; Adulteration
Recommendations on quantities, symbols and measurement units for publications in ACQUAL: preparation of tables and graphs by W. Richter (53-53).
Graduation ceremony at JRC-IRMM of the first graduates of the Euromaster Measurement Science in Chemistry (57-57).