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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.3, #3)
2nd EURACHEM Workshop on Measurement Uncertainty in Chemical Analysis
by Wolfhard Wegscheider (pp. 90-90).
Michael Brodsky, member of the Advisory board of our Journal, has been honoured with the Amethyst Award
by Paul De Bièvre; Helmut Günzler (pp. 91-91).
Introduction to measurement uncertainty in chemical analysis by A. Williams (pp. 92-94).
This paper reviews the experience of the use of the Eurachem Guide and gives a brief overview of the principles of evaluating uncertainty. This is followed by discussion of the comments received on the Guide and highlights some of the issues that need to be considered in the next version.
ISO uncertainty and collaborative trial data by S. L. R. Ellison (pp. 95-100).
The possibility of using interlaboratory study repeatability and reproducibility estimates as the basis for measurement uncertainty estimates is discussed. It is argued that collaborative trial reproducibility is an appropriate basis for estimating uncertainty in routine testing provided certain conditions are met by the laboratory. The primary shortcomings relate to establishment of traceability and consequent estimation of bias associated with the method, and quantitatively establishing the relevance to the single laboratory. Approaches to resolving both difficulties are proposed, the former via full implementation of trueness determination suggested in ISO 5725 : 1994 or by independent checks on individual accuracy and precision, the latter via a reconciliation procedure. The paper also discusses other factors including sampling and sample pre-treatment, change in sample matrix, and the influence of level of analyte.
Keywords: Key words Measurement uncertainty; Collaborative study; Bias; Trueness
Estimating measurement uncertainty: reconciliation using a cause and effect approach by S. L. R. Ellison; V. J. Barwick (pp. 101-105).
A strategy is presented for applying existing data and planning necessary additional experiments for uncertainty estimation. The strategy has two stages: identifying and structuring the input effects, followed by an explicit reconciliation stage to assess the degree to which information available meets the requirement and thus identify factors requiring further study. A graphical approach to identifying and structuring the input effects on a measurement result is presented. The methodology promotes consistent identification of important effects, and permits effective application of prior data with minimal risk of duplication or omission. The results of applying the methodology are discussed, with particular reference to the use of planned recovery and precision studies.
Keywords: Key words Measurement uncertainty; Validation; Reconciliation; Cause and effect analysis
Shall we consider covariances? by W. Bremser; Werner Hässelbarth (pp. 106-110).
The impact of correlation between the reference points used in determination of a functional relationship by application of least-squares techniques is discussed. Calculations are based on an improved effective-variance approach to the general least-squares problem. From a simulation study, general conclusions about the impact of correlation are drawn. Standard addition as an example of a method using correlated data is discussed in detail.
Keywords: Key words Uncertainty; Calibration; Weighted regression; Covariances
The interpretation and use of stability data in the certification of reference materials by K. Heydorn (pp. 111-114).
Stability tests are carried out on candidate reference materials in order to ascertain that the certification values continue to be valid a reasonable time after completion of the certification analysis. These tests are also used for recommending storage conditions, as well as the duration of storage before certification values need be rechecked. BCR (Community Bureau of Reference) reference materials do not normally have an expiry date, but rely on stability monitoring throughout the lifetime of the certified material. The 1997 version of the BCR Guidelines for the production and certification of reference materials does, however, take into account the necessity of limiting the validity of a certification, when degradation of the material during storage cannot be ignored. This paper discusses an example of significant degradation taking place between the time of completion of the certification analysis and the issue of a formal certificate. Various options are presented together with an account of their influence on the certified values and their uncertainties.
Keywords: Key words Certified reference materials; Stability; Validity
Uncertainty – The key topic of metrology in chemistry by Werner Hässelbarth (pp. 115-116).
At the Second EURACHEM Workshop on Measurement Uncertainty in Chemical Analysis the author had the pleasure of chairing a working group on chemical metrology. This note presents some propositions arising from the preparation of, as well as from the discussion at and after, the working group session.
Keywords: Key words Uncertainty; Metrology; Comparability; Traceability; Validation; Bias
Uncertainty of sampling in chemical analysis by M. Thompson (pp. 117-121).
Uncertainty of sampling is the contribution from sampling errors to the combined uncertainty associated with an analytical measurement when the measurand is the concentration of the analyte in the 'target', the total bulk of material that the sample is meant to represent. Of the errors considered to contribute to uncertainty, random errors of sampling, characterised by precision, are much more accessible to investigation than those due to bias. Where an approximation to random sampling can be achieved, realistic precisions can normally be estimated. In some instances reproducibility precision is significantly greater than repeatability precision, and the contribution of between-sampler variations to sampling uncertainty must be acknowledged. However, the collaborative trial of a sampling method is an expensive and difficult exercise to execute. A system of internal quality control for routine sampling can be introduced. Fitness for purpose has been defined in terms of the required combined uncertainty of sampling and analysis.
Keywords: Key words Uncertainty; Sampling; Fitness for purpose; Collaborative trial; Internal quality control
Influence of two grinding methods on the uncertainty of determinations of heavy metals in atomic absorption spectrometry/electrothermal atomisation of plant samples by G. Stringari; Ivo Pancheri; Frank Möller; Osvaldo Failla (pp. 122-126).
Chemical analyses of trace elements are affected by relatively high analytical errors due to the different steps of the laboratory procedures: samples grinding, mineralisation and instrumental measurements. In the present communication, the influence of the grinding phase on the global uncertainty of Pb, Cd, Ni and Cr determinations in plant samples by the classical method of atomic absorption spectrometry/electrothermal atomisation (AAS-ETA) after dry ashing is quantified. Two grinding machines, a planetary mill with balls and jars of agate versus a stainless steel grinder were compared by analysing leaf samples of cucumber, strawberry, kiwivines, apple trees and grapevines from agricultural experimental plots under controlled conditions. Variance components due to the difference between grinding methods and experimental plots were estimated. Further, the simultaneous effects of the grinding methods on all considered metals have been evaluated by analysis of variance. With the stainless steel grinder, on average, higher levels of the considered heavy metals were obtained (up to 67% of the mean values). On average, the increments were similar for metals contained in steel (Ni and Cr) and those not contained (Pb and Cd). The true causes of these differences need further investigation to determine whether the higher metal detection is due to possible contamination, to a different grinding quality or to other reasons. Finally, the grinding methods did not seem to affect the combined uncertainty of the analyses.
Keywords: Key words Atomic absorption spectrometry/electrothermal atomisation; Grinding machines; Trace elements; Uncertainty; Variance components
In- and off-laboratory sources of uncertainty in the use of a serum standard reference material as a means of accuracy control in cholesterol determination by André Henrion (pp. 127-130).
Repeated subsampling or a hierarchical design of experiments combined with an analysis of variance (ANOVA) is demonstrated to be a useful tool in the determination of uncertainty components in amount-of-substance measurements. With the reference material of human serum as investigated here for total cholesterol, besides several in-laboratory sources of uncertainty, a vial-to-vial effect which can be regarded as an off-laboratory source was found to be significant. This knowledge might be essential when the material is used for calibration and for the self-assessment of a laboratory.
Keywords: Key words Experimental design; Analysis of variance (ANOVA); Determination of uncertainties; Human serum; Cholesterol; Standard reference material
To what extent can an uncertainty calculation be general? by I. Kuselman (pp. 131-133).
It is argued that results of uncertainty calculations in chemical analysis should be taken into consideration with some caution owing to their limited generality. The issue of the uncertainty in uncertainty estimation is discussed in two aspects. The first is due to the differences between procedure-oriented and result-oriented uncertainty assessments, and the second is due to the differences between the theoretical calculation of uncertainty and its quantication using the validation (experimental) data. It is shown that the uncertainty calculation for instrumental analytical methods using a regression calibration curve is result-oriented and meaningful only until the next calibration. A scheme for evaluation of the uncertainty in uncertainty calculation by statistical analysis of experimental data is given and illustrated with examples from the author's practice. Some recommendations for the design of corresponding experiments are formulated.
Keywords: Key words Analysis uncertainty; Uncertainty in uncertainty; Statistical estimations; Analytical method validation; Design of experiment