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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.13, #2)


Design of experiment for evaluation of uncertainty from sampling in the framework of the fitness for purpose concept: a case study by Ilya Kuselman (pp. 63-68).
The design of an experiment for the evaluation of sampling uncertainty in the framework of the fitness for purpose concept is described in terms of probabilities (risks of the user) of type 1 and type 2 errors in decisions concerning the significance of effects influencing the sampling uncertainty and the measurement results. As a case study, an experiment based on the duplicate method for quantification of the sampling uncertainty and inhomogeneity (heterogeneity) of a melt of tin-free bronze produced in a 10-ton reflective oven is analyzed. The melt is defined as the sampling target. It is shown that the number of such targets (melts), the number of samples under analysis and the number of replicate analyses can be minimized, i.e., the size and cost of the experiment can be reduced, if the user knows which risks are acceptable. When inhomogeneity of the sampling target has a systematic character, like the decrease of the mass fraction of aluminum from the beginning to the end of the melt pouring in the case study, the inhomogeneity effect can be separated from the sampling uncertainty and evaluated according to the user’s needs.

Keywords: Sampling uncertainty; Inhomogeneity; Design of experiment; Fitness for purpose; Tin-free bronze


A new internal quality control chart based on biological variation by Abdurrahman Coskun; Mustafa Serteser; Arno Fraterman; Ibrahim Unsal (pp. 69-75).
The instruments and analytical methods currently used in clinical laboratories today have better precision and stability than those used in the past. With the development of chemical and immunological methods and instrumentation technology, test precision has increased. However, the application of the Westgard multirule procedure to control sera in the Levey-Jennings chart may not be useful or cost-effective. We devised a new test-specific decision limit for accepting or rejecting runs based on the data of within-subject biological variation and prepared a control chart. We then applied these new limits and control chart to a group of tests performed in our laboratory. With the exception of the tests for albumin, total protein, and calcium, for which desirable performance standards could not be attained with the current technology and methodology, the application of these control limits was cost-effective and convenient. It is estimated that the value of healthy within-subject biological variations is constant, irrespective of the methodology, the area in which the study has been performed, and the number of subjects included in the study. We believe that control limits based on biological variation are reliable and cost-effective, and may be useful in modern and accredited laboratories.

Keywords: Biological variation; Decision interval; Levey-Jennings chart; Quality control; Quality management; Six Sigma; Westgard multirules


Traceable reference values for routine drinking water proficiency testing: first experiences by Michael Koch; Frank Baumeister (pp. 77-82).
Proficiency testing (PT) results have been used to improve traceability in chemical drinking water analysis. With a generalized least-square regression the mass concentrations of As and Sb were calculated in a drinking water that had been used to prepare proficiency testing samples by a spiking procedure. From the mass concentrations in the matrix and the spiked amounts, reference values with an uncertainty budget could be calculated without the need for reference measurements. The degree to which these reference values can be regarded as traceable is discussed. The results showed slight deviations in some samples between reference values and consensus means.

Keywords: Proficiency testing (PT); Traceability; Reference value; Drinking water


A practical procedure for assigning a reference value and uncertainty in the frame of an interlaboratory comparison by Stefaan Pommé; Yana Spasova (pp. 83-89).
A method is suggested for the calculation of a reference value and its uncertainty to be used in the frame of an interlaboratory comparison (ILC). It is assumed that the reference value of the measurand is determined independently from the ILC round. It is derived from a limited set of measurement results obtained from one or several expert laboratories. The procedure involves three stages: (1) check of the experimental data and possible corrections; (2) check of the consistency of data, and possibly increase of the uncertainties in order to attain internal consistency; (3) choice between fully, partially or un-weighted mean.

Keywords: Interlaboratory comparison; Reference value; Mean; Uncertainty


Estimation of the efficiencies and uncertainties of the extraction and cleanup steps of pesticide residue determination in cucumber using 14C-carbaryl by Osman Tiryaki; Dilan Baysoyu (pp. 91-99).
The estimation of the uncertainty of measurement has, nowadays, become an integral part of analytical results. The uncertainty and efficiency of extraction and cleanup are very important components of a pesticide residue analytical method. In this work, for a quick review of extraction and cleanup efficiency and to evaluate the individual analysis steps during method adaptation, 14C-carbaryl was applied at all fortification levels. Then, further analyses, such as ethylacetate extraction, filtration, evaporation and cleanup, were performed. The calibration of gel chromatograph column, performed with both 14C-carbaryl and fortification mixture (dichlorvos, malathion and chlorpyrifos), showed that pesticide fractions came through the column between the 8- and 23-mL fractions. The overall recovery of 14C-carbaryl after the extraction and cleanup step was 0.91, with the relative uncertainty of 0.084. Using the “bottom-up” approach, the uncertainty of extraction u cEX and cleanup u cGPC were 0.033 and 0.107, respectively. The combined standard uncertainty u c associated with the described analytical method was 0.112. Similar values were obtained using the alternative “top-down” approach: uncertainty of extraction u ct1 was 0.039, uncertainty of cleanup u ct2 was 0.108 and the combined standard uncertainty u cAV was 0.081. Both approaches showed that the uncertainty of cleanup was the main source of combined standard uncertainty.

Keywords: 14C-pesticide; Uncertainty of extraction and cleanup; GPC calibration; Cleanup efficiency


Collaborative sampling trial in the context of quality assurance in the German marine monitoring programme for the North Sea and the Baltic Sea by Michael Gluschke (pp. 101-107).
This paper presents the assessment of a collaborative trial in sampling in the Baltic Sea in the framework of quality assurance in the German marine monitoring programme for the North Sea and the Baltic Sea. The objective of investigations was to determine the influence of sampling on analytical results for selected monitoring parameters and to harmonize the procedure for sampling of sea water to a large extent. In these studies the staff of three vessels took replicate sea water samples, 1 m below the surface and below the halocline, at two monitoring stations. Mass concentration mean values for different nutrient parameters were obtained from each sample, all in one laboratory. Data produced from the hierarchical design were treated with robust analysis of variance (ANOVA) to generate uncertainty estimates, as standard uncertainties (“u” expressed as standard deviation), for geochemical variation (s geochem), primary sampling (s sampling), and chemical analysis (s analysis). Geochemical variation dominated the total variance in all cases. Sampling and analytical uncertainties contributed together up to 15% of the total variance and had a relative measurement uncertainty (u%) of less than 2% for all the parameters investigated. Thus for this study the sampling protocol and the analytical method could be regarded as fit-for-purpose.

Keywords: Sampling; Harmonization; Collaborative sampling trial; Baltic Sea; Quality assurance; Marine monitoring

The 1st International Proficiency Testing Conference, Romania 2007 by Graziela Guslicov; Paul De Bièvre (pp. 109-110).
On “statistics and measurement results in chemistry” by S. L. R. Ellison; A. Williams (pp. 111-112).
D. Brynn Hibbert: Quality Assurance for the Analytical Chemistry Laboratory by Katrice Lippa; John Sieber; David Duewer (pp. 115-116).
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