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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.9, #10)
A priori evaluation of the adequacy of reference materials by Ilya Kuselman (pp. 591-596).
The concept of adequacy of reference materials (RMs) to samples under analysis is discussed. It is emphasized that effectiveness of RM as a measurement standard can depend dramatically on assessment of the RM adequacy. A score is proposed for quantitative adequacy evaluation that formalizes the existing analytical knowledge. The score is based on the comparison of the compositions and physico-chemical parameters of RM with those of a sample, choosing the parameters influencing the analytical results. The score allows us to find the most adequate RM before analysis, using RM catalogues and prior information about the sample and analytical method.
Keywords: Reference materials; Samples; Analytical methods; Adequacy; Selection
Correlation between repeated measurements: bane and boon of the GUM approach to the uncertainty of measurement by W. Haesselbarth; W. Bremser (pp. 597-600).
With measurement uncertainty estimation accounting for all relevant uncertainty contributions, the results of measurements using the same procedure on different objects or samples may no longer be considered as being independent, and correlations have to be taken into account. For this purpose, a simple approximation for the estimation of covariances is derived and applied to the estimation of uncertainty for some basic combinations of two measurement results. This covariance estimate is also applied to the estimation of uncertainty for the mean value of the results of replicate measurements on the same object or sample.
Keywords: Uncertainty; GUM approach; Correlation; Covariances; Repeated measurements
Developments in external quality assessment for clinical microbiology laboratories by Michael A. Noble (pp. 601-604).
There has been considerable development in external quality assessment programs for clinical microbiology laboratories. Over the last 5 years samples have progressively become more clinically realistic and relevant. Programs are tending to produce more samples that look and act like real samples. As programs change to a broader range of challenge targets, the observed rate of errors is substantially greater for pre-analytic and post-analytic challenges than with analytic phase challenges. Rather than focusing only on analytic phase procedures, schemes should target to a greater extent pre-analytic and post-analytic aspects of the laboratory cycle.
The uncertainty in the calculation of the amount of blocked and reactive lysine in milk products, as determined by the furosine method by Esther Campos Giménez; Thierry Bénet; Lionel Spack (pp. 605-614).
The measurement uncertainty in the calculation of the amount of blocked and reactive lysine (as determined by the furosine method) was evaluated according to the procedure described in the Eurachem/CITAC guide. The analytical method involves the chromatographic determination of lysine and furosine after acid hydrolysis. The calculation of blocked and reactive lysine in the initial protein is based on known conversion factors. The estimation of the uncertainty was performed in two steps: (1) determination of the uncertainty in the chromatographic determination of lysine and furosine, and (2) determination of the uncertainty in the calculation of blocked and reactive lysine. The individual contributions to the final uncertainty were identified, quantified, and combined in uncertainty budgets. The largest contribution to the calculation of blocked lysine came from estimating the conversion factor of blocked lysine into furosine during acid hydrolysis. For the calculation of reactive lysine, the main contribution came from the chromatographic determination of lysine. The uncertainty estimates were compared to available validation data (in-house and collaborative standard deviations of reproducibility).
Keywords: Measurement uncertainty; Uncertainty budget; Lysine; Furosine; Amino acids
International Measurement Evaluation Programme (IMEP); IMEP-14: Picturing the performance of analytical laboratories measuring trace elements in sediment by I. Papadakis; L. Van Nevel; E. Vendelbo; J. Norgaard; P. Taylor (pp. 615-623).
The International Measurement Evaluation Programme (IMEP) is an interlaboratory comparison scheme founded, owned and co-ordinated by the Institute for Reference Materials and Measurements (IRMM) since 1988. IMEP-14, for the first time in the series, is focusing on trace elements in sediment. Reference values for ten elements stating total concentrations and expanded uncertainties according to GUM were established. In total, 239 laboratories from 43 countries in five continents participated in the comparison, and the degree of equivalence between the results from the laboratories and the reference values is presented graphically. Identical samples were distributed to nine National Measurement Institutes within the frame of the CCQM-P15 pilot study. Participation in the IMEP-14 interlaboratory comparison was offered to laboratories in the EU new member states and acceding countries according to the IRMM’s “Metrology in Chemistry support programme for EU new member states and acceding countries”.
Keywords: Interlaboratory comparison; Sediment analysis; Uncertainty; Traceability; Isotope dilution mass spectrometry; EU new member states and acceding countries; TrainMiC
Sieving of sediments for subsequent analysis of metal pollution: results of a German interlaboratory study by Michael Gluschke; Peter Woitke; Jörg Wellmitz; Peter Lepom (pp. 624-628).
An interlaboratory study was carried out to get information on repeatability and reproducibility of sieving, and how sieving influences the variability of analytical measurements. Thirteen laboratories took part, sieving three marine sediment samples to get particle size fractions (<20/>20 μm; <63/>63 μm), and determine their percent by weight. Repeatabilities between 2 and 13% and reproducibilities between 25 and 75% were obtained. Differences in the sieving procedures applied by the participants may be proposed as a reason for the high variability in the results. The determination of metals revealed that they accumulated in the below 20 μm fraction. These findings support the common practice in German sediment monitoring programmes of using the <20 μm fraction of sediments for metal analysis. The reproducibilities of the analyses of metals in sediments were between 5 and 15%, and indicate a significant influence of sieving on the results.
Keywords: Sieving; Sediments; Interlaboratory study; Metals; Marine monitoring
Measurement uncertainty in water microbiology may not estimate the laboratory’s precision by H. E. Tillett; D. Sartory (pp. 629-632).
It has been proposed that microbiology laboratories introduce a measurement uncertainty for organism counts. Suggested methods of achieving this involve studying replicate counts from subsamples. This will overwhelmingly estimate the natural variation in numbers of organisms present in different subsamples, and the different characteristics of strains of organisms within the species or group (e.g. the coliform bacteria) being counted. Thus any measure of the laboratory’s competence, in terms of precision, will be masked. In chemistry there is likely to be much more complete homogeneity of an analyte in a test material and these replicate measures will illustrate whether the laboratory is competent in achieving accurate results.
Keywords: Measurement uncertainty; Microbiology; Water quality; Statistics
Uses and abuses of PT by Nick Boley (pp. 633-634).
Proficiency testing (PT) is a very valuable quality tool for laboratories making all kinds of analytical measurements, in chemistry, microbiology and laboratory medicine. Compared with other quality tools, which are largely concerned with INPUTS (e.g. staff training, calibration, documentation), PT is concerned with OUTPUTS (i.e. the actual results produced by the laboratory). The quality of routine measurements and results, which would normally be reported to customers, both internal and external, should be demonstrated. This is a major reason for many laboratories participating in PT, even if they are not accredited, or use very few, if any, other quality tools. This is important, for it enables the laboratory manager to measure the efficacy of the overall quality system, and to determine if the inputs to this are appropriate. This should be done in a spirit of learning—it was discussed at the previous workshop whether PT schemes are “teachers” or “policemen”, and the consensus was that MOST are teachers (or at least try to be). Learning from mistakes, investigating problems, implementing corrective actions and then observing whether thisis successful in the next PT represents what many in our profession would regard as a proper and effective use of PT. Although this may be the intended primary use of PT by laboratories, we must not forget that PT has other valid uses. In particular, the use of PT to demonstrate competence to third parties—accreditation bodies, regulators, and customers—has become more important in the last few years. However, this development, however valuable, can lead to abuse of PT scheme data. This paper will highlight briefly some examples of both proper use and abuse of PT. These are taken from my experience with PT schemes over many years.
Proficiency testing in analytical chemistry, microbiology and laboratory medicine: working group discussions on current practice and future directions by Brian Brookman; Ioannis Papadakis; Alan Squirrell; Steve Evans; Steve Ellison; Ulf Örnemark; Nick Boley; Anita Schmidt; Ellen van Berkel; Barry Tylee; Mike Noble; Michael Koch; Dan Tholen (pp. 635-641).
A summary of the working group (WG) discussions on proficiency testing (PT) and external quality assessment (EQA) held at the joint EURACHEM/CITAC/EQALM workshop, Bracknell, UK, 16–18 February 2003 is provided. The nine WG’s covered a range of issues concerned with current practice and future directions; PT/EQA as a tool for regulators (WG1); PT/EQA as a tool for accreditation (WG2); evaluation of performance and uncertainty (WG3); frequency of PT/EQA participation (WG4); selection of appropriate PT/EQA schemes (WG5); added value of PT/EQA and cost benefit evaluation (WG6); global harmonisation and rationalisation (WG7); new technical areas and challenges in PT/EQA (WG8); and accreditation of PT/EQA providers (WG9). Participants with different backgrounds were on each WG in order to capture a range of views and experience from different sectors. The discussions reflected on the keynote lectures and built, in many cases, on discussions at previous workshops in 2000 and 2002.
Keywords: Accreditation; External quality assessment; International harmonisation; Proficiency testing; Uncertainty
Measurement uncertainties and specified limits: what is logical or common sense in chemical measurements?
by Alain Marschal (pp. 642-643).