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Zusammenfassung
If the value of a derived measurement standard is assigned by comparison with a reference standard of the same quantity, the uncertainty is increased by the additional uncertainty on the difference measurement. This basic fact has lead to the general belief that the uncertainty of derived standards is always larger than that of the reference standards. However, if the value of a derived standard is assigned by comparison with several independent reference standards using an appropriate average, the increase of uncertainty due to the uncertainty on difference measurement may be counterbalanced by the the well-known decrease of uncertainty through averaging. The gain of accuracy made possible by this mechanism is restricted to second-generation standards. Further gain through iteration is prevented by correlation between standards derived from the same set of reference standards. As a consequence, the concept of metrological hierarchy levels, relating to traceability chains, becomes questionable for traceability networks.
Uncertainty propagation via preparation, calibration and comparison steps in traceability networks
(1999)
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.
A first international 26Al interlaboratory comparison was initiated and evaluation of the results of the eight participating accelerator mass spectrometry (AMS) laboratories on four synthetic samples shows no difference in the sense of simple statistical significance. However, certain interlaboratory bias is observed with more detailed statistical analyses. Some laboratories seem to underestimate the uncertainties of their measurements, and others may overestimate measurement reproducibility.
KALIBRIERUNSICHERHEIT?
(2004)
After finishing Part I of the first international 26Al interlaboratory comparison with accelerator mass spectrometry (AMS) laboratories [S. Merchel, W. Bremser, Nucl. Instr. and Meth. B 223224 (2004) 393], the evaluation of Part II with radionuclide counting laboratories took place. The evaluation of the results of the seven participating laboratories on four meteorite samples shows a good overall agreement between laboratories, i.e. it does not reveal any statistically significant differences if results are compared sample-by-sample. However, certain interlaboratory bias is observed with a more detailed statistical analysis including some multivariate approaches.
Deoxynivalenol (DON), nivalenol (NIV) and zearalenone (ZEN) are toxic secondary metabolites produced by several species of Fusarium fungi. These mycotoxins are often found together in a large variety of cereal-based foods, which are regulated by maximum content levels of DON and ZEN. To date, suitable certified reference materials (CRM) intended for quality control purposes are lacking for these Fusarium mycotoxins. In order to overcome this lack, the first CRM for the determination of DON, NIV and ZEN in naturally contaminated wheat flour (ERM®-BC600) was developed in the framework of a European Reference Materials (ERM®) project. This article describes and discusses the whole process of ERM®-BC600 development, including material preparation, homogeneity and stability studies, and an interlaboratory comparison study for certification. A total of 21 selected expert laboratories from different European countries with documented expertise in the field of mycotoxin analysis took part in the certification study using various gas and liquid chromatographic methods. The certified values and their corresponding expanded uncertainties (k=2) were assigned in full compliance with the requirements of ISO Guide 35 and are as follows: 102±11 µgkg-1 for DON, 1000±130 µgkg-1 for NIV and 90±8 µgkg-1 for ZEN.
Acitivities of ISO/REMCO
(2011)
Mycotoxins are important non-anthropogenic food and feed contaminants, which can be present on almost every agricultural commodity. Effective consumer protection therefore essentially depends on food surveillance by reliable quantitative analysis enabled by appropriate quality control. Certified (matrix) reference materials (CRMs) are versatile tools to support Quality assurance. However, in the case of ochratoxin A (OTA), a hepato- and nephrotoxic mycotoxin, which is regulated in various foods, there is a lack of suitable CRMs. This lack has now been overcome by the development of two European Reference Materials (ERM®) for the Determination of OTA in roasted coffee (ERM®-BD475) and red wine (ERM®-BD476). This article discusses the material preparation process as well as the results of homogeneity and stability testing. Furthermore, the results of the inhouse certification studies carried out at BAM Federal Institute for Materials Research and Testing are presented and discussed. Interlaboratory comparison studies involving selected expert laboratories with documented Expertise in the field of mycotoxin analysis were conducted to confirm the certified values determined by BAM. The certified ochratoxin A values and their corresponding expanded uncertainties (k = 2) were assigned in full compliance with the requirements of ISO Guide 35 and are as follows:
(6.0 ± 0.6) µg kg-1 for roasted coffee, ERM®-BD475, and (0.52 ± 0.11) µg L-1 for red wine, ERM®-BD476.
The 2010 CCQM-K79 'Comparison of value-assigned CRMs and PT materials: Ethanol in aqueous media' is the second key comparison directly testing the chemical measurement services provided to customers by National Metrology Institutes (NMIs) and Designated Institutes (DIs). CCQM-K79 compared the assigned ethanol values of proficiency test (PT) and certified reference materials (CRMs) using measurements made on these materials under repeatability conditions. Nine NMIs submitted 27 CRM or value-assigned PT materials for evaluation. These materials represent many of the higher-order reference materials then available for this commercially and forensically important measurand.
The assigned ethanol mass fraction in the materials ranged from 0.1 mg/kg to 334 mg/kg. All materials were stored and prepared according the specifications provided by each NMI. Samples were processed and analyzed under repeatability conditions by one analytical team using a gas chromatography with flame ionization detection (GC-FID) method of demonstrated trueness and precision.
Given the number of materials and the time required for each analysis, the majority of the measurements were made in two measurement campaigns ('runs'). Due to a shipping delay from one NMI, an unanticipated third campaign was required. In all three campaigns, replicate analyses (three injections of one preparation separated in time) were made for one randomly selected unit of each of the 27 materials. Nine of the 27 materials were gravimetrically diluted before measurement to provide solutions with ethanol mass fraction in the established linear range of the GC-FID method. The repeatability measurement value for each analyzed solution was estimated as the mean of all replicate values. The within- and between-campaign variance components were estimated using one-way ANOVA. Markov Chain Monte Carlo Bayesian analysis was used to estimate 95% level-of-confidence coverage intervals for the mean values.
Uncertainty-weighted generalized distance regression was used to establish the key comparison reference function (KCRF) relating the assigned values to the repeatability measurements. On the basis of leave-one-out cross-validation, all of the assigned values for all 27 materials were deemed equivalent at the 95% level of confidence. These materials were used to define the KCRF.
Parametric bootstrap Monte Carlo was used to estimate 95% level-of-confidence coverage intervals for the degrees of equivalence of materials, d ± U95(d), and of the participating NMIs, D ± U95(D). Because of the very wide range of ethanol mass fraction in the materials, these degrees of equivalence are expressed in percent relative form: %d ± U95(%d) and %D ± U95(%D). The median of the absolute values of the %D for the participating NMIs is less than 0.05% with a median U95(%D) of less than 1%. These results demonstrate that the participating NMIs have the ability to correctly value-assign CRMs and proficiency test materials for ethanol in aqueous media and similar measurands.
Much has been published over the last years on finding the best (consensus) reference value (RV) for a given set of laboratory data attained in inter- or key comparisons (the latter with a specific view to the CIPM MRA). However, the discussion is ongoing. Except cases where real reference values are available from other sources, the quality of the (consensus value is of major importance in proficiency testing, method Validation, or reference material certification.
A distribution-free, self-consistent RV estimation approach has been developed which is based on the single criterion of participant's compatibility, at the cost of adjusted uncertainties. The final result is a fully compatible data set with minimum Overall variance, and adjusted uncertainties for those values reported which had to be made compatible with the (key comparison) reference value.
Any outlier discussion becomes obsolete, including considerations concerning different degrees of robustness of the various location and .dispersion estimators available. The penalty for "badly" performing laboratories is a large attributed uncertainty which forces them to either improve the measurement or adjust (i.e. reduce) their performance Claims. The evaluation algorithm is described. An example from a CCQM comparison is given, and consequences for the performance Claims of the participants discussed.