Forgot your password?
New user?
New Window Opens on the Secret Life of Microbes: Scientists Develop First Microbial Profiles of Ecosystems
CAS announces the release of SciFinder Scholar for Mac OS X
Press Releases
Press Releases
| Check out our New Publishers' Select for Free Articles |
Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.15, #9)
A metrological traceability chain prevents circular reasoning in measurement design
by Paul De Bièvre (pp. 491-492).
Comparison of ED-XRF and LA-ICP-MS with the European reference method of acid digestion-ICP-MS for the measurement of metals in ambient particulate matter by Richard J. C. Brown; Kym E. Jarvis; Benoit A. Disch; Sharon L. Goddard; Elke Adriaenssens; Natacha Claeys (pp. 493-502).
A comparison study of the measurement of metals in ambient particulate matter collected on air filters, using energy-dispersive X-ray fluorescence (ED-XRF), laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) and wet chemical digestion followed by ICP-MS analysis according to the European ‘reference method’ EN14902, is presented. Whilst it is shown that the methods have a low systematic bias with respect to each other, overall they do show a high random variability, and when considered individually using regression methods, some analytes have shown bias with respect to the EN14902 method. The low systematic bias observed is not unexpected since both the ED-XRF and LA-ICP-MS methods have been calibrated using results from the EN14902 technique. The uncertainty of each analysis has been estimated and compared with the data quality objectives for uncertainty specified in the relevant European air quality legislation. This has tentatively shown that approximately 75% of the analyses using ED-XRF and LA-ICP-MS meet the requirements of the legislation. However, improvements in repeatability and calibration methods for both ED-XRF and LA-ICP-MS would be needed before these methods were truly applicable for routine use in air quality measurements of this type.
Keywords: ED-XRF; LA-ICP-MS; Air quality legislation; Ambient air; Metals
Internal quality controls applied in inductively coupled plasma mass spectrometry multi-elemental analysis in the second French Total Diet Study by S. Millour; L. Noël; R. Chekri; C. Vastel; A. Kadar; T. Guérin (pp. 503-513).
In 2006, the French Food Safety Agency (AFSSA) started the second French Total Diet Study to assess exposure to essential and non-essential elements through dietary intake by analysing 1352 food samples. A total of 21 elements were analysed to date by Inductively Coupled Plasma Mass Spectrometry after closed vessel microwave digestion, in 660 samples corresponding to at half of the study. This work presents and discusses the results of the various internal quality controls (IQC) applied to ensure that the analytical procedure is correctly carried out, from digestion to analysis, and to enable analytical chemists to validate the results obtained. The whole IQC allows to estimate uncertainties according to elements and to select those that should be discarded from the study or only given as indicative values for elements that were not within defined quality limits.
Keywords: Total Diet Study; Minerals and essential trace elements; ICP-MS; Multi-elemental analysis; Closed microwave digestion; Internal quality controls
Interpretation of the measurement results near the detection limit in gamma-ray spectrometry using Bayesian statistics by M. Korun; P. Maver Modec (pp. 515-520).
The requirement that the true value of an activity cannot be negative is used for the transformation of raw observed values, which can be positive or negative, into the expected activity values. The probability distribution of the activity values is a truncated Gaussian distribution, and the expected value and the variance of the activity values are derived from the observed value and its standard deviation. It has been shown that the standard deviation of the activity values is smaller than the standard deviation of the observed value and that the ratio of the standard deviation of the activity values and the expected value is less than unity. Since the expected activity value is larger than the original observed value, and the standard deviation of the activity values is smaller than the standard deviation of the observed value, the additional information, that the activity cannot be negative, leads to an improvement in the result. However, since the expected activity value depends on the standard deviation of the observed value, conservatively assessed standard deviation lead to a bias of the expected activity values.
Keywords: Gamma-ray spectrometry; Bayesian statistics; Observed value; Expected value; Variance
Measurement of small quantities: further observations on Bayesian methodology by R. Willink (pp. 521-527).
This article discusses the measurement of the sum of small positive quantities each estimated in separate analyses. It extends criticism recently given of ‘objective Bayesian’ methodology (Accred Qual Assur 15:181–188, 2010) and identifies two troublesome effects: the inherent bias in an analysis for a single quantity is compounded when the measurand is the sum of such quantities, and the precaution of overestimating measurement variability can actually make the resulting interval of measurement uncertainty less reliable! Unacceptable results are obtained. A cause of this behavior is identified, and a distinction between ‘objective’ and ‘subjective’ Bayesian statistics is discussed.
Keywords: Measurement uncertainty; Improper distribution; Non-informative distribution; Prior distribution; Propagation of error
Comments to EURACHEM/CITAC guide “Measurement uncertainty arising from sampling” by Peter-Theodor Wilrich (pp. 529-531).
The EURACHEM/CITAC Guide “Measurement uncertainty arising from sampling” deals with the design and analysis of experiments for the evaluation of the sampling and analytical standard deviation when a defined sampling and analytical method is used for the determination of the concentration, expressed as mass fraction (mg/kg), of an analyte in a specified material. The Guide recommends reporting the relative expanded uncertainty and using it directly, i.e. it implicitly assumes that the standard deviation is proportional to the mass fraction even in case the experimental data do not support this assumption. Example A1 (and some of the other examples of the Guide) demonstrates that this can result in extreme levels of underestimation or overestimation of the uncertainty of measurement results. Hence, such recommendations should be avoided!
Keywords: Measurement uncertainty; Relative expanded uncertainty; Sampling
Reply to comments on EURACHEM/CITAC guide “Measurement uncertainty arising from sampling” by Michael H. Ramsey; Stephen L. R. Ellison (pp. 533-535).
The Eurachem/CITAC Guide on ‘Measurement uncertainty arising from sampling’ describes a number of methods and approaches that can be used for the estimation of this uncertainty. A recent comment upon this Guide by Wilrich questions the expression of the measurement uncertainty in a form that is relative to the concentration, rather than just as an absolute number (i.e. as relative expanded uncertainty rather than expanded uncertainty), in one of the worked examples. This reply argues that the measurement results from the ‘duplicate’ method cannot reliably distinguish between constant standard deviation and constant relative standard deviation over the range observed in the example and that the most appropriate model must accordingly rely on prior knowledge. Since extensive prior knowledge indicates that the precision of sampling and of chemical analysis both tend to increase as a function of concentration, the body of the Guide recommends expression as a relative standard deviation. It is acknowledged that this assumption should be restated with the results of the worked examples as well as in its current position in the main body of the text, in a future edition of the Guide.
Keywords: Measurement uncertainty; Relative expanded uncertainty; Sampling
EU regulation for accreditation by Vagn Andersen; Martina Bednarova (pp. 537-538).
Accreditation in the European Union has since January 2010 been regulated by the EU Regulation on Accreditation and Market Surveillance, which establishes a legal framework for the provision of accreditation services in Europe.
Keywords: Europe; Accreditation; Regulation (EC) No. 765/2008; European co-operation for Accreditation
Developments in accreditation of flexible scopes in Europe by Martina Bednarova; Christina Waddington (pp. 539-542).
Accreditation of testing and calibration laboratories is the most frequently used type of accreditation. European Co-operation for Accreditation (EA) as the European network of recognised national accreditation bodies covers accreditation of all types of conformity assessment bodies. In Europe, the number of accredited testing and calibration laboratories covered under the EA Multilateral Agreement is about 15,000 nowadays.