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.17, #1)
Cost effective, robust estimation of measurement uncertainty from sampling using unbalanced ANOVA by Peter D. Rostron; Michael H. Ramsey (pp. 7-14).
There is an increasing appreciation that the uncertainty in environmental measurements is vitally important for their reliable interpretation. However, the adoption of methods to estimate this uncertainty has been limited by the extra cost of implementation. One method that can be used to estimate the random components of uncertainty in the sampling and analytical processes requires the collection of duplicate samples at 10% of the primary sampling locations and duplicating the analyses of these samples. A new program has been written and applied to a modified experimental design to enable a 33% reduction in the cost of analysing this 10% subset, whilst accommodating outlying values. This unbalanced robust analysis of variance (U-RANOVA) uses an unbalanced rather than the balanced experimental design usually employed. Simulation techniques have been used to validate the results of the program, by comparison of the results between the proposed unbalanced and the established balanced designs. Comparisons are also made against the seed parameters (mean and standard deviation) used to simulate the parent population, prior to the addition of a proportion (up to 10%) of outlying values. Application to a large number of different simulated populations shows that U-RANOVA produces results that are effectively indistinguishable from the results produced by the accepted balanced approach and are equally close to the true (seed) parameters of the parent normal population.
Keywords: Uncertainty; Unbalanced design; Duplicate sample; Robust ANOVA; Accommodating outliers; Optimized uncertainty
Validation interlaboratory trial for ISO 12010: Water quality—Determination of short-chain polychlorinated alkanes (SCCP) in water by Sabine Geiß; Max Schneider; Gerhild Donnevert; Jürgen W. Einax; Nicole Lettmann; Aurélien Rey; Herbert Lepper; Birgit Körner; Theodor Prey; Bettina Hilger; Moana Engelke; Marie-Pierre Strub; Herve Adrien; George Sawal; Dirk Löffler; Tom Schillings; Ines Hussy; Philipp Steinbichl; Sigrid Scharf; Alexander Ruderisch; Jorge E. Olmos; F. Javier Santos; Arantxa Bartolome; Josep Caixach (pp. 15-25).
A validation interlaboratory trial was carried out to prepare ISO 12010: Water quality—Determination of short-chain polychlorinated alkanes (SCCP) in water—Method using gas chromatography/mass spectrometry (GC-MS) and electron capture negative ionisation (ECNI). The task was to determine the sum of short-chain polychlorinated n-alkanes with carbon chain lengths of C10–C13 and a chlorine content between 49% (g/100 g) and 67% (g/100 g) in water by GC-ECNI-MS. The quantification had to be performed by multiple linear regression as described in ISO/DIS 12010, the compulsory method. Samples distributed were real river samples, and waste water spiked with a target concentration of 0.4 and 0.6 μg/L for the sum of SCCPs, i.e. a concentration around the environmental quality target level according to the European Water Framework Directive. The different types of water samples tested were surface water with <150 mg/L suspended matter, surface water with 0.5 g/L suspended matter, and filtered waste waster. The interlaboratory trial included the extraction of the water samples, a column chromatographic clean up, a concentration step, and integration of chromatographic unresolved complex mixtures as well as the calibration and quantification by multiple linear regression. The reproducibility standard deviation of the standard concentration was 11.9%. Reproducibility standard deviations of concentrations in the three different water samples between 27.8 and 34.2% were achieved by 10–12 participating laboratories from six countries.
Keywords: Chloroalkanes C10–C13 ; Chloroparaffins; Priority substances; SCCP; PCA; European water framework directive; ISO 12010
Fitness for purpose in relation to specification limits by Manuel Alvarez-Prieto; Juan Jiménez-Chacón (pp. 27-34).
Frequently, the ultimate goal of an analytical measurement is to assess compliance in respect to a specification limit. So, the necessary requirement is to make a correct decision on the grounds of the measurement result. It is suggested a practical methodology to attain fitness for purpose results when specification limits are established for compliance assessment. It is based on a comparison between fitness for purpose uncertainty functions that define a high probability of conclusive correct decisions and a characteristic uncertainty function which describes the performance of the analytical system along concentration in a given condition of measurement. To attain fitness for purpose results, different replication programs are applied. Determinations of Zn, Mn and Cd in potable water are discussed as case studies. They represent different cases concerning relative position of detection and specification limits. A definition of fitness for purpose when specification limits are relevant is given. Quality of analytical results and analytical procedures should be associated with the concept of fitness for purpose.
Keywords: Fitness for purpose; Specification limits; Measurement uncertainty; Performance characteristics; Compliance assessment
Capability of laboratories to determine core nutrients in foods: results of an international proficiency test by Stephen W. C. Chung; Jeff C. H. Tran; W. W. Wong (pp. 35-44).
A proficiency test (PT) to assess the capabilities of laboratories to determine nutrients in a biscuit sample was carried out in September 2009. The need for such interlaboratory comparison arose from an increasing nutrition labeling requirements of different countries around the world. Forty-eight laboratories worldwide participated in this PT program for the determination of total lipids, saturated fat, trans-fat, protein, sugars, total dietary fiber (TDF), ash and sodium in food. This program was organized under the auspices of the Asia-Pacific Laboratory Accreditation Cooperation (APLAC). APLAC considers PT programs as one of the objective means of assessing the performance of the accredited laboratories. The program is one of the APLAC PT series whose primary purposes are to establish mutual agreement on the equivalence of the operation of APLAC member laboratories and to provide an opportunity to identify testing deficiency so that corrective actions can be taken, if necessary. The results of this program indicated that the consensus mean values estimated by robust statistics were in good agreement with the corresponding values obtained in the homogeneity tests except for trans-fat and TDF. The relative standard deviations of participant results for trans-fat and TDF were also found to be higher (19 and 24%, respectively) than the other test parameters. The relative standard deviations for other test parameters were less than 10%. One-half of the laboratories obtained satisfactory z-scores $$ (left| z
ight| < 3) $$ for all their reported results. Although participants were instructed to provide the measurement uncertainties of their reported results, only 27.5% of them had done so. Albeit many of the measurement uncertainties appear to be reasonable, some of them were found to vary widely, indicating that some laboratories still have difficulties in estimating measurement uncertainties. Although the overall measurement capability can be considered to be satisfactory, there is still room for improvement of analytical procedures.
Keywords: Asia-Pacific Laboratory Accreditation Cooperation; Food; Laboratory performance; Nutrient; Performance assessment; Proficiency test; Measurement uncertainty
Comparison of three electrochemical end-point detection methods to assay potassium dichromate by coulometric titration by Toshiaki Asakai; Akiharu Hioki (pp. 45-52).
Coulometric titrations with three electrochemical end-point detection methods were performed to assay potassium dichromate as a standard for oxidation–reduction titration. The assay as an oxidizing agent was carried out with ferrous ions produced by electrolytically reducing ferric ions. Three end-point detection methods were employed and compared with each other: constant potential amperometry, potentiometry, and constant voltage biamperometry (a dead-stop method). The last one was found to provide high accuracy in the coulometric titration of potassium dichromate. Solution form samples were also measured to confirm the possible existence of chromium(III) in potassium dichromate by both coulometric titration and ion-exchange chromatography with inductively coupled plasma time-of-flight mass spectrometry.
Keywords: Coulometric titration; ICP-TOFMS; Ion chromatography; Electrochemical end-point detection; Potassium dichromate; Certified reference material
Uncertainty estimation of anions and cations measured by ion chromatography in fine urban ambient particles (PM2.5) by Manuel A. Leiva G.; Ma. Consuelo Araya; Ana Maria Alvarado; Rodrigo J. Seguel (pp. 53-63).
The present work presents a measurement uncertainty evaluation according to Guide to the Expression of Uncertainty in Measurement (GUM) of the concentration of the cations K+ and Li+ and anions NO 3 −2 and SO 4 −2 in fine airborne particulate matter, refers to particles less than 2.5 μm in diameter (PM2.5), as measured by ion chromatography (US-EPA 300 method). The GUM method is not typically used to report uncertainty. In general, the analytical results only report the measurement’s standard deviation under repetition as an uncertainty; thus, not all sources of uncertainty are considered. In this work, the major sources of uncertainty regarding the measurements were identified as contributions to linear least square regression lines, repeatability, precision, and trueness. The expanded uncertainty was approximately 20% for anions and cations. The largest contribution to uncertainty was found to be repeatability.
Keywords: Uncertainty estimation; Air quality; Anions; Cations; PM2.5; Ambient particles; GUM
Method validation for determination of 13 elements in honey samples by ICP-MS by Maria Chudzinska; Anna Debska; Danuta Baralkiewicz (pp. 65-73).
A method for simultaneous ICP-MS determination of 13 elements in three types of honey from Poland is described. The method was validated, and the uncertainty budget was set up. The results obtained for the relative expanded uncertainties U rel (k = 2) were 15.1% for Al, 18.6% for B, 18.8% for Ba, 7.9% for Ca, 24.4% for Cd, 7.24% for Cu, 7.9% for K, 4.8% for Mg, 8.3% for Mn, 12.7% for Na, 14.9% for Ni, 12.5% for Pb and 13.4% for Zn. Traceability of the measurement results was established based on the use of the corn flour CRM INCT-CF-3 and the apple leaves CRM SRM 1515 and by analyzing spiked samples. Recovery rates between 94% (Zn) and 107% (Na) were found. The detection limits of all elements studied showed the suitability of the procedure for routine analyses. Summarizing it can be concluded that the described analytical procedures to measure the mass fractions of 13 elements in honey samples with established traceability and evaluated uncertainty allow to obtain reliable and internationally comparable results.
Keywords: Honey; Multielement analysis; ICP-MS; Traceability; Uncertainty; Validation parameters
Presentation of differential laser-induced fluorimetry as a reference measurement procedure for determination of total uranium content in ores and similar matrices by D. P. S. Rathore; Manjeet Kumar; P. K. Tarafder (pp. 75-84).
The metrological principle of ‘differential technique in laser-induced fluorimetry’ analysis is discussed and recommended as a reference measurement procedure for determination of total uranium content in ores and similar matrices. The estimated relative expanded uncertainty values obtained for uranium content in standard IAEA samples are, S 1, 0.04 g/kg, S 2, 0.06 g/kg, S 3, 0.04 g/kg, and for S 4, 0.10 g/kg, respectively. These low uncertainty values obtained for uranium show high metrological quality of differential technique. This reference measurement procedure guarantees the quality of an analytical result (accuracy, high precision, reliability, comparability, and traceability). Laser-induced fluorimetry will be useful for the analysis of uranium in ores, certification of reference materials, borehole core assay, and other diverse applications in nuclear fuel cycle. Differential technique in spectrophotometry/laser fluorimetry has inherent high metrological quality. In principle, laser-induced fluorimetry is an ideal technique for the very accurate determination of uranium by the use of appropriate fluorescence-enhancing reagents and methodology depending upon the concentration of uranium and sample matrices.
Keywords: Laser-induced fluorimetry; Differential technique; Reference measurement procedure; Measurement uncertainty; Standard materials
Principles for constructing notation in unit systems and their application to the SI by Marcus P. Foster (pp. 85-92).
The SI is the world’s premier scientific instrument and, like all instruments, must be updated as science and technology advance. The CIPM is planning to redefine four SI base units in terms of invariants of nature and has already dubbed this revision the ‘New SI’. This title will confuse most SI users, who are impacted only by the structure and notation of the SI, and neither of these has changed. A truly ‘New SI’, to be practical in the information age, would have at minimum an unambiguous, and preferably a machine- and user-friendly, notation. These and other principles for constructing a notation are proposed, and the SI historical and current notations are examined against these. Several levels of remediation are suggested.
Keywords: SI units; Parsing; Notation; Ambiguity; Character set; Precedence
What exactly is uncertainty? by Michael Thompson (pp. 93-94).
Uncertainty is defined in VIM3 as a ‘parameter’ but that, in my view, is a mistake that detracts from the clarity of the concept. Trying to overcome the resulting difficulties while retaining ‘parameter’ has brought about progressive amendments to the definition, and an increasing list of footnotes that have failed to resolve the issue. Surely the uncertainty of a result is the density function (or mass function) that best describes the probability of possible values of the measurand.
Keywords: Uncertainty; Definition; Probability density function; Confidence band
On the use of the mean squared error as a proficiency index by Ismael Castelazo; Yoshito Mitani (pp. 95-97).
The mean squared error is presented as a convenient parameter to be used in assessing laboratories participating in proficiency tests. Its main advantages and disadvantages are presented and compared with the z score and the normalized error. A proficiency index is proposed as the ratio of an estimate of the mean squared error over a reference uncertainty.
Keywords: Proficiency tests; Performance index; Mean squared error
General guidance for the value assignment of reference materials: a proposed scope for the revision of ISO Guide 35
by Angelique Botha (pp. 99-100).
TDRM/TDLM workshop on reference materials and laboratory accreditation at the AOAC annual meeting 2011
by Hendrik Emons; Jane Weitzel; John Budin; Melissa Phillips; Catherine Rimmer; Donna Zink (pp. 101-105).