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, #2)
EURACHEM/CITAC workshop on recent developments in measurement uncertainty
by Alex Williams (pp. 111-113).
Development of metrological models for internal standard single-point and multi-point calibrations for the assessment of tear gas weapons compliance with legislation by Daniel M. Silveira; Pedro A. S. Salgueiro; M. Filomena G. F. C. Camões; Ricardo J. N. Bettencourt da Silva (pp. 115-127).
Tear gases are the most widely used non-lethal weapons, both by security forces and by the general public. The pepper spray, whose active agent is capsaicin, is the only self-defence aerosol allowed in Portugal, where capsaicin concentration must be below 5 g/100 mL. The cost-effective evaluation of the compliance of self-defence weapons with legislation involves the use of two measurement procedures with increasing quantitative capability. Samples are first assessed by preliminary measurement procedure based on single-point GC–MS calibration. Whenever the measurement uncertainty from this assessment makes evaluation inconclusive, the evaluation of sample compliance using multi-point GC–MS calibration is performed. Metrological models including sound criteria for the evaluation of sample compliance with legislation were developed for both measurement procedures. Such models include the evaluation of the impact of instrumental performance, calibration model, sample dilution and standards preparation on measurement uncertainty. The relative expanded uncertainty, in the studied range (capsaicin 3–7 g/100 mL), of measurements supported in single-point calibrations ranged from 10 to 22% and the ones supported on multi-point calibrations from 8 to 12% depending on capsaicin concentration and daily GC–MS repeatability. Measurements are fit for the intended use since they present a relative expanded uncertainty smaller than a target value of 30, or 15%, for measurements supported in single- or multi-point calibrations, respectively.
Keywords: Internal standard; Single-point calibration; Multi-point calibration; Uncertainty; Capsaicin; Tear gas weapons
Uncertainty of standard addition experiments: a novel approach to include the uncertainty associated with the standard in the model equation by Anna-Lisa Hauswaldt; Olaf Rienitz; Reinhard Jährling; Nicolas Fischer; Detlef Schiel; Guillaume Labarraque; Bertil Magnusson (pp. 129-138).
A new model equation for determining the measurement result in standard addition experiments was derived and successfully applied to the quantitative determination of rhodium in automotive catalysts. Existing equations for standard addition experiments with gravimetric preparation were changed in order to integrate the novel idea of including the uncertainty associated with the standard into the model equation. Using this novel equation combined with the ordinary least squares algorithm for the regression line also yielded a new formula for the associated measurement uncertainty. This uncertainty accounts for the first time for the uncertainty associated with the standard. The derivation for the model equation and the resulting associated measurement uncertainty is shown for gravimetric standard addition experiments both with and without an internal standard.
Keywords: Standard addition; Measurement uncertainty; Internal standard; Gravimetric preparation; Metrological traceability
The estimation and use of measurement uncertainty for a drug substance test procedure validated according to USP <1225> by M. L. Jane Weitzel (pp. 139-146).
The vision for metrology at the United States Pharmacopeia (USP) is for international recognition, harmonization, and official acceptance of all USP reference standards that are based on sound, scientific, metrological principles, such as measurement uncertainty (MU). Pharmaceutical testing laboratories will need to estimate the MU for their test procedures. This paper demonstrates how to estimate MU for a test procedure validated following the requirements in USP General Chapter <1225>, Validation of Compendial Procedures. The test procedure determines the assay and impurities for a pharmaceutical drug substance. Pharmaceutical manufacturing companies are required to test the drug substance used in their drug products. In this example, a new test procedure was developed and validated following the requirements in USP <1225>. The MU was estimated using the data from the test procedure validation. The MU estimate determined that the test procedure was fit for use with one determination, avoiding the expense of duplicate determinations. The MU estimate was used to set up decision rules for comparing test data to the assay and impurity specifications. The information from the MU estimate was used to monitor the routine use of the test procedure using control charts that assess accuracy and precision of each run based upon actual test procedure performance capability.
Keywords: Drug substance; Estimation; Reporting; United States Pharmacopeia
Assessment of the determination of water-soluble ionic composition of atmospheric aerosols from the analysis of crossed halves of filters by Ricardo J. N. Bettencourt da Silva; Alejandro Arias; Cristina M. R. R. Oliveira; M. Filomena G. F. C. Camões (pp. 147-157).
The water-soluble ionic composition of atmospheric aerosols is a key parameter for estimating its anthropogenic or natural origin. This evaluation depends on the reliability of the performed measurements. The differential approach for the evaluation of the measurement uncertainty was used for estimating, separately, the uncertainty associated with the extraction step, by difference from the observed intermediate precision and the combination of all the other uncertainty components affecting the estimated intermediate precision. The intermediate precision was estimated from the difference of results of the analysis, in different days, of several pairs of filters resulting from cross-changing of their halves. The precision associated with the symmetry of filters division, affecting the homogeneity of studied combined pairs of filters, was subtracted from the observed dispersion of results. The resulting detailed model of measurement performance allowed defining strategies for cost of analysis or magnitude of measurement uncertainty reduction. Measurements are fit for the analysis of urban aerosols since the expanded relative measurement uncertainty is smaller than a target value of 30 %.
Keywords: Atmospheric aerosols; Ionic composition; Method validation; Measurement uncertainty; Crossed filter halves
Uncertainty estimation to evaluate mass balances on a combustion system by Paula Teixeira; Helena Lopes; Ibrahim Gulyurtlu; Nuno Lapa (pp. 159-166).
Mass balances of ash and potassium for a fluidized bed combustor were performed incorporating measurement uncertainties. The total output mass of ash or a chemical element should be equal to the mass in the input fuel; however, this is not often achieved. A realistic estimation of recovery uncertainty can support the reliability of a mass balance. Estimation of uncertainty helps to establish a reliable evaluation of the recovery ratio of ash mass and elemental mass. This may clarify whether any apparent lack in closing the mass balance can be attributed to uncertainties. The evaluation of measurement uncertainty for different matrices, namely coal, biomass, sand and ashes from different streams was based on internal quality control data and external quality control data, namely analysis of samples from proficiency tests or use of a certified reference material. The evaluation of intermediate precision and trueness allowed the estimation of measurement uncertainty. Due to the different physic and chemical characteristics of the studied matrices, the uncertainty of precision was evaluated using R-charts of data obtained from the analysis of duplicates for the majority of samples. This allowed evaluating sample heterogeneity effects. The instrumental acceptance criterion was also considered and included in the combined uncertainty. The trueness was evaluated using data from several proficiency tests and from analysis of a certified reference material or sample spiking. Statistically significant bias was included.
Keywords: Uncertainty budget; Precision; Trueness; Mass balance
The content of chlorides in concrete designs: a probabilistic approach to compliance assessment with due regard to the measurement uncertainty by Wilfried Hinrichs (pp. 167-175).
A major factor for the durability of concrete is the protection against corrosion of reinforcing steel. One out of some corrosion agents are dissolved chlorides. Therefore, their contents are limited in the raw materials and subject to a compliance assessment. The concrete standard EN 206-1 provides both a semi-probabilistic calculation method for the design of concrete as regards the input of chlorides through the components and a decision rule for compliance assessment. This paper describes a full probabilistic approach to the compliance assessment taking into account the heterogeneity of the components and the uncertainty of testing. The content of Cl− is defined as the sum of gamma-distributed parameters. The uncertainty data are taken from the test procedures and from effects due to sampling procedures. The proposed approach is quantified for a high-performance and for a simple concrete. It allows calculating upper limits of chloride contents according to the client’s requirements for conformity and/or for producer’s/consumer’s risks. Additionally discussed are the particular financial consequences of incorrect decisions. Major results of the application of the probabilistic approach are that the upper limits of the chloride contents are comparable to those of the semi-probabilistic one and that the risks for high-performance concretes are not negligible. The decision rule given in EN 206-1 is easy to apply but not unambiguous. The probabilistic approach permits to define concepts for decision rules based on the probability of conformity or on the appropriate risks. For risk-based concepts, the individual performance of the testing laboratory can be taken into account.
Keywords: Compliance; Measurement uncertainty; Decision rule; Risk; Profit
Quantifying uncertainty in determination of polysaccharides in glycoconjugate vaccines based on in-house validation data by Susanna Murtas; Andrea Gaggioli; Christina von Hunolstein (pp. 177-182).
The measurement uncertainty in quantifying polysaccharides in glycoconjugate vaccines was estimated in order to ascertain compliance of these products in the batch release procedure of an Official Medicines Control Laboratory. A single-laboratory validation approach of an internal analytical procedure to quantify the total saccharide by the way of the sialic acid (N-Acetylneuraminic acid) concentration in a meningococcal glycoconjugate vaccine by High Performance Anion Exchange Chromatography coupled with Pulsed Amperometric Detection is reported. Uncertainty components from precision, bias and other sources were evaluated. It turned out that uncertainty is dominated by the precision contribution, while bias was mainly due to the non-recovery from hydrolysis. As a consequence, a correction factor was considered. At a confidence level of approximately 95%, the expanded uncertainty of the total polysaccharide concentration in a vaccine, when measured using the internal method, was found to be 11.5%.
Keywords: Quantifying uncertainty; Bias; Precision; Glycoconjugate vaccines
Measurement uncertainty of food carotenoid determination by M. Graça Dias; M. Filomena Camões; Luísa Oliveira (pp. 183-189).
For consistent interpretation of an analytical method result it is necessary to evaluate the confidence that can be placed in it, in the form of a measurement uncertainty estimate. The Guide to the expression of Uncertainty in Measurement issued by ISO establishes rules for evaluating and expressing uncertainty. Carotenoid determination in food is a complex analytical process involving several mass transfer steps (extraction, evaporation, saponification, etc.), making difficult the application of these guidelines. The ISO guide was interpreted for analytical chemistry by EURACHEM, which includes the possibility of using intra- and interlaboratory information. Measurement uncertainty was estimated based on laboratory validation data, including precision and method performance studies, and also, based on laboratory participation in proficiency tests. These methods of uncertainty estimation were applied to analytical results of different food matrices of fruits and vegetables. Measurement uncertainty of food carotenoid determination was 10–30% of the composition value in the great majority of cases. Higher values were found for measurements near instrumental quantification limits (e.g. 75% for β-cryptoxanthin, and 99% for lutein, in pear) or when sample chromatograms presented interferences with the analyte peak (e.g. 44% for α-carotene in orange). Lower relative expanded measurement uncertainty values (3–13%) were obtained for food matrices/analytes not requiring the saponification step. Based on these results, the saponification step should be avoided if food carotenoids are not present in the ester form. Food carotenoid content should be expressed taking into account the measurement uncertainty; therefore the maximum number of significant figures of a result should be 2.
Keywords: Food; Carotenoids; HPLC; Measurement uncertainty
Metrological traceability for benzo[a]pyrene quantification in airborne particulate matter by Francesca Rolle; Valter Maurino; Michela Sega (pp. 191-197).
Airborne particulate matter (PM) represents one of the most important sources of urban pollution due to its physical and chemical properties. There is a great concern for PM dangerous effects on human health because particles can deeply penetrate into the respiratory system, carrying the contaminants adsorbed onto their surface. Polycyclic aromatic hydrocarbons (PAHs) are a class of organic contaminants that can be adsorbed onto PM and can have harmful effects on health, due to their particular chemical structure. The International Agency for Research on Cancer (IARC) classified PAHs as potential carcinogenic agents and benzo[a]pyrene (BaP) as carcinogenic to humans. In this paper, the development of a metrologically traceable procedure for the quantification of BaP in airborne PM, performed at the Italian National Institute of Metrological Research (Istituto Nazionale di Ricerca Metrologica—I.N.Ri.M.), is presented. The identification and quantification of BaP in PM samples were carried out by gas chromatography coupled with mass spectrometry (GC–MS). Metrological traceability was established in all the procedure steps, after performing the method validation. Suitable certified reference materials (CRMs) were used both to validate the analytical method and to calibrate the GC–MS. The measurement uncertainty was evaluated by identifying and taking into account all the relevant sources deriving from the whole procedure steps.
Keywords: Metrological traceability; Particulate matter; Polycyclic aromatic hydrocarbons; Measurement uncertainty; Traceability chain
Indoor air quality: validation and setting up quality control for determination of anions and cations in particulate matter by Maria Trancoso; Ana Sousa; Filomena Mouro; Maria C. Freitas; Susana Almeida; Nuno Canha (pp. 199-206).
In recent years, the indoor air quality has been studied more frequently due to an increasing concern within the scientific community on the effects of indoor air quality upon health. The indoor air quality studies of schools have a large impact in both health and educational performance of children since they constitute a sensitive group with higher risk than adults, particularly vulnerable to pollutants due to their undeveloped airways. A total of 14 basic schools located in Lisbon city, Portugal, were selected for sampling the total particulate matter (TPM) by passive deposition into polycarbonate filters and to assess the indoor air quality. Compared to automatic samplers, this passive sampling method represents an easier and cheaper way to assess several indoor air quality environments with no interference in the classroom activities. The procedure was performed on four different campaigns during 2009–2010. The filter loads were measured by gravimetry with a 0.1-μg sensitivity balance and, afterwards, the TPM water-soluble ions content was assessed by ionic chromatography (Cl−, NO3 −, PO4 3− and SO4 2−); flame absorption (Na+, K+, Mg2+ and Ca2+). The performance characteristics of the methods, namely specificity, limit of detection, limit of quantification, working range, precision and trueness were evaluated. Measurement uncertainty was expressed in terms of precision and trueness. Precision under intralaboratory reproducibility conditions was estimated from triplicate analysis. The trueness component was estimated in terms of overall recovery using the reference material SPS-NUTR WW2 Batch 107, from Spectrapure Standards, Oslo, Norway, for anions and the certified reference material CRM 1643e, from NIST, Gaithersburg, MD for cations. Measurement uncertainty of the results obtained with the methods described in this work fulfilled the relative differences (RD) defined by the anion−cation balance in the extraction solutions of the particulate matter. Target RD values were defined: RD < 0.05.
Keywords: Indoor air; Measurement uncertainty; Anion−cation balance; Quality control
A comparison in the evaluation of measurement uncertainty in analytical chemistry testing between the use of quality control data and a regression analysis by João A. Sousa; Alexandra M. Reynolds; Álvaro S. Ribeiro (pp. 207-214).
The evaluation of measurement uncertainties has been widely applied to the calibration of measurement instruments, whereas its application to tests, despite increasing requirements, is a more recent phenomenon. The generalization of the evaluation of measurement uncertainties to tests has been a gradual process, in line with changes in the requirements of the normative framework that regulates the accreditation of tests laboratories and also as the perceived good practices have evolved. The sole identification of the relevant sources of uncertainty was followed by the requirement to provide a simplified estimate of the measurement uncertainty, and it is now an accepted requirement to properly evaluate the expanded measurement uncertainty associated with any tests. In this study, the evaluation of measurement uncertainty associated with the determination of sulfate in water will be attempted using a procedure that includes linear regression, with the regression parameters provided with associated uncertainties, and a Monte Carlo method applied as a validation tool of the conventional mainstream evaluation method, concerning the approximations in terms of linearization of the model and the assumed shape of the output distribution introduced by this approach.
Keywords: Measurement uncertainty; Chemical metrology; Regression analysis; Monte Carlo method; GUM uncertainty framework
No prospect for change by Gary Price (pp. 215-219).
A recent discussion by Martin Milton in the November issue of this journal was critical of many published objections to the proposed new International System (SI) measurement units (and in particular the definition of the mole) on the grounds that many objectors had proposed new terms in attempts to clarify thinking and that it is impractical to achieve consensus on such proposals. This discussion in response argues that those criticisms miss the point of the objectors’ arguments and are perhaps more appropriately directed at both the current and the new SI itself. A primary example of a neologism in the SI is the term “amount of substance.” The many substantive problems with the new SI remain unaddressed by its proponents. Many important consequences of exactly fixing multiple inter-dependent fundamental physical constants as the basis of the world’s measurements would appear not to have been considered by the global institutions responsible for the world’s measurement units.
Keywords: New SI; Measurement units; Mole; Policy
Making progress in a practical discipline by Martin J. T. Milton (pp. 221-222).
The proposals to redefine four of the base units of the SI led to an intense exchange of views in the pages of ACQUAL. It should be kept in mind that metrology is a practical discipline.
Keywords: SI ; Traceability ; Mole
Reliability of measurement results in food microbiology: the contribution of reference laboratories in the European Union and of international/European standardization by Bertrand Lombard (pp. 223-229).
The contribution of reference laboratories in the European Union and of European/international standardization to the reliability of food microbiology measurement results is discussed. A set of European Union reference laboratories has been established. Each of them coordinates a network of national reference laboratories which, in turn, coordinate networks of laboratories in charge of official testing and sometimes own checks in each European Union country. Their contribution to the reliability of food microbiology measurement results is illustrated by three food safety cases: Listeria monocytogenes, coagulase positive staphylococci and milk/milk products. The contribution of European/international standardization focuses on two topics: method validation and measurement uncertainty. The standards covering these topics—EN ISO 16140 and ISO/TS 19036—are briefly discussed, and an update given on their ongoing revision.
Keywords: Measurement reliability; Food microbiology; Reference laboratories; CEN; ISO; Method validation; Measurement uncertainty