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.16, #4-5)
Chemical modelling of multicomponent mixtures: quality assurance is more than just equilibrium data quality assessment by Peter M. May; Montserrat Filella (pp. 179-184).
Despite the large amount of data available, the great effort put into searches for the ‘best’ parameters and many comparative modelling studies, considerable uncertainties continue to plague chemical thermodynamics. An important factor in this ongoing failure has been the notion that the problem can be solved by better assessment of data quality on a case-by-case basis. This approach has proved strikingly unsuccessful. A different methodology must therefore be found to meet the general requirements of thermodynamic modelling in aquatic chemistry. This paper discusses current practices in quality assessment of thermodynamic data and the problems associated with them. It outlines a general approach which might address the above problem based on two concepts: (i) using large databases to store as much of the available data as possible in the form that it appears in the literature along with an assessed ‘score’ or ‘measure of information content’ and (ii) then using automatic mechanisms informed by this score to produce the thermodynamically consistent datasets needed for modelling calculations.
Keywords: Equilibrium constants; Speciation modelling; Multicomponent mixtures; Quality assurance; Aquatic chemistry
Uncertainty analysis of correlated stability constants by C. Ekberg; S. Englund; J.-O. Liljenzin (pp. 185-189).
Every attempt of using a computer to model reality has two main uncertainties: the conceptual uncertainty and the data uncertainty. The conceptual uncertainty deals with the choice of model selected for the simulation and the data uncertainty is about the precision and accuracy of the input data. They are often determined experimentally and may thus be encumbered by a number of uncertainties. Normally when treating uncertainties in input data these data are treated as independent variables. However, since many of these parameters are determined together they are actually correlated. This paper focuses on chemical stability constants, a most important parameter for chemical calculations based on speciation. Commonly in the literature they are at best given with an uncertainty interval. We propose to also give the covariance matrix thus giving the opportunity to really assess correlations. In addition we discuss the effect of these correlations on speciations.
Keywords: Uncertainty analysis; Correlations; Americium complexation; Speciation
pH buffer assessment and Pitzer’s equations by Petra Spitzer; Paola Fisicaro; Guenther Meinrath; Daniela Stoica (pp. 191-198).
Traceable measurement of the quantity pH in media outside the limits of the recent IUPAC recommendation requires an alternative to the Bates–Guggenheim convention. The Pitzer’s approach to ion interactions is widely appraised and offers a suitable approach to the estimation of single-ion activity coefficients. For this purpose, the underlying electrolyte data need to be consistent and the uncertainty in existing Pitzer’s data needs to be assessed. Such an assessment is attempted in this study for pH in potassium acetate buffer medium. The re-evaluation of Pitzer’s data from potassium nitrate data revealed inconsistencies and wide uncertainty margins. The uncertainty contribution from binary Pitzer’s parameters to an assessment of pH in seawater resulted in an uncertainty in pH of about 0.03 (95% confidence interval). This contribution has to be taken into account in the overall uncertainty budget. Lack of appropriate data for other pH buffer systems (e.g., phosphate buffers) is noted.
Keywords: pH measurement; Traceability; Measurement uncertainty; Pitzer’s equation; Resampling algorithms
Chemometrics in f-element speciation: a metrological challenge? by Stefan Lis; Günther Meinrath (pp. 199-205).
The UV–Vis spectra of the f-elements, especially the lanthanides, are known for their rather sharp absorption bands showing systematic and reproducible changes upon coordination with suitable ligands. These absorption bands have been used as indicators allowing interpretation of the processes in solution. Modern absorption spectrometers in combination with digital recording of the spectra allow a precise collection of a large number of absorptions for many samples. The number of absorption thus collected easily amounts to several thousands of individual data. Chemometric techniques, especially factor analysis, are occasionally used to extract information from these data sets on basis of Bouguer–Lambert–Beer law. Recent studies indicate the sensitivity of parameter values obtained from uncritical use of chemometric techniques to various influence and nuisance factors. On the basis of selected examples, the effects of parameter correlations, residual correlations, and measurement uncertainty introduced by volume operations are demonstrated. Using the ISO Guide to the Expression of Uncertainty as a convention for assessing measurement uncertainty, formation constants lg K derived from UV–Vis spectra of f-elements should be associated with a measurement uncertainty u of at least u = 0.15 (k = 1).
Keywords: f-elements; UV–Vis spectroscopy; Chemometrics; Factor analysis; Guide to the Expression of Uncertainty
Uncertainties in chemical modelling of solubility, speciation and sorption by C. Ekberg; A. Ödegaard-Jensen (pp. 207-214).
Chemical analyses become more and more expensive to perform, and more and more research is based on numerical simulation. However, all numerical models are subject to uncertainties, either in the implementation of the model towards reality or simply that the desired input data are not explicitly known without uncertainties. These uncertainties will affect the predictability of any model and thus it is of vital interest that the effect of these uncertainties is known. Especially if the result of the simulations is a topic upon which serious decisions are going to be made. In this paper, we outline a simple and rather straight forward approach to the effect of uncertainties on such simple chemical calculations as solubility of a solid phase in a given water and the chemical speciation of a solution. In addition, we also touch upon the much more complicated matter of uncertainties in sorption modelling—a subject which will be enlightened in much greater detail in an upcoming NEA publication in the matter.
Keywords: Uncertainty analysis; Sensitivity analysis; Solubility; Speciation; Sorption; LJUNGSKILE; SENVAR
Trace element complexation by humic substances: issues related to quality assurance by Montserrat Filella; Wolfgang Hummel (pp. 215-223).
Despite the amount of data available and the effort that has been put into studying the binding of trace elements by humic substances, there is still a significant amount of uncertainty surrounding the validity and applicability of the existing values in solving particular problems in the environmental, toxicity and industrial fields. This paper discusses the problems associated with the characterisation of humic binding and proposes a general approach for quality assessing trace element-humic substances complexation data, which includes the normalisation of raw experimental data prior to any data treatment (modelling).
Keywords: Humic substances; Metal binding; Metal complexation; Quality assurance; Aquatic chemistry
Stochastic ensembles of thermodynamic potentials by Rainer Feistel (pp. 225-235).
The provision of uncertainty estimates along with measurement results or values computed thereof is metrologically mandatory. This is in particular true for observational data related to climate change, and thermodynamic properties of geophysical substances derived thereof, such as of air, seawater or ice. The recent International Thermodynamic Equation of Seawater 2010 (TEOS-10) provides such properties in a comprehensive and highly accurate way, derived from empirical thermodynamic potentials released by the International Association for the Properties of Water and Steam (IAPWS). Currently, there are no generally recognised algorithms available for a systematic and comprehensive estimation of uncertainties for arbitrary properties derived from those potentials at arbitrary input values, based on the experimental uncertainties of the laboratory data that were used originally for the correlations during the construction process. In particular, standard formulas for the uncertainty propagation which do not account for mutual uncertainty correlations between different coefficients tend to systematically and significantly overestimate the uncertainties of derived quantities, which may lead to practically useless results. In this paper, stochastic ensembles of thermodynamic potentials, derived from randomly modified input data, are considered statistically to provide analytical formulas for the computation of the covariance matrix of the related regression coefficients, from which in turn uncertainty estimates for any derived property can be computed a posteriori. For illustration purposes, simple analytical application examples of the general formalism are briefly discussed in greater detail.
Keywords: Uncertainty; Correlation; Thermodynamic potential; Fundamental thermodynamic equation; Covariance matrix; Error propagation; Thermodynamic properties; Water; Ice; Seawater
Thermodynamic description of retention mechanism in liquid chromatography by Bogusław Buszewski; Renata Gadzała-Kopciuch; Monika Michel (pp. 237-244).
The chemical and physical properties of the stationary and the mobile phase determine the retention and selectivity of the separated molecules in chromatographic process. It should explain the specific and non-specific interactions in the chromatographic system. Special emphasis should be placed on column selection through the adequate choice of column dimensions (diameter and length) and the type of stationary phase with required parameters, which contributes the system effectiveness. However, column effectiveness is unable to guarantee a satisfactory level of substance separation. For this reason, the interactions between the analyte, the stationary and the mobile phase need to be investigated to determine the retention factors of the separated substances and the optimal parameters of the chromatographic system. According to the principles of thermodynamics, in particular molecular interactions, the chromatographic process has to be carried out with a high level of control. This work can be treated a brief tutorial devoted to thermodynamics of liquid chromatography process with a special emphasis on molecular interactions between analyte species and the components of the mobile and stationary phases.
Keywords: Thermodynamic description of retention; Molecular modeling liquid chromatography
Purity assessment for 17β-estradiol by Siu-kay Wong; Tin-yau Law; Yee-Lok Wong (pp. 245-252).
Purity assessment for high-purity organic substances is regarded as one of the core competences of national metrology institutes. For this reason, Consultative Committee for Amount of Substance—Metrology in Chemistry (CCQM) regularly organized intercomparison studies to compare and demonstrate the degree of equivalence of national measurement capabilities for purity determination. This paper reports the method development work for the participation in a recent CCQM key comparison on purity assessment for estradiol. An indirect consecutive estimation approach was adopted to determine the purity of the estradiol sample, i.e., through the identification and quantification of all possible impurities present in the sample. In the course of assessment, a variety of analytical techniques were applied. The most challenging part was to identify and estimate the amount of unknown organic-related substance present in the testing material. For the moisture determination, a gas chromatography with thermal conductivity detection (GC–TCD) method was employed to supplement the results obtained using coulometric Karl Fischer titration technique. Inorganic impurities and volatile organic compounds residual were analyzed using inductively coupled plasma mass spectrometry (ICP–MS) and gas chromatography–mass spectrometry (GC–MS) techniques, respectively, and their contributions were found to be comparatively insignificant.
Keywords: 17β-estradiol; Purity assessment; CCQM study
How to remove barriers to medicines trade in emerging economies: the role of medicines regulatory authority in Serbia by Gordana Pejović; Jovan Filipović; Ljiljana Tasić (pp. 253-261).
The primary role of European pharmaceutical legislation is to safeguard public health while encouraging the development of the pharmaceutical industry, creation of single market for pharmaceuticals with removed barriers to medicines trade. This article presents some important characteristics of pharmaceutical legislation in Serbia, the potential EU candidate country, and the latest improvements of existing national pharmaceutical legislation. Additional explanation is given for the Serbian negotiations with World Trade Organization and measures taken for the harmonisation with Agreements on Technical barriers to Trade (TBT), especially with Agreement on Trade-Related Intellectual Property Rights. Current status of Serbian pharmaceutical market is described, with the highlight of domestic manufacturers’ capacity to comply with European Union requirements. National medicines agency is described in the framework of European medicines quality infrastructure. The role and responsibility of national medicines authority to assure access to quality, safe and efficacious medicines on Serbian market is described. The special emphasis is given to agency’s role in harmonisation process, in terms of removing unnecessary barriers to medicines trade in Serbia.
Keywords: Medicines agency; Quality infrastructure; Barriers to trade; Access to medicines
A basic measurement model for reference material preparation by Jongseon Park; Gyeonghee Nam; Byungjoo Kim; Euijin Hwang; JongOh Choi (pp. 263-266).
The basic model $$ x_{ ext{CRM}} = x_{ ext{char}} + delta x_{ ext{bb}} + delta x_{ ext{sts}} + delta x_{ ext{lts}} $$ in the ISO Guide 35 for the evaluation of uncertainty is reviewed. In the basic model, the authors of the Guide assume that the homogeneity and stability studies are designed in such a way that the values of the error terms are zero, but their uncertainties are not. Based on their assumption, it is shown that the basic model in the Guide requires output quantity x CRM to be employed as input quantities to define δx bb, δx sts, and δx lts, which results in circular reasoning on the value of CRM. In order to revise the basic model in the Guide, a proposed basic model Y = X is derived from the general model Y = X(1 + b′T) in the Guide as the special case of T = 0. The uncertainty evaluation for the proposed model is discussed. It is shown that it is not necessary to evaluate the uncertainty due to the between-bottle inhomogeneity u bb separately, when a single method is employed in the characterization. The u bb is shown to be one of the uncertainty components of the certified value x char, because x char is the mean value of at least 10 bottles.
Keywords: Uncertainty; Measurement model; Reference material; ANOVA
European analytical column no. 39. Analytical chemistry and bioanalytical chemistry: a yet unshaped social relationship
by George Horvai; Paul Worsfold; Bo Karlberg; Jens E. T. Andersen (pp. 267-269).
Comment on “Recommendations on quantities, symbols and measurement units for publications in ACQUAL: preparation of tables and graphs”
by Richard J. C. Brown (pp. 271-272).