Experimental design and data evaluation considerations for comparisons of reference materials

The analysis of reference materials (RMs) can help assess the equivalence of chemical measurement processes. When two or more RMs are available for a given measurand, confidently establishing the equivalence of measurement processes requires the RMs to be capable of yielding equivalent results. Evaluating the degrees of equivalence among RMs that differ in analyte quantity and perhaps matrix composition requires an approach other than that used to assess results for samples of a single material. We have more than a decade of experience with an approach that compares the assigned values of RMs to a simple linear model of the relationship between those values and measurement results ideally made under repeatability conditions. In addition to accessing the metrological equivalence of specific RMs, the equivalence of the value-assignment capabilities of the organizations that issue the RMs can also be accessed. This report summarizes our experience with the design of and analysis of studies using this approach and provides numeric and graphical tools for estimating degrees of equivalence. We divide the required tasks into four steps: (1) design, (2) measurement, (3) definition of a reference function, and (4) estimation of degrees of equivalence. We regard the experimental design and measurement tasks as most critical to the eventual utility of the comparison, since creative mathematics cannot fully compensate for poor planning or erratic measurements.     

A new approach to the preparation of biological reference materials for trace metals

Summary Preparation and certification of a new, nondefatted biological reference material (RM) for trace metals is reported. LUTS-1 is a homogeneous slurry of lobster hepatopancreas containing 85% (w/w) moisture and 8.2% (w/w) lipids (55% dry weight basis). This RM should prove of considerable use in validating analytical procedures involving faunal field samples containing significant lipid content.

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Neue Methode zur Herstellung von biologischen Referenzmaterialien für Spurenmetalle

     

Evaluation of analytical interlaboratory comparisons and certification of reference materials

A procedure for the evaluation of results of interlaboratory comparisons of trace multi-component determinations is presented. The distribution of interlaboratory comparison data may differ significantly from the normal distribution and, therefore, various types of median and modal values approximate their central values better than the mean. The simple median and its confidence interval, calculated on the basis of a nonparametric distribution after the rejection of outlying results, is shown to be the optimal indicator of the central values. The total procedure consists of three parts: statistical evaluation of the data, classification of the results according to a special set of analytical criteria, and graphic presentation of the data. The application of the results of such comparisons for certification of analytical reference materials is discussed.     

A convenient and economic approach to achieve SI-traceable reference values to be used in drinking-water interlaboratory comparisons

Metrologically traceable reference values add an essential benefit to interlaboratory comparisons: unlike consensus values, they can be used to establish national and international comparability. Furthermore, the participating laboratories obtain a reliable and unbiased benchmark to check their results for accuracy. Usually, metrologically traceable reference values are obtained by so-called primary methods which demand excessive efforts at great expense. Within the framework of two national drinking-water interlaboratory comparisons (proficiency testing rounds), a new approach to provide metrologically traceable reference values was applied. It is solely based on existing data which were collected during the comparison itself. Lead (Pb) measurements serve as an example to show how metrologically traceable reference values were derived from the lead amount added during sample preparation and the amount of lead already present in the drinking-water matrix used to prepare these samples. Within this approach, the matrix content is calculated in a way similar to a standard addition experiment. An uncertainty budget for the reference value was set up which describes the link to the corresponding SI units. Isotope dilution mass spectrometry (IDMS) as a primary method was used to validate this approach in the case of cadmium, chromium, copper, lead, and nickel.     

A synopsis of different approaches to the certification of reference materials

Proliferation of Biological and Environmental Reference Materials (RMs), agencies and individuals preparing and characterizing them for a wide variety of inorganic and organic chemical constituents, makes it pertinent to look at the various certification approaches followed. This synopsis presents amalgamated versions of several of the major certification approaches, for natural-matrix RMs, alluding to some of the many factors that significantly affect the conduct and outcome of the analytical characterization exercises.     

Improvement of the traceability of reference materials certified for purity: evaluation of a semi-preparative high-performance liquid chromatography approach

A semi-preparative high-performance liquid chromatography process was evaluated as a tool to quantitatively determine the purity or percentage mass fraction content (% m/m) of organic compounds. The method is simple and does not require the identification and subsequent quantitation of organic-related structure impurities. A protocol was developed and tested on four reference materials certified for purity from 95% m/m to 99.3% m/m. Comparing the purity results of each certified reference material using the new approach with their respective certified values showed no significant analytical bias. Semi-preparative high-performance liquid chromatography has proved the potential to be a primary method directly traceable to mass with an uncertainty statement written down also in terms of mass with expanding uncertainty ranging from 0.8% to 1.3% m/m compared to 0.3 to 2.0% m/m for the certified purity values at the 95% confidence interval.     

Some aspects of the evaluation of measurement uncertainty using reference materials

To ensure and to confirm the required traceability according to the definition given in the International Vocabulary of Basic and Standard Terms in Metrology, three main aspects need to be considered in practice: “stated reference”, “unbroken chain of calibration” and “stated uncertainty”. For a certain spectrochemical result, each of the aspects above mentioned is highly dependent on measurement uncertainty, both on its magnitude and how it was estimated. The paper describes the experience of the Romanian National Institute of Metrology (INM) in estimating measurement uncertainty during certification of reference materials, in metrological calibration and during specific analytical processes. Practical examples of the use of reference materials or certified reference materials issued by the INM to estimate measurement uncertainty are discussed for their applicability in spectrochemical and turbidity analysis. Some aspects of the use of analysis of variance (ANOVA) to obtain additional information on the components of measurement uncertainty and to identify the magnitude of individual random effects are presented.     

Some aspects of the evaluation of measurement uncertainty using reference materials

 In practice there are three aspects that need to be considered in order to achieve the required traceability according to its definition: the 'stated reference', the 'unbroken chain of calibrations' and the "stated uncertainty". For a certain chemical result, each of these aspects highly depends on the measurement uncertainty, both on its magnitude and how it was estimated. Therefore, the paper describes the experience of the Romanian National Institute of Metrology in estimating measurement uncertainty during the certification of reference materials (RMs), in metrological activities (calibration, pattern approval, periodical verification, etc.), as well as during the analytical measurement process. Practical examples of estimation of measurement uncertainty using RMs or certified reference materials are discussed for their applicability in spectrophotometric and turbidimetric analysis. Use of the analysis of variance to obtain some additional information on the components of measurement uncertainty and to identify the magnitude of individual random effects is described. Received: 12 November 1999 / Accepted: 25 February 2000     

Some aspects of the evaluation of measurement uncertainty using reference materials

To ensure and to confirm the required traceability according to the definition given in the International Vocabulary of Basic and Standard Terms in Metrology, three main aspects need to be considered in practice: "stated reference", "unbroken chain of calibration" and "stated uncertainty". For a certain spectrochemical result, each of the aspects above mentioned is highly dependent on measurement uncertainty, both on its magnitude and how it was estimated. The paper describes the experience of the Romanian National Institute of Metrology (INM) in estimating measurement uncertainty during certification of reference materials, in metrological calibration and during specific analytical processes. Practical examples of the use of reference materials or certified reference materials issued by the INM to estimate measurement uncertainty are discussed for their applicability in spectrochemical and turbidity analysis. Some aspects of the use of analysis of variance (ANOVA) to obtain additional information on the components of measurement uncertainty and to identify the magnitude of individual random effects are presented.     

New approach to evaluation of the stress-strain properties of nanolayers of solid materials

The review focuses on results obtained in the development of a new approach to determination of the stress-strain properties (tensile strength, yield point, plastic deformation under uniaxial stretching) of nanolayers of solid materials. The approach is based on analysis of parameters of the microrelief generated by strains in polymer films with hard thin coatings. A significant increase in the tensile strength and ductility of noble metal coatings under uniaxial stretching at the metal layer thickness less than 30 nm is demonstrated for the first time. This substantiates the assumption of a specific state of nanolayers of solid materials. The developed method also enables evaluation of the effect of characteristic defects in solids on their stress-strain behavior. It was found that the stress-strain properties of nanocoatings depend on the physical state of the polymer substrate. A possible mechanism of the revealed phenomena is suggested.     

Certification of reference materials for inorganic trace analysis: the INCT approach

This paper presents the work done by the Institute of Nuclear Chemistry and Technology (INCT), Warsaw on a procedure of the certification of matrix reference materials (CRMs) for inorganic trace analysis. The INCT has been involved in preparation and certification of that type of CRMs since 1986 till now. The certification of CRMs is performed on the basis of statistical evaluation of the data obtained from the worldwide interlaboratory comparison. The initially adopted certification procedure has been developed, and the final shape is presented and discussed. The modifications are connected with the new demands of the international standards. The results of analysis of candidate CRMs obtained by the potentially primary procedures based on radiochemical neutron activation analysis (RNAA) and results of analysis of CRM accompanying candidate RMs are applied in the certification process for quality assurance purpose.     

The establishment of a quality evaluation system of reference materials

Reference materials have been applied widely to ensure the traceability, comparability and reliability of measurement results. To achieve this purpose, the quality of reference materials (RMs) themselves is surely an important aspect to be pay attention to. A quality evaluation system of RMs has been established through the project “The National Sharing Platform of Reference Materials” in China to give a reliable assessment on the quality of RMs from various sources including the accuracy and comparability of their property values, which is very useful to promote the appropriate selecting and using of RMs in China. Through the application of National Metrology Institute calibration and measurement capabilities on the basis of the international mutual recognition arrangement, it can also provide a powerful supplement to the current activities such as the accreditation of RM producers in the construction of a global harmonized quality control and assurance system of RMs.     

Analytical problem-solving, reference materials, and multivariate quality control: A chemometrics approach

Summary Breakthroughs in sensor technology have augmented the chemist's measurement repertoire by introducing new kinds of detectors with improved selectivity and the capacity to perform simultaneous multi-species measurements. Thus, the electronic revolution has qualitatively and quantitatively changed the data matrices to which the analyst/problem-solver has access. The new chemical subdiscipline of chemometrics is developing powerful mathematical and statistical data analysis tools to exploit the electronic windfall and enhance data interpretation. Principal component analysis and graphical procedures have been used to examine the multivariate suitability of current reference materials in matching the concentration ranges and matrices for various food analyses. Principal component analysis has been useful in developing and exploring quality control information for the routine analysis laboratory.     

Availability of reference materials: COMAR the database for certified reference materials

COMAR is the international database for certified reference materials. A new user-friendly web-based version, COMAR², has been developed by BAM and CONET Consulting AG which will be available in December 2002. The advantages of COMAR² are briefly explained.Presented at the International ILAC/IAF Conference on Accreditation in Global Trade, 23-25 September 2002, Berlin, Germany     

Preparation of the Bovine Kidney and Bovine Muscle reference materials and the certification of element contents from interlaboratory comparisons

Results obtained in interlaboratory comparisons on candidate reference materials Bovine Kidney and Bovine Muscle, in which more than 32 laboratories participated employing atomic spectrometry, electrochemical, and nuclear analytical techniques, have been evaluated using combined statistical and analytical considerations. Certified and/or information values for the elements Br, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, P, Pb, Rb, Se, and Zn have been established. Information on contents of the elements Al, As, Au, Cs, Li, Ni, Sb, Sn, Sr, and V is also presented.     

A comprehensive survey of the INAA of reference materials

The nature of summary reports now being prepared for issuance, summarizing the results of detailed INAA Advance Prediction Computer Program (APCP) calculations for many widely used reference materials, is discussed and illustrated. For each material, the best photopeak % _rel value (where it is10%) is shown for each of the 12 condition sets (t_i/t_d/t_c) calculated from the input elemental composition. These summary reports will be very useful to groups that use various reference materials as multi-element standards.     

Microheterogeneity evaluation of polycyclic aromatic hydrocarbons in particulate standard reference materials

With the emergence of highly sensitive analytical techniques, the microanalysis of natural-matrix materials employing smaller sample sizes is increasingly more common, which subsequently warrants a homogeneity assessment for the individual components at the appropriate sampling level. Pressurized liquid extraction (PLE) in combination with gas chromatography/mass spectrometry (GC/MS) has been used to determine the sampling constants and evaluate the relative homogeneity of trace levels of polycyclic aromatic hydrocarbons (PAHs) for two previously certified particulate standard reference materials, SRM 1649a Urban Dust and SRM 1650b Diesel Particulate Matter, in the milligram sampling range. Fluoranthene, pyrene, benz[a]anthracene and benzo[e]pyrene within SRM 1650b Diesel Particulate Matter were deemed to be homogeneous, based on relatively small sampling constants (K _S<100 mg), whereas the larger sampling constants (K _S>100 mg) obtained for all PAHs in SRM 1649a Urban Dust suggest more material heterogeneity. The material heterogeneity of ten individual PAHs (phenanthrene, anthracene, fluoranthene, pyrene, benz[a]anthracene, benzo[k]fluoranthene, benzo[e]pyrene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene and benzo[ghi]perylene) was also described via nonlinear relationships (i.e., power law) between subsampling error S _s (%) and sample mass, which are used to predict analyte-specific minimum sample masses that result in a specific level of analytical uncertainty. Electronic supplementary material Supplementary material is available for this article at and is accessible for authorized users.