Analytical quality control program used by the trace elements in foods and diets sub-network of the FAO European cooperative network on trace elements

Summary The analytical quality control program employed by the ongoing FAO study on 14 trace elements in nationally representative staple foods of European countries is presented. The analytical quality control used is based on: 1) precautions taken to avoid trace element contaminations or losses during sampling and sample handling; and, 2) on methods used to guarantee that the actual determinations yield correct results. The precautions are presented. A number of certified biological reference materials (RMs) were used to validate the analytical methods employed. The following staple food RMs were also prepared: wheat flour, potato powder, animal muscle (pork) and milk powder. They were tested for homogeneity and subjected to an interlaboratory comparison study on the basis of which recommended values for trace element concentrations were defined. Further, the mean relative standard deviation for the 95% confidence limits of the medians in all RMs was below 5% for Ca, Mg and Zn; below 10% for Mn; below 15% for Fe, Cu and Se; and below 25% for Mo and Ni. These RMs were used to control the analytical quality of the trace element determinations in the actual samples. It is concluded that important contaminations were avoided in sampling and sample handling and that use of the RMs described was necessary to guarantee the analytical quality of the results.     

Some considerations regarding reference materials and their role in environmental monitoring. Plenary lecture.

After a brief introduction indicating the principal function of reference materials (RMs) in ensuring that the analytical measurement programme is performing reliably, the availability, different types and sources of information on RMs are described. Next, the correspondence of available RMs to present sample demands is discussed, particularly with regard to the adequacy of matrices and analytes, the availability of RMs for 'difficult' trace elements, and the preparation and certification of speciation RMs. The role of RMs in quality control is indicated, and their usage and certain problems which can thereby arise, including total dissolution, homogeneity and their representativeness in relation to samples, are discussed. The application of truly representative RMs in low-level trace element work, as a means of quality control of sampling and storage procedures, is mentioned. Strategies open to the analyst in the total absence of certified RMs are presented, and the importance of publication of independent results by alternative methods for such materials, as a means of establishing consensus values, is stressed. In the situation where results for a particular analyte obtained by different groups for the same (but not identical) biomedical or environmental system differ markedly, a progressive multi-stage exchange scheme is proposed, which is designed to reveal whether the differences arise from analytical factors, sampling effects or real system differences.     

Global perspectives for secondary reference materials for analysis of food and related biological samples

Primary, secondary and tertiary reference materials (RM) play an important role in quality controls of analytical measurements. Logistics of preparation and proper use of primary and secondary RMs are presented. Tertiary (i.e. in-house) control materials are useful as substitutes in the absence of recognized primary or secondary RMs. The lack of interdisciplinary interaction during development of RMs (e.g. in specific areas such as foods), has an important impact on limiting the usefulness of certain types of RMs. The abundance of RMs in some countries and regions appears to have little effect on the existing paucity in RMs in other regions, and the underlying causes are outlined. The ability of a laboratory to produce good quality in-house RMs traceable to recognized primary or secondary RMs is a direct measure of its quest for reliable analytical data. Therefore many laboratories should be encouraged to engage in secondary and tertiary RM activities designed to answer specific measurement problems. In this context, assistance (e.g. practical training opportunities) in identifying simple methods of analyses for their efficacy in determining specific analytes is a source of help that can be extended to countries experiencing limitations in laboratory instrumentation.     

Reference materials in the Russian system of accreditation of analytical laboratories

 This article is devoted to the role of reference materials (RMs) in chemical analysis and their main applications in analytical laboratories. The principal requirements of the RMs used in accredited laboratories in the Russian Analytical Laboratories Accreditation System (SAAL) are presented. These include the basic regulatory and metrological requirements of RMs. Finally, a review of the provision of RMs used for the analytical control of various test objects is presented. Received: 9 August 1998 / Accepted: 9 November 1998     

Use of neutron activation analysis and LC–ICP-MS in the development of candidate reference materials for As species determination

Arsenic (As) is widely found in marine organisms, mainly as organic arsenical compounds. With the introduction of regulations for As species in foodstuffs and for environmental control, it has become more important to quantify the amount of arsenic species present. To address this concern new reference materials (RMs) for validation and quality assurance of As speciation measurements are being developed, including a tuna fish tissue and a robalo liver tissue. Instrumental neutron activation analysis (INAA) is used in this work as a proven analytical tool for As determinations and for validating the As mass fraction determined by liquid chromatography coupled to inductively coupled mass spectrometry (LC–ICP-MS) in extracts of the candidate RMs and other fish samples including certified RMs. Various methods for the extraction of water-soluble As species were evaluated. The best results were acquired after methanol/acetone/water extraction yielding in 93 % extractable As in the tuna RM. This procedure was used for the LC–ICP-MS studies. The results demonstrate that INAA can account for 100 % of the distribution of As species in analytical processes. It complements LC–ICP-MS for the validation of the characterization of As species in the development of RMs for such species. Excellent agreement of measured results in certified RMs with the reported values confirms the validity of the applied procedures.     

Recent developments in food-matrix Reference Materials at NIST

Since 1996, the National Institute of Standards and Technology (NIST) has developed several food-matrix Standard Reference Materials (SRMs) characterized for nutrient concentrations. These include SRM 1544 Fatty Acids and Cholesterol in a Frozen Diet Composite, SRM 1546 Meat Homogenate, SRM 1548a Typical Diet, SRM 1566b Oyster Tissue, SRM 1846 Infant Formula, and SRM 2383 Baby Food Composite. Three additional materials--SRM 1946 Lake Superior Fish Tissue, SRM 2384 Baking Chocolate, and SRM 2385 Spinach--are in preparation. NIST also recently assigned values for proximate (fat, protein, etc.), individual fatty acid, and total dietary fiber concentrations in a number of existing SRMs and reference materials (RMs) that previously had values assigned for their elemental composition. NIST has used several modes for assignment of analyte concentrations in the food-matrix RMs, including the use of data provided by collaborating laboratories, alone and in combination with NIST data. The use of data provided by collaborating food industry and contract laboratories for the analysis of food-matrix RMs has enabled NIST to provide assigned values for many analytes that NIST does not have the resources or analytical expertise to measure.     

Environmental lead: need for reference materials

The status of the availability and appropriateness of standard reference materials (RMs) currently available for Pb in paint, dust, soil and drinking water is reviewed. The need for RMs which will be most appropriate for environmental Pb analysis in the context of the analytical techniques used, government regulations and health effects, is assessed. Also, the importance of factors such as bioavailability, sample size, homogeneity and composition that should be considered in developing appropriate RMs certified for Pb in paint, dust, soil and drinking water is stressed.     

Measurements for certification of chlortetracycline reference materials within the European Union Standards, Measurements and Testing programme

The experimental design of a material certification programme is described. The matrix reference materials (RMs) comprised chlortetracycline (CTC)-containing and CTC-free lyophilised porcine liver, kidney and muscle produced under the European Commission's Standards Measurements and Testing (SMT) programme. The aim of the certification programme was to determine accurately and precisely the concentration of CTC and 4-epi-chlortetracycline (epi-CTC) contained in the RMs. A multi-laboratory approach was used to certify analyte concentrations. Participants (n = 19) were instructed to strictly adhere to previously established guidelines. Following the examination of analytical performance criteria, statistical manipulation of results submitted by 13 laboratories, (6 withdrew) allowed an estimate to be made of the true value of the analyte content. The Nalimov test was used for detection of outlying results. The Cochran and Bartlett tests were employed for testing the homogeneity of variances. The normality of results distribution was tested according to the Kolmogorov-Smirnov-Lilliefors test. One-way analysis of variance (ANOVA) was employed to calculate the within and between-laboratory standard deviations, the overall mean and confidence interval for the CTC and epi-CTC content of each of the RMs. Certified values were within or very close to the target concentration ranges specified in the SMT contract. These studies have demonstrated the successful production and certification of CTC-containing and CTC-free porcine RMs.     

Measurements for certification of chlortetracycline reference materials within the European Union Standards, Measurements and Testing programme

The experimental design of a material certification programme is described. The matrix reference materials (RMs) comprised chlortetracycline (CTC)-containing and CTC-free lyophilised porcine liver, kidney and muscle produced under the European Commission’s Standards Measurements and Testing (SMT) programme. The aim of the certification programme was to determine accurately and precisely the concentration of CTC and 4-epi-chlortetracycline (epi-CTC) contained in the RMs. A multi-laboratory approach was used to certify analyte concentrations. Participants (n = 19) were instructed to strictly adhere to previously established guidelines. Following the examination of analytical performance criteria, statistical manipulation of results submitted by 13 laboratories, (6 withdrew) allowed an estimate to be made of the true value of the analyte content. The Nalimov test was used for detection of outlying results. The Cochran and Bartlett tests were employed for testing the homogeneity of variances. The normality of results distribution was tested according to the Kolmogorov-Smirnov-Lilliefors test. One-way analysis of variance (ANOVA) was employed to calculate the within and between-laboratory standard deviations, the overall mean and confidence interval for the CTC and epi-CTC content of each of the RMs. Certified values were within or very close to the target concentration ranges specified in the SMT contract. These studies have demonstrated the successful production and certification of CTC-containing and CTC-free porcine RMs.     

Development of a proficiency testing scheme for a limited number of participants in the field of natural water analysis

A proficiency testing (PT) scheme was developed for a limited number of analytical laboratories participating in the analysis of natural water in Israel. Three fit-for-intended-use reference materials (RMs) were prepared for a pilot PT from natural water: RM-blank and two in-house reference materials (IHRMs) with seven analytes fortified to achieve different levels of analyte concentrations. The measurands for the PT participants were the traceable spike values certified in the IHRMs, i.e., added mass concentrations of the analytes. The RM-blank and IHRMs were found to be homogeneous and stable over 11 days, the time necessary for the experimental part of the PT. The RMs were distributed to the PT participants as unknown test items similar to routine samples. The test results were evaluated using their deviations from the IHRM-certified values. Eight Israeli laboratories took part in the interlaboratory comparison. Individual laboratory performance and metrological compatibility of the PT results of the participants, as a local group of laboratories, were evaluated for every analyte.     

Size-selective poorly soluble particulate reference materials for evaluation of quantitative analytical methods

Owing to the absence of readily available certified particulate reference materials (RMs), most analytical methods used to determine particulate contaminant levels in workplace or other environments are validated using solution RMs, which do not assess the robustness of the digestion step for all forms and sizes of particles in a sample. A library of particulate RMs having a range of chemical forms and particle sizes is needed to support a shift in method evaluation strategies to include both solution and particulate RMs. In support of creating this library, we characterized bulk and physically size separated fractions of beryllium oxide (BeO) particles recovered from the machining fluid sludge of an industrial ceramic products grinding operation. Particles were large agglomerates of compact, crystalline BeO primary particles having diameters on the order of several micrometers. As expected, the particle surface area was independent of sieve size, with a range from 3.61 m(2)/g (53-63-microm fraction) to 4.82 m(2)/g (355-600-microm fraction). The density was near the theoretical value (3.01 g/cm(3)). The data support more detailed characterization of the sludge materials for use as size-selective RMs. This work illustrates an approach that can be used to develop RMs that are difficult to digest.     

Reference materials and certified reference materials for spectrometry in Romania

Several reference materials (RMs) and certified reference materials (CRMs) are widely used in Romania as measurement standards in different spectrochemical measurements. Among them, single element standard solution certified for their mass concentration play a key role in ensuring the required traceability of results expressed in this measurement unit. A short review of the locally available elemental RMs and CRMs used in atomic spectrometry or in other analytical techniques where aqueous standard solutions are required (usually called RMs or CRMs for spectrometry) is given. The experience of the INM in preparation and certification of such materials is described. Some aspects regarding their use for ensuring the accuracy and for confirmation of the traceability of analytical measurements, especially through calibration and metrological validation of main instrument performances, are discussed.     

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.     

Reference materials and certified reference materials for spectrometry in Romania

Several reference materials (RMs) and certified reference materials (CRMs) are widely used in Romania as measurement standards in different spectrochemical measurements. Among them, single element standard solution certified for their mass concentration play a key role in ensuring the required traceability of results expressed in this measurement unit. A short review of the locally available elemental RMs and CRMs used in atomic spectrometry or in other analytical techniques where aqueous standard solutions are required (usually called RMs or CRMs for spectrometry) is given. The experience of the INM in preparation and certification of such materials is described. Some aspects regarding their use for ensuring the accuracy and for confirmation of the traceability of analytical measurements, especially through calibration and metrological validation of main instrument performances, are discussed.     

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.     

Reference materials in environmental trace organic analysis

 The paper reviews the application of reference materials (RMs) in environmental analysis of trace organic pollutants. The problems related to RM preparation, stability, use, etc. are critically discussed. Importance of analytical quality assurance and the role RMs play in the process are emphasised. Received: 18 May 1999 · Accepted: 27 December 1999     

Twenty five years of reference material activity at Agriculture and Agri-Food Canada

In the mid 1970s, the available RMs, notably Bowen's Kale and Orchard Leaves and Bovine liver from National Bureau of Standards (NBS), although of great benefit, were overwhelmingly insufficiently representative, in respect of matrix and elemental composition, of the wide range of natural products submitted for analysis and in worldwide commerce. To provide additional coverage, an RM development project was initiated with input from cooperating analysts leading to an Agriculture and Agri-Food Canada/National Institute of Standards and Technology (NIST) cooperative venture and development of a total of 12 different agricultural/food RMs. With a total of 303 concentration values for 34 elements and a wide range of matrix components such as ash, silica, protein, fat, carbohydrate and fiber, these RMs significantly augment the world repertoire of biological control materials. A final material under consideration is a highly reliable, discrete, synthetic RM for quality control and calibration. This paper summarizes the research and developmental activities undertaken during the past quarter of a century related to RM development at Agriculture and Agri-Food Canada and includes a short historical background, conceptual considerations, preparation, physical characterization, homogeneity estimation, chemical characterization, calculation of recommended reference values and associated uncertainties, methodology development and application, and performance of inorganic analytical methods in a multielement, multilaboratory, collaborative characterization campaign.     

Twenty five years of reference material activity at Agriculture and Agri-Food Canada

In the mid 1970s, the available RMs, notably Bowen’s Kale and Orchard Leaves and Bovine liver from National Bureau of Standards (NBS), although of great benefit, were overwhelmingly insufficiently representative, in respect of matrix and elemental composition, of the wide range of natural products submitted for analysis and in worldwide commerce. To provide additional coverage, an RM development project was initiated with input from cooperating analysts leading to an Agriculture and Agri-Food Canada/National Institute of Standards and Technology (NIST) cooperative venture and development of a total of 12 different agricultural/food RMs. With a total of 303 concentration values for 34 elements and a wide range of matrix components such as ash, silica, protein, fat, carbohydrate and fiber, these RMs significantly augment the world repertoire of biological control materials. A final material under consideration is a highly reliable, discrete, synthetic RM for quality control and calibration. This paper summarizes the research and developmental activities undertaken during the past quarter of a century related to RM development at Agriculture and Agri-Food Canada and includes a short historical background, conceptual considerations, preparation, physical characterization, homogeneity estimation, chemical characterization, calculation of recommended reference values and associated uncertainties, methodology development and application, and performance of inorganic analytical methods in a multielement, multilaboratory, collaborative characterization campaign.     

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.     

Inhibited phenol ionization in reverse micelles: confinement effect at the nanometer scale

We found that the absorption spectra of 2-acetylphenol (2-HAP), 4-acetylphenol (4-HAP), and p-nitrophenol (p-NPh) in water/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane reverse micelles (RMs) at various W(0) (W(0) = [H(2)O]/[surfactant]) values studied changed with time if (-)OH ions were present in the RM water pool. There is an evolution of ionized phenol (phenolate) bands to nonionized phenol absorption bands with time and this process is faster at low W(0) values and with phenols with higher bulk water pK(a) values. That is, in bulk water and at the hydroxide anion concentration used, only phenolate species are observed, whereas in AOT RMs at this fixed hydroxide anion concentration, ionized phenols convert into nonionized phenol species over time. Furthermore, we demonstrate that, independent of the (-)OH concentration used to prepare the AOT RMs, the nonionized phenols are the more stable species in the RM media. We explain our results by considering that strong hydrogen-bonding interactions between phenols and the AOT polar head groups result in the existence of only nonionized phenols at the AOT RM interface. The situation is quite different when the phenols are dissolved in cationic benzyl-n-hexadecyldimethylammonium chloride RMs. Therein, only phenolates species are present at the (-)OH concentrations used. The results clearly demonstrate that the classical definition of pH does not apply in a confined environment, such as in the interior of RMs and challenge the general idea that pH can be determined inside RMs.