Matrix Reference Materials Research Articles

Towards the development of a fossil bone geochemical standard: An inter-laboratory study

Ten international laboratories participated in an inter-laboratory comparison of a fossil bone composite with the objective of producing a matrix and structure-matched reference material for studies of the bio-mineralization of ancient fossil bone. We report the major and trace element compositions of the fossil bone composite, using in-situ method as well as various wet chemical digestion techniques. For major element concentrations, the intra-laboratory analytical precision (%RSD r) ranges from 7 to 18%, with higher percentages for Ti and K. The %RSD r are smaller than the inter-laboratory analytical precision (%RSD R; <15–30%). Trace element concentrations vary by ∼5 orders of magnitude (0.1 mg kg −1 for Th to 10,000 mg kg −1 for Ba). The intra-laboratory analytical precision %RSD r varies between 8 and 45%. The reproducibility values (%RSD R) range from 13 to <50%, although extreme value >100% was found for the high field strength elements (Hf, Th, Zr, Nb). The rare earth element (REE) concentrations, which vary over 3 orders of magnitude, have %RSD r and %RSD R values at 8–15% and 20–32%, respectively. However, the REE patterns (which are very important for paleo-environmental, taphonomic and paleo-oceanographic analyses) are much more consistent. These data suggest that the complex and unpredictable nature of the mineralogical and chemical composition of fossil bone makes it difficult to set-up and calibrate analytical instruments using conventional standards, and may result in non-spectral matrix effects. We propose an analytical protocol that can be employed in future inter-laboratory studies to produce a certified fossil bone geochemical standard.

Online measurement of water content in candidate reference materials by acousto-optical tuneable filter near-infrared spectrometry (AOTF-NIR) using pork meat calibrants controlled by Karl Fischer titration

Certified reference materials (CRMs) are prepared from a wide variety of matrices. Water removal is an excellent way of achieving increased matrix lifetime and hence CRM stability. High-speed acousto-optical tuneable filter near-infrared spectrometry (AOTF-NIR) has been implemented for measurement of water content in powder matrix reference materials in amber glass vials. Almost 50,000 spectra were collected from 1300 to 2100 nm with a 2 nm increment for powders of meat, rye grass, potato, cotton seed and sugar beet. The AOTF-NIR instrument was placed in a capping machine, with a measurement frequency of 10–15 vials/min and a trigger signal for reproducible collection of spectra. The calibrants comprised 19 pork meat powder samples equilibrated with different hygrostatic solutions or subjected to oven drying to achieve different water concentrations. Mixtures of powders with different water content were also prepared in order to obtain a calibration range from 0.5 to 8.3% water (m/m). All calibration samples were measured by volumetric Karl Fischer titration (V-KFT), accredited under ISO 17025. The calibrants were then measured by AOTF-NIR together with the samples. Multiplicative scatter correction (MSC) was applied to the absorbance spectra in order to correct for the scattering of light in the different powders and scattering effects from the vials. A partial least squares regression model (PLS) based on two principal components was created and applied for prediction of water content in the samples with a standard error of 0.5% water (m/m). The AOTF-NIR has the potential of rapidly monitoring a large number of samples of different materials with good accuracy as demonstrated by the good agreement with V-KFT. Nevertheless, it is necessary to expand the number of calibration models for different vial sizes as it turns out that MSC cannot correct properly for the influence of scattering of light due to the different vial sizes, in this case 100 mL and 10 mL vials. The influence of the matrix seems not to be critical because the prediction of the water content in a wide variety of matrices was successful using the meat powder as a universal calibrant.

Metrological concepts for enhancing the reliability of food and nutritional measurements

Metrology is a specialized discipline that deals with the science of measurements, irrespective of the field of application and regardless of the size of the measurement uncertainty. Incorporating metrological approaches into the measurement process enhances the reliability of analytical findings. These developments include: high-quality reference standards, validated methods, robust sampling practices, proven calibration approaches, natural matrix reference materials, speciation chemistry, assessment of measurement uncertainty and establishment of traceability links, certified reference materials to facilitate one aspect of traceability, and proficiency testing, among others.

Improving data quality in food composition databanks: a EuroFIR contribution

Food composition databanks (FCDBs) should provide nutrient composition data comparable over time at national and international levels. However, the linkage between national database compilers and permanent structures to support the upgrading and monitoring of nutrient values in foods are far from satisfactory. This paper focuses on European efforts to improve the quality of nutrient values entered into FCDBs, emphasizing initiatives under the EU Network of Excellence: European Food Information Resource (EuroFIR – NoE). The general concept of quality assurance is described and results obtained in the project’s first year are presented. A survey among EuroFIR partners aimed at evaluating the current situation in comparability of nutrient values suggests that an integrated approach has two requirements: the implementation of a quality management system (QMS) and a harmonized data-quality assessment system (DQAS) to select values from different sources. The use of reference materials (RMs) is a key criterion in deciding on comparability and reliability of candidate nutrient values. Consequently, results of a survey on food matrix reference materials are presented. These suggest that developments in RMs for nutrient analysis in foods have a great impact on the quality of data to be included in FCDBs.

An interlaboratory study to evaluate potential matrix reference materials for herbicides in water

Matrix reference materials (MRM) are essential tools for the validation of analytical protocols. Nowadays, there are no such materials for the determination of herbicides in water. Pesticides stored in acetonitrile and stored on solid phase extraction (SPE) cartridges previously percolated with a water sample spiked with triazines and phenylureas have proven to be good candidates for reference materials because of their satisfactory stability under appropriate temperature conditions. To evaluate the behaviors of these materials containing pesticides and to be analyzed by liquid chromatography, a collaborative study including 15 laboratories has been organized. Observed reproducibility on candidate materials after the removal of extreme results was 16.1% for the vials with pesticides in acetonitrile (at around 0.125 mg/L) directly analyzed, 29.2% for a water sample spiked with the pesticides (at around 0.5 μg/L) analyzed after preconcentration on a cartridge and 26.7% for the cartridges previously percolated with a water containing the pesticides (250 mL at around 0.5 μg/L for each pesticide) analyzed after elution. Such dispersion values are quite compatible with the requirement of a further certification for such materials.

Assuring the reliability of mass spectrometry for the routine determination of traces of doping in horse-urine

This paper describes an analytical method using a nuclear-related technique for the detection of forbidden doping substances in the urine of race horses. The proposed method, adapted from the Methode Alcaline Sur C-18 developed by the French Laboratoire de Controle Antidopage, is based on gas chromatography separation followed by mass spectrometry (GC-MS). The method was validated for caffeine, identified as the most frequent doping substance in the Brazilian horseracing activity. This validation is also a major requirement to achieve ISO/IEC 17025 laboratory accreditation. The validation has led to several metrological challenges because the decisions are largely based on qualitative results (“false-positive” and/or “false-negative”) and the degree of accuracy, as well as the traceability had to be determined in the absence of certified matrix reference materials.

Feasibility study of a reference material for water chemistry: Long term stability of triazine and phenylurea residues stored in vials or on polymeric sorbents

Matrix Reference Materials (MRM) are essential tools for the validation of analytical protocols. Nowadays, there are no such materials for the determination of herbicides in water. So, a feasibility study of a MRM for the analysis of triazines and phenylureas in water was carried out. Different kinds of candidates MRM were prepared: solutions of pesticides diluted in acetonitrile and stored in sealed vials or stored at the dry state after the evaporation of the solvent to dryness, pesticides stored on two different types of polymeric solid-phase extraction (SPE) sorbents after the percolation of drinking or river waters spiked with pesticides. The stability of these candidates MRM stored at various temperatures (room temperature, 0.5 °C or −18 °C) was studied over a period of approximately 1 year. Two different levels of concentration were studied for each kind of material. During the storage, some samples of each different MRM candidate were monthly analyzed by liquid chromatography. Results showed that, among the candidate materials, some of them presented satisfactory enough stability to consider a further certification. They were either pesticides in solution in sealed vials or pesticides stored on cartridges after the percolation of spiked water samples. However, it was shown that these different MRM candidates had to be stored at a temperature lower than 0.5 °C.

Development of a new standard reference material: SRM 1955 (homocysteine and folate in human serum)

Total homocysteine (tHCY) and folate are interrelated biomarkers for arteriosclerosis and coronary heart disease. Although many different methods for both tHCY and folate are clinically available, the intermethod and interlaboratory results are often poor, resulting in the need for a matrix reference material and reference methods. The National Institute of Standards and Technology (NIST) has developed isotope dilution liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/ tandem mass spectrometry (LC/MS/MS) methods for determination of tHCY and several folate forms including 5-methyltetrahydrofolic acid (5MT) and folic acid (FA). Additionally, a method for simultaneous measurement of tHCY, 5MT, and FA has been developed and validated. In collaboration with the Centers for Disease Control and Prevention (CDC), mass spectrometric methods and methods used in clinical laboratories have been applied to characterize a new Standard Reference Material (SRM), SRM 1955, "Homocysteine and Folate in Human Serum," containing low, medium, and high levels of tHCY and 5MT. Additionally, FA, 5-formyltetrahydrofolic acid (5FT), vitamin B12, and total folate values are provided. Use of the new SRM should improve clinical measurements and will permit traceability to internationally recognized certified reference materials, as described by European Directive 98/79/EC on in vitro diagnostic medical devices.

Suitability of a fully 13C isotope labeled internal standard for the determination of the mycotoxin deoxynivalenol by LC-MS/MS without clean up

Very often, the accuracy of quantitative analytical methods for the determination of mycotoxins by liquid chromatography (LC)-mass spectrometry (MS) and LC-MS/MS is limited by matrix effects during the ionization process in the MS source. Stable isotope labeled standards are best suited to correct for matrix effects and to improve both the trueness and the precision of analytical methods employing LC-MS and LC-MS/MS. This paper describes the successful use of fully 13C isotope labeled deoxynivalenol [(13C15)DON] as an internal standard (IS) for the accurate determination of DON in maize and wheat by LC electrospray ionization MS/MS. To show the full potential of (13C15)DON as IS, maize and wheat extracts were analyzed without further cleanup. Subsequent to calibration for the LC-MS end determination, DON was quantified in matrix reference materials (wheat and maize). Without consideration of the IS, apparent recoveries of DON were 29+/-6% (n=7) for wheat and 37+/-5% (n=7) for maize. However, the determination of DON in the reference materials yielded 95+/-3% (wheat) and 99+/-3% (maize) when (13C15)DON was used as an IS for data evaluation.

The natural matrix reference material database of the International Atomic Energy Agency: Reference materials in support of dietary investigations

Certified reference materials (CRMs) are the recognized tools to make the results of analytical measurements comparable and traceable to international standards. The use of CRMs in quality control and quality assurance of analytical determinations and in development of newer analytical methodologies is widely recognized. The International Atomic Energy Agency (IAEA) maintains a database on internationally available CRMs of natural matrices. This database is periodically updated, and presently documents about 2700 reference materials. Several food and clinical natural matrix materials certified for nutritionally essential substances are included. The IAEA database serves as a useful indicator of the status of current developments in natural matrix CRMs in this field. This article provides an account of the available data on natural matrix reference materials related to diet and create an awareness among the prospective end-users to enable the wider adaptation of these CRMs for validation purposes.

Screening, optimization and validation of microwave-assisted extraction for the determination of persistent organochlorine pesticides

In the present study, a microwave-assisted extraction method has been investigated for the extraction of persistent organochlorine pesticides from sediment in comparison to the classical Soxhlet extraction. The extraction procedure has been screened by a complete factorial design for statistically significant parameters. Thereafter, the identified parameters, composition of extraction solvent (mixtures of n-hexane–acetone), extraction temperature and time were optimized by a Box–Behnken design. Finally, the optimized microwave-assisted procedure has been validated by extraction of two different matrix reference materials, a sediment and a mallow powder. The data obtained for both materials were in good agreement with those obtained by Soxhlet extraction and published values. Special emphasis has been given to an accurate determination of p, p′-DDT, since DDT is known as instable during GC-injection. In order to evaluate the DDT-degradation, 13C 12- p, p′-DDT-solutions were analyzed and the degradation rates during GC-injection were calculated. Furthermore, 13C 12- p, p′-DDT was added to the sediment samples prior to extraction and the degradation rates during the optimization experiment have been investigated.

Determination of self-absorption corrections for gamma analysis of environmental samples: comparing gamma-absorption curves and spiked matrix-matched samples

Accurate determination of the massic activity of gamma-emitting radionuclides in environmental samples, particularly sediments and soils, cannot be achieved without taking into account sample self-absorption. The extent of self-absorption in the sample will depend on a number of factors including sample composition, density, sample size and gamma-ray energy. The preferred method for correcting for this effect is to use spiked or natural matrix reference materials that match each sample type to be analysed. However, for laboratories that must measure a wide variety of sample matrices this method is too costly and time-consuming. Another technique commonly used is to make direct gamma-ray transmission measurements for each sample. This method, while more practical, still requires a minimum of three measurements to be made for each sample analysed. A more convenient method is to prepare sets of gamma-absorption curves. This approach involves making a series of direct transmission measurements for samples of varying densities but similar type. Sets of matching samples, both spiked and unspiked, were prepared and density correction factors determined using the direct transmission method and the spiked sample approach. It was found that, for soil and sediment samples, these two methods typically differed by 5–9% for higher energy gamma rays and by 12–15% for the 59.54 keV 241Am peak. Gamma-absorption curves were also derived and, for the admittedly limited dataset, 95% confidence intervals of ±7% for the curve generated using the spiked samples method were obtained.

Certified reference materials to support a European thematic strategy for soil protection: the EUROSOILS (IRMM-443) experience.

The recent proposal of thematic strategy for soil protection by the European Commission outlines that knowledge of problems associated with soil has increased considerably based on surveys, monitoring systems and data networks. Although this information is very useful, its value for the policy-making process is limited due to a lack of comparability. From this unsatisfying situation the need arises to develop an EU-wide monitoring system accompanied by suitable matrix reference materials. Experiences gained from the development and certification of IRMM-443 (The EUROSOILS) show how to approach the complex problem of providing suitable (i.e. representative) soil CRMs for the arising analytical needs to support existing and upcoming EU legislation. This paper presents briefly the quantitative findings of a certification exercise (adsorption coefficients for atrazine, lindane and 2,4-D, as well as pH in suspension). A comparison of these results with those of an intercomparison exercise organised in 1989 by the German UBA on very similar soils shows the significant improvement in the determination of soil adsorption coefficients that was induced by the EUROSOIL project. Indicative values of total and organic carbon content and nitrogen according to ISO Standards are also presented.Furthermore, the idea of reference soils and derived reference materials (for analytical purposes) as well as reference matrices (substrates for ecotoxicological testing) is highlighted and set into the context of horizontal standardisation of methods.

The need for reference materials when monitoring nitrate intake.

Whether dietary exposure to nitrate metabolites is detrimental or beneficial to human health has long been a matter of controversy. In spite of no consistent epidemiological evidence, nitrate metabolites are associated with the formation of carcinogenic-nitrosamines and gastric cancer. Furthermore, recent studies demonstrate that ingested nitrate plays a role in host defence against gastrointestinal pathogenic bacteria. Analytical values of nitrate content in foods are essential for estimating nitrate intake. The analytical process is of paramount importance for assessing human nitrate exposure and for establishing a link between these exposures and the current and future observed health effects. Therefore, the quality assurance of the measurement process is crucial to obtaining reliability, comparability and traceability of results. Certified Reference Materials (CRM) should play a role in the consistency of the measurement process. However, the availability of nitrate CRMs is still poor. When food monitoring is demanded, an approach could be to use In House Reference Materials (IHRM), prepared at a high metrological level, and all preparation steps should be quality driven. IHRMs were prepared, and available CRMs were used to provide traceability of the process. The homogeneity of IHRM was evaluated using an appropriate statistical design. The stability was monitored using an isochronous method. The material shelf life and storage conditions are presented. HPLC was optimised for the determination of nitrates in four vegetable categories. When a suite of IHRMs were used, the response of the HPLC system was linear over the range 1 to 8 mg L(-1). The detection limit for these compounds was 0.2 microg L(-1) and the determination limit 1.2 microg L(-1). The relationship between measurement uncertainty and critical points of the analytical process is presented. The differences in observed relative uncertainty between food categories could reflect current limitations in the food matrix reference materials.

Improvements in efficiency of production and traceability for certification of reference materials

Issues of current interest to certified reference material producers are addressed. Alternative strategies for certification of matrix reference materials are discussed and the benefits of adopting a flexible, cost-effective approach are described. The difficulty of undertaking homogeneity testing where certification is to be carried out with definitive techniques capable of providing very small measurement uncertainty is discussed. Methodology is described which combines conventional screening of the candidate material for homogeneity with an additional, precise assessment of homogeneity based on isotope dilution mass spectrometry measurements. A systematic procedure for evaluating the commutability (horizontal traceability or scope) of matrix reference materials has been evaluated and shows that in some circumstances matrix effects may be less pervasive than is generally believed. This offers the possibility, especially for trace analysis applications, of more efficient use of existing reference materials without compromising measurement reliability. Vertical traceability of matrix reference material data is of growing interest but is difficult to achieve with present interlaboratory certification exercises. A modification is described which attempts to address this issue. It also offers the possibility of improved identification of outliers and reduced variation of data between the participating laboratories.

Determination of total mercury in environmental and biological samples using k0-INAA, RNAA and CVAAS/AFS techniques: Advantages and disadvantages

The accuracy and precision of the results obtained for total mercury in various environmental and biological samples and certified reference materials (CRMs) by various analytical methods, including k 0-instrumental neutron activation analysis (k 0-INAA), radiochemical neutron activation analysis (RNAA) and cold vapour atomic absorption (and atomic fluorescence) spectrometry (CVAAS/AFS) used in routine analysis in our laboratory, were investigated. Three natural matrix reference materials (RMs) from the International Atomic Energy Agency (IAEA), five CRMs from the Institute for Reference Materials and Measurements (IRMM), six CRMs from the National Institute of Standards and Technology (NIST) and one from the Jožef Stefan Institute (IJS) were analyzed. The results obtained show good agreement between certified or assigned values, and between the methods used, except for some data obtained by k 0-INAA in biological samples. This can be explained by losses during irradiation in semi-open systems (irradiation in plastic ampoules) and/or spectral interferences.

K0-NAA quality assessment by analysis of different certified reference materials using the KAYZERO/SOLCOI software

A suite of natural matrix reference materials (RMs) were used to assess the quality of analytical results obtained by k0-instrumental neutron activation analysis (k0-INAA) at the Jožef Stefan Institute (IJS). Five certified reference materials (CRMs) from the Institute for Reference Materials and Measurements (IRMM), two standard reference materials (SRMs) from the National Institute of Standards and Technology (NIST), three RMs from the International Atomic Energy Agency (IAEA) and one RM from IJS were analyzed. Altogether, results for twenty-four elements in inorganic matrices and twenty-nine elements in organic matrices, obtained by k0-INAA, were compared to certified values. Results obtained show good agreement with certified or assigned values except for Fe, La, Nd, Sm and U in inorganic matrices, and Ag, Al and Cr in organic matrices.

Preparation of Gaseous Standard Mixtures: Methods for Controlling the Amount of Components Generated in the Process of Thermal Decomposition of Immobilized Compounds

Gaseous standard mixtures play an important role in characterizing and checking the applicability of new analytical procedures. In this case gaseous standard mixtures can be treated as a special kind of reference material (validation process requires the use of so-called matrix reference materials).The selection of a method of generation of gaseous standard mixtures with the desired characteristics depends on the nature of the analyte and diluent gas, as well as on the required concentration of analytes in the mixture. Recently, dynamic methods of generation of gaseous standard mixtures are becoming more and more common. These include the method based on thermal decomposition of immobilized compounds. The generation of the measured component takes place as a result of heating of a sample of solid support with chemically modified surface. During a chemical reaction initiated by sufficiently high temperature, the immobilized compound undergoes decomposition or rearrangement accompanied by the release of a specific volatile compound. Various amounts of the analyte per unit time can be obtained by adjusting the conditions of the thermal decomposition process.This article describes the possibility of varying the amount of a measured component through the adjustment of conditions of drying the solid support or of the process of chemical modification of its surface. The principle of this technique is exemplified by the generation of a mixture containing ethene as the analyte. Porous glass was used as a solid support. The proposed technique was used for the generation of gaseous standard mixtures for the calibration of a thermal desorber-gas chromatograph-flame ionization detector (TD-GC-FID) system.

Interlaboratory comparisons - demonstrating degree of equivalence

The role of matrix reference materials in the process of demonstrating the degree of equivalence of measurement results obtained from intercomparisons is outlined, reviewing exemplary selected experience gained at BAM regarding the determination of organic contaminants in environmental matrices. The specific characteristics of reference materials employed in the process of demonstrating equivalence between laboratories in the course of proficiency testing as well as the development, comparison and validation of methods are elaborated. The demand is for series of appropriately characterised samples which are fit for the purpose and it is seen from representative examples that the utilisation of such tailor-made RM designed to tackle the specific need of an analytical problem dominates over certified matrix reference materials in this context. Concluding, the role of certified matrix reference materials in quality assurance is briefly looked at both from the user’s and providers’ points of view.

CCQM-P13 pilot study. Metals in artificial food digest

The accuracy of elemental analyses in complex matrices is usually assessed by analysis of a suitable matrix reference material. The reference value is ascribed by consensus mean and by application of primary methods of analysis. However, the quality of this value will be affected by problems such as matrix-induced interferences, moisture corrections and heterogeneity.Pilot study CCQM-P13 was undertaken to assess the capabilities ofNational Metrology Institutes to analyse Ca, Cu and Cd in an acidic solution that simulates the digest of a food sample. This study filled the gap between the analysis of a gravimetrically prepared calibration solution and the analysis of an unknown in a complex matrix requiring extensive sample preparation. Having an independent reference value, with a small uncertainty, allowed a more rigorous estimation of the reliability of the institutes' analysis and uncertainty estimates, without including issues around sample digestion.The reference values were: 1.6617 ± 0.0020 µmol/g for Ca, 7.037 ± 0.012 nmol/g for Cu and 45.57 ± 0.10 pmol/g for Cd (expanded uncertainties are quoted with coverage factor of 2). The other elements in the matrix were: Na (~25 µg/g), K (~90 µg/g), Cl (~120 µg/g), Fe (~100 ng/g), Mg (~5 µg/g), P (~5 µg/g), Sn (~80 ng/g) and Zn (~200 ng/g).Twelve international laboratories, representing eight countries, determined the amount content of the analytes. A range of techniques that include ID-ICP-MS (high resolution and collision cell), ICP-MS, ICP-OES, AAS, voltametry and potentiometry were used. The results for this pilot study averaged: 1.654 ± 0.058 µmol/g for Ca (n = 10), 7.26 ± 0.53 nmol/g for Cu (n = 12) and 45.2 ± 5.1 pmol/g for Cd (n = 11) where the values associated with the averages are the standard deviations of n participants.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by theCCQM Working Group on Inorganic Analysis (IAWG), according to theprovisions of the Mutual Recognition Arrangement (MRA).