Known purity—the fundament of element determination by atomic spectrometry

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Abstract

Millions of measurements are performed each year by liquid based analytical atomic spectrometry to support healthcare, diagnostic tests, environmental monitoring, material assay, product development and safety. Despite the effort to develop absolute methods, most methods still depend on calibration solutions, which are gravimetric mixtures of high purity solvents and high purity (source material) metals or compounds. As in the real world ideal purity does not exist, the impurity of the solvent and the purity of the source material needs to be known. The impurity of a solvent with respect to one analyte can be measured rather easily and with low limits of determination. In contrast the measurement of the purity of the source material, i.e., the mass fraction of the main constituent in a high purity metal, is more difficult to determine. It becomes even more difficult when the source material is not a pure metal but a compound since problems regarding stoichiometry arise additionally. Although the major producers of calibration solutions make a special effort to determine the purity of the source material, the actual purity statement is often incomplete or not demonstrated. The main reason for this situation is the complexity and high effort necessary to fully characterize such a material. This problem holds to a very wide extent also for the primary standards for element determination at the National Metrology Institutes and Designated Institutes (NMIs and DIs). It is the task of the NMIs and DIs to realise and disseminate primary standards for providing traceability to the International System of Units (SI). The primary elemental standards at the NMIs should provide the link to secondary standards produced by commercial producers and other independently prepared standards for element determination. Without such primary standards, elemental calibration solutions may vary and, depending on the uncertainty required, comparability of measurement in time and space results cannot be achieved.

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EMRP-project SIB09 elements, primary standards for challenging elements

An attempt to overcome this situation is a new project in the framework of the European Metrology Research Programme (EMRP). Based on stakeholder needs, EMRP aims to strengthen the collaboration of the European National Metrology Institutes and Designated Institutes (NMIs and DIs) in the areas of industry, energy, SI broader scope (SIB), environment, health and new technologies. The EMRP is jointly funded by the EMRP participating countries within EURAMET (the European body representing NMIs and DIs) and the European Commission.

The project described here is working on methodology for the production of “Primary Standards for Challenging Elements” (SIB09 Elements). The consortium consists of seven funded and two unfunded NMIs/DIs and to date two research excellence grants (REG) at a university and a research centre. In addition several collaborators contribute to the project. A complete list of all participants in the project is given in Table 1. Their geographical distribution in Europe and the logo of the project is given in Fig. 1.

Table 1 Project partners

No.	Participant type	Short name	Organisation legal full name	Contact	Country
1	Funded JRP-partner	BAM	Bundesanstalt fuer Materialforschung und – pruefung	Heinrich Kipphardt	Germany
				Silke Richter
				Jochen Vogl
2	Funded JRP-partner	BRML	Biroul Roman de Metrologie Legala	Mirella Buzoianu	Romania
3	Funded JRP-partner	INRIM	Istituto Nazionale di Ricerca Metrologica	Luigi Bergamaschi	Italy
				Giancarlo D'Agostino
4	Funded JRP-partner	LGC	LGC Limited	Mike Sargent	United Kingdom
				Heidi Goenaga-Infante
5	Funded JRP-partner	LNE	Laboratoire national de métrologie et d'essais	Paola Fisicaro	France
6	Funded JRP-partner	PTB	Physikalisch-Technische Bundesanstalt	Detlef Schiel	Germany
				Olaf Rienitz
7	Funded JRP-partner	SMU	Slovenský metrologický ústav	Michal Mariassy	Slovakia
8	Unfunded JRP-partner	CENAM	Centro Nacional de Metrologia – CENAM	Rocio Arvizu Torres	Mexico
				Yoshito Mitani
9	Unfunded JRP-partner	HRMF	Hellenic Republic Ministry of Finance	Eugenia Lampi	Greece
10	REG	UGent	Ghent University, Department of Analytical Chemistry	Frank Vanhaecke	Belgium
11	REG	IFW	Leibniz Institute for Solid State and Materials Research	Volker Hoffmann	Germany

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Fig. 1 Geographical distribution of the project partners.

This research project aims to provide the technical basis for resolving the lack of primary standards for element determination in a sustainable way within the scope of worldwide activity of the National Metrology Institutes running parallel to the project. Methodology will be developed for realising fit for purpose primary standards for most of the challenging elements frequently used in elemental analysis. The methods will be developed and applied for selected guide elements for which Al, Mg, Zn, Mo and Rh were chosen.

The research of the project is based on three technical pillars. First there will be the development of procedures for purity analysis with respect to metallic and non-metallic impurities of high purity materials with focus on efficient methods and challenges in non-metal determination. The second pillar will be matrix investigations, involving direct determination of the main components as a universal approach, purification to establish blank materials, measurement of the isotopic composition and to develop efficient methods for dealing with isotopic composition. In the third pillar work will be on the dissemination of the primary standards through loss free decomposition, linking two solutions of similar composition and to link a solution to a solid material directly with small uncertainty.

The various tasks have been organized within 5 work packages as is shown in Fig. 2.

Fig. 2 Structure of the project.

Methods for impurity analysis (WP1)

The aim of the work package is to develop methods for metallic and non-metallic impurity analysis of high purity materials and to demonstrate their applicability on selected guide elements that are representative for a group of elements with similar behaviour. A consequence (or by-product) of the general method development is the availability of well characterized materials for specific elements.

The guide elements in this work package are Al, Mg, Zn. Al and Mg are of great importance for the industry for light weight construction materials as used in car and plane construction. Zn is an element with an interesting prospect for energy production via reduction of water or use of ZnBr₂. Additionally Mg and Al are relevant in clinical chemistry, especially with respect to Alzheimer’s disease.

To serve as a primary standard for element determination the total purity of a high purity material needs to be measured. In order to achieve a sufficiently small uncertainty (i.e. < 0.01%) this involves determining all possible impurity contributions and to subtract their sum from the ideal purity of 100%.

The general challenge of this type of impurity analysis lies in its comprehensiveness and complexity. Efficient methods to tackle this problem are urgently needed. An additional challenge comes from the fact that it is very difficult to achieve a sufficient homogeneity for the high purity materials to be analysed. The latter is often simply ignored (which might render the analyses less useful even if state-of-the-art methodology has been used).

Special attention needs to be paid to the determination of non-metallic impurities. Although they often dominate the impurity statement for a high purity material, it is an unfortunate matter of fact that they are usually ignored, because they are difficult to measure. The choice of methods providing SI traceable results for H, N and especially O determination that are independent of calibration with matrix reference materials is limited to a small number of hot extraction methods. However, even for hot extraction and combustion analysis (for C and S) a versatile calibration basis with small uncertainty and SI traceability is still not available. Also, for elements forming on the one side stable oxides and having on the other side high vapour pressures, extraction efficiency for oxygen is a crucial point.

For the development activities of this WP also candidate materials must be obtained and tested for general suitability. Methodology developed must be applied to them and the results on the individual materials must be compiled for further dissemination.

Elemental and isotopic characterisation (WP2)

The aim of this work package is to develop and validate new and improved methods for determinations of the elemental and isotopic composition of primary elemental standards.

Direct determination of the main component of a primary elemental standard is an essential alternative to the impurity assessment techniques developed in WP1. The methodology also serves to establish traceability between those primary standards and different elements. The aim of this task is to evaluate the possibility and conditions for use of main component assay methods to establish SI traceability for elemental solutions. Attention will be paid to satisfactory evaluation of rounded amperometric titration curves in EDTA assay. Assays traceable to EDTA material of known composition will be developed and evaluated using materials of known composition prepared in WP1.

The determination of isotope abundance ratios is urgently needed in establishing traceability in quantitative elemental analysis because they play a fundamental role in two aspects of the traceability chain: 1. by measuring the isotope composition of primary and secondary calibration standards and the analyte in matrix reference materials or other samples, and 2. by the application of isotope dilution mass spectrometry (IDMS) for transfer of calibrations between primary and secondary standards and reference measurements.

The top level of traceability is being realized by primary calibration standards for the determination of element concentrations but these high purity substances may undergo isotope fractionation processes during production. Additionally for many elements there are isotope variations in nature. Therefore the isotope composition between primary calibration standard and secondary calibration standard might differ from each other, but certainly they differ from the isotopic composition of the analyte in many samples. Many measurement principles in chemical analysis are on a molar basis or are isotope selective and therefore a difference in the isotope composition between sample and calibration standard will lead to biased results. Also the conversion from mass fraction (e.g. mg kg⁻¹) to amount content (e.g. mol kg⁻¹) and vice versa leads to biased results unless the isotope composition is being considered.

Most determinations of isotope abundance ratios are based on mass spectrometry which offers the potential for measuring the ratios with very small uncertainties. However, the instruments are subject to mass discrimination effects which require mass calibration or correction of the spectrometer. A key task for this work package is to evaluate potential approaches for this purpose with sufficiently small uncertainty for measuring the isotope composition of primary calibration standards as well as the other applications mentioned above. The more fundamental approach, with potential for smallest uncertainties, is based on mass bias calibration of the spectrometer for a specific application using isotope mixtures of known composition which are prepared gravimetrically from pure isotopes of the target element. This is time consuming and expensive. A comparison will, therefore, be made with alternative approaches for making mass bias corrections which, whilst having larger uncertainties, are less time consuming and are potentially feasible for rapid application to a much larger group of elements. Possible approaches to be evaluated include mass bias calibration with mixtures of commercially available “pure” isotopes, mass bias correction using a nearby isotope of another element, and mass bias correction using the “isotopic double spiking” technique.

The elements selected as models for development and optimisation of new isotopic measurement procedures are Mg and Mo. Mg will be used to evaluate procedures for ultra-high purification of single isotopes and gravimetric preparation of isotopic calibration mixtures for mass bias correction of Mg isotope ratio measurements. Mo is an element for which isotopes of reasonable purity are available commercially but for which ultra-high purification would present much greater challenges. Unlike Mg, it also has sufficient isotopes to allow evaluation of the “isotopic double spiking” technique. Both elements show significant isotopic variation in nature. Hence the availability of these reference materials will also improve the uncertainties achievable by IDMS measurements for applications such as materials science, biological/clinical applications, and environmental measurements. The improved methodology for mass bias corrections will make a major contribution to WP3 which uses the MC-ICP-MS technique for certification of elemental calibration solutions.

Standard solutions for challenging elements (WP3)

Elemental solutions with a mass concentration of 1 g L⁻¹ are used in almost every chemical laboratory for calibration. The solutions are the carrier of traceability and are directly provided by the NMIs to the customers or indirectly via commercial calibration solution producers which are linked with the primary standards of the NMIs. A relative uncertainty of 0.3% for the concentrations of the customer solutions turned out to be the present demand. Considering the certification and linking procedures the primary solutions of the NMIs need to have an uncertainty ≤0.1%. At present only calibration solutions of a few elements meet these requirements whereas solutions for many essential elements are nonexistent. The reasons are the lack of reliable primary standards and of methods for the solid–liquid transfer of the primary elemental materials. To perform this transfer is a main task of this work package.

For practical use, the primary solid elemental materials (e.g., the metals characterized in WP1) have to be converted to a measurable form which is in almost every case a liquid solution. Primary standard solutions are needed for dissemination and for calibration purposes as well. The main challenge of this work is to assure that the primary solid material is completely dissolved and brought into the primary solution. This is in particular problematic for hardly dissolvable solid materials as the refractory elements and for materials which form volatile compounds during the digestion process. The refractory elements considered here are examples for such a challenging solid–liquid transfer and there are no appropriate dissolution methods available so far.

The results of a CCQM-pilot study, CCQM-P46, clearly underpin this present situation. Primary Rh solutions prepared by the NMIs showed relative discrepancies of about 0.5% from the reference value. This is much higher than the requirements mentioned above and could be caused by incompleteness of the digestion process and insufficient purity assessment of the primary materials.

Measurement methods providing highest precision ≤0.1% are needed for dissemination. For that purpose the elemental content of secondary solutions made from any solid elemental material of unknown purity has to be attributed by comparing measurement with primary solutions. Secondary solutions can be provided by the NMI to the customers as transfer standards. Alternatively instrumental neutron activation analysis (INAA) should be investigated concerning its suitability for the measurement of the elemental content of the solutions directly against a primary solid material, without the need to dissolve the latter. This technique has the potential to rationalize solid–liquid transfers absolutely in particular in the case of elements which are difficult to digest. Such an approach has not yet been tested for that purpose.

Creating impact (WP4)

The key technical aim of the Joint Research Program (JRP) is the development of efficient methods to overcome the existing lack of demonstrated primary standards for element determination aiming to induce international activities with the JRP and its collaborators as core.

Knowledge transfer within the metrological community will be realised by reporting at the regular meetings which take place at both European and global metrology organisations. These bodies are respectively the Inorganic Analysis Working Groups of EURAMET and CCQM/1,2/. A database of currently available primary elemental materials and solutions in Europe will be established. It will be accessible for metrology institutes and will give the opportunity to realise traceability for elements which are not at the disposal of an NMI. In a second step this database will be extended to a worldwide level within the framework of the inorganic analysis working group of CCQM. Knowledge transfer to the scientific community is realised via conference participation (e.g. EUROANALYSIS 2013). Two workshops for stakeholders especially focused on producers of calibration solutions and reference materials will be held to establish dissemination of primary standards via secondary standards to the field laboratories.

Standardisation bodies also belong to the stakeholders, e.g. CEN/TC 230, which underpinned the importance of this project for the implementation of standards in the field of water analysis. Demonstrating traceability to SI (e.g. via use of standards that are linked to primary standards) is also an essential part for implementing ISO/IEC 17025. The JRP aims to provide the basis for realisation of such standards.

Training is provided during the stakeholder workshops.

More detailed information about EMRP and the project can be found on the following webpages/3,4/:

/1/http://www.euramet.org

/2/http://www.bipm.org/en/committees/cc/ccqm/

/3/http://www.emrponline.eu/

/4/http://www.ptb.de/emrp/sib09.html

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