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Organisationseinheit der BAM
The CCQM-K88 key comparison was organized by the Inorganic Analysis Working Group of CCQM to test the abilities of the national metrology institutes to measure the mass fraction of lead in lead-free solder containing silver and copper. National Metrology Institute of Japan (NMIJ), National Institute of Metrology of China (NIM) and Korea Research Institute of Standards and Science (KRISS) acted as the coordinating laboratories. The participants used different measurement methods, though most of them used inductively coupled plasma optical emission spectrometry (ICP-OES) or isotope-dilution inductively coupled plasma mass spectrometry (ID-ICP-MS). Accounting for relative expanded uncertainty, comparability of measurement results was successfully demonstrated by the participating NMIs for the measurement of the mass fraction of lead in lead-free solder at the level of 200 mg/kg.
It is expected that metals at mass fractions greater than approximately 100 mg/kg in lead-free solder containing silver and copper can be determined by each participant using the same technique(s) employed for this key comparison to achieve similar uncertainties mentioned in the present report.
The determination of the mass fractions of bromide, sulfate, and lead as well as the isotopic composition of the lead (expressed as the molar mass and the amount fractions of all four stable lead isotopes) in an aqueous solution of sodium chloride with a mass fraction of 0.15 g/g was the subject of this comparison. Even though the mass fractions ranged from 3 μg/g (bromide) to 50 ng/g (lead), almost all results reported agreed with the according KCRVs.
The aim of this comparison was to demonstrate the capability of national metrology institutes to measure elemental mass fractions at a level of w(E) ≈ 1 g/kg as found in almost all mono-elemental calibration solutions. These calibration solutions represent an important link in traceability systems in inorganic analysis. Virtually all traceable routine measurements are linked to the SI through these calibration solutions. Every participant was provided with three solutions of each of the three selected elements chromium, cobalt and lead. This comparison was a joint activity of the Inorganic Analysis Working Group (IAWG) and the Electrochemical Analysis Working Group (EAWG) of the CCQM and was piloted by the Physikalisch-Technische Bundesanstalt (PTB, Braunschweig, Germany) with the help of the Bundesanstalt für Materialforschung und -prüfung (BAM, Berlin, Germany), the Centro Nacional de Metrología (CENAM, Querétaro, Mexico) and the National Institute of Standards and Technology (NIST, Gaithersburg, USA).
A small majority of participants applied inductively coupled plasma optical emission spectrometry (ICP OES) in combination with a variety of calibration strategies (one-point-calibration, bracketing, calibration curve, each with and without an internal standard). But also IDMS techniques were carried out on quadrupole, high resolution and multicollector ICP-MS machines as well as a TIMS machine. Several participants applied titrimetry. FAAS as well as ICP-MS combined with non-IDMS calibration strategies were used by at least one participant. The key comparison reference values (KCRV) were agreed upon during the IAWG/EAWG meeting in November 2011 held in Sydney as the added element content calculated from the gravimetric sample preparation. Accordingly the degrees of equivalence were calculated. Despite the large variety of methods applied no superior method could be identified. The relative deviation of the median of the participants' results from the gravimetric reference value was equal or smaller than 0.1% (with an average of 0.05%) in the case of all three elements.
Within the scope of a project of the 'European Association of National Metrology Institutes' (EURAMET), a European metrological dissemination System (network) providing traceable reference values assigned to matrix materials for validation purposes is described and put to the test. It enables testing laboratories (TL) to obtain comparable results for measurements under the 'EU Water Framework Directive 2000/60/EC' (WFD) and thus, to comply with a core requirement of this very directive. The dissemination system is characterized by the fact that it is available to all laboratories throughout Europe which intend to perform measurements in the context of the WFD and that it can ensure sustainable metrological traceability to the International System of Units (SI) as a reference point for the measurement results. This Dissemination system is set up in a hierarchical manner and links up the level of the national metrology institutes (NMI) with that of the TLs via an intermediate level of calibration laboratories (CL) by comparison measurements. The CLs are expert laboratories with respect to the measurement of the analytes considered here (within the project, the CLs are called potential calibration laboratories (PCL)) and are additionally involved in the organization of comparison measurements within the scope of regional quality assurance (QA) systems.
Three comparison measurements have been performed to support the approach. A total of about 130 laboratories participated in this exercise with the focus on the measurement of the priority substances Pb, Cd, Hg, and Ni defined in the WFD. The elemental concentrations in the water samples roughly corresponded to one of the established environmental quality standards (EQS), the annual average concentration (AA-EQS), which is defined in the daughter Directive 2008/105/EC of the WFD. It turned out that a significant number of TLs still need to improve their measurement methods in order to be able to fulfill the Minimum requirements of the WFD, in particular, with regard to the elements Cd and Hg probably due to their low EQS values.
Furthermore, it became obvious that the hierarchical Dissemination system suggested here actually corresponds to the measuring capabilities of the three participating Groups (NMIs, PCLs, and TLs).
A complete metrological traceability system for measurement results of chemical analysis was set up. Core components are pure substances (national standards) characterised at the highest metrological level, primary solutions prepared from these pure substances and secondary solutions deduced from the primary solutions and intended for sale. The relative uncertainty of the element mass fraction of the primary substances and solutions is < 0.01 and < 0.05%, respectively. For the certification of transfer solutions and for stability testing, a precision measurement method for element contents has been developed by means of optical emission spectrometry (ICP OES) by which uncertainties between 0.1 and 0.05% can be achieved. The dissemination to field laboratories is effected with the aid of a calibration laboratory of the German Calibration Service (DKD) which certifies the element content of the secondary solutions with an uncertainty <= 0.3%. Calibration with these solutions enables the user to establish traceability of his measurement results to the International System of Units (SI). Currently, the system comprises Cu, Fe, Bi, Ga, Si, Na, K, Sn, W, and Pb.
Gravimetrically prepared mono-elemental reference solutions having a well-known mass fraction of approximately 1 g/kg (or a mass concentration of 1 g/L) define the very basis of virtually all measurements in inorganic analysis. Serving as the starting materials of all standard/calibration solutions, they link virtually all measurements of inorganic analytes (regardless of the method applied) to the purity of the solid materials (high-purity metals or salts) they were prepared from. In case these solid materials are characterized comprehensively with respect to their purity, this link also establishes direct metrological traceability to The International System of Units (SI). This, in turn, ensures the comparability of all results on the highest level achievable. Several national metrology institutes (NMIs) and designated institutes (DIs) have been working for nearly two decades in close cooperation with commercial producers on making an increasing number of traceable reference solutions available. Besides the comprehensive characterization of the solid starting materials, dissolving them both loss-free and completely under strict gravimetric control is a challenging problem in the case of several elements like molybdenum and rhodium. Within the framework of the European Metrology Research Programme (EMRP), in the Joint Research Project (JRP) called SIB09 Primary standards for challenging elements, reference solutions of molybdenum and rhodium were prepared directly from the respective metals with a relative expanded uncertainty associated with the mass fraction of Urel(w)<0.05 %. To achieve this, a microwave-assisted digestion procedure for Rh and a hotplate digestion procedure for Mo were developed along with highly accurate and precise inductively coupled plasma optical emission spectrometry (ICP OES) and multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) methods required to assist with the preparation and as dissemination tools.
CCQM-K143 is a key comparison that assesses participants’ ability to prepare single element calibration solutions. Preparing calibration solutions properly is the cornerstone of establishing a traceability link to the International System of Units (SI), and therefore should be tested in order to confirm the validity of CCQM comparisons of more complex materials. CCQM-K143 consisted of participants each preparing a single copper calibration solution at 10 g/kg copper mass fraction and shipping 10 bottled aliquots of that solution to the coordinating laboratory, the National Institute of Standards and Technology (NIST). The masses and mass fraction for the prepared solutions were documented with the submitted samples.
The solutions prepared by all participants were measured at NIST by high performance inductively coupled plasma optical emission spectroscopy (HP-ICP-OES). The intensity measurements for copper were not mapped onto values of mass fraction via calibration. Instead, ratios were computed between the measurements for copper and simultaneous measurements for manganese, the internal standard, and all subsequent data reductions, including the computation of the KCRV and the degrees of equivalence, were based on these ratios. Other than for two participants whose measurement results appeared to suffer
from calculation or preparation errors, all unilateral degrees of equivalence showed that the measured values did not differ significantly from the KCRV. These results were confirmed by a second set of ICP-OES measurements performed by the Physikalisch-Technische Bundesanstalt (PTB). CCQM-K143 showed that participants are capable of preparing calibration solutions starting from high purity, assayed copper metal.
Similar steps are involved when preparing solutions for other elements, so it seems safe to infer that similar capabilities should prevail when preparing many different, single-element solutions.
For the first time, an ab initio calibration for absolute Mg isotope ratios was carried out, without making any a priori assumptions. All quantities influencing the calibration such as the purity of the enriched isotopes or liquid and solid densities were carefully analysed and their associated uncertainties were considered. A second unique aspect was the preparation of three sets of calibration solutions, which were applied to calibrate three multicollector ICPMS instruments by quantifying the correction factors for instrumental mass discrimination. Those fully calibrated mass spectrometers were then used to determine the absolute Mg isotope ratios in three candidate European Reference Materials (ERM)-AE143, -AE144 and -AE145, with ERM-AE143 becoming the new primary isotopic reference material for absolute isotope ratio and delta measurements. The isotope amount ratios of ERM-AE143 are n(25Mg)/n(24Mg) = 0.126590(20) mol/mol and n(26Mg)/n(24Mg) = 0.139362(43) mol/mol, with the resulting isotope amount fractions of x(24Mg) = 0.789920(46) mol/mol, x(25Mg) = 0.099996(14) mol/ mol and x(26Mg) = 0.110085(28) mol/mol and an atomic weight of Ar(Mg) = 24.305017(73); all uncertainties were stated for k = 2. This isotopic composition is identical within uncertainties to those stated on the NIST SRM 980 certificate. The candidate materials ERM-AE144 and -AE145 are isotopically lighter than ERM-AE143 by 1.6 ‰ and 1.3 ‰, respectively, concerning their n(26Mg)/n(24Mg) ratio. The relative combined standard uncertainties are ≤0.1 ‰ for the isotope ratio n(25Mg)/n(24Mg) and ≤0.15 ‰ for the isotope ratio n(26Mg)/ n(24Mg). In addition to characterizing the new isotopic reference materials, it was demonstrated that commonly used fractionation laws are invalid for correcting Mg isotope ratios in multicollector ICPMS as they result in a bias which is not covered by its associated uncertainty. Depending on their type, fractionation laws create a bias up to several per mil, with the exponential law showing the smallest bias between 0.1 ‰ and 0.7 ‰.
The continuous improvement of analytical procedures using multi-collector technologies in ICP-mass spectrometry has led to an increased demand for isotope standards with improved homogeneity and reduced measurement uncertainty. For magnesium, this has led to a variety of available standards with different quality levels ranging from artefact standards to isotope reference materials certified for absolute isotope ratios. This required an intercalibration of all standards and reference materials, which we present in this interlaboratory comparison study. The materials Cambridge1, DSM3, ERMAE143, ERM-AE144, ERM-AE145, IRMM-009 and NIST SRM 980 were cross-calibrated with expanded measurement uncertainties (95% confidence level) of less than 0.030‰ for the δ25/24Mg values and less than 0.037‰ for the δ26/24Mg values. Thus, comparability of all magnesium isotope delta (δ) measurements based on these standards and reference materials is established. Further, ERM-AE143 anchors all magnesium δ-scales to absolute isotope ratios and therefore establishes SI traceability, here traceability to the SI base unit mole. This applies especially to the DSM3 scale, which is proposed to be maintained. With ERM-AE144 and ERM-AE145, which are product and educt of a sublimation-condensation process, for the first time a set of isotope reference materials is available with a published value for the apparent triple isotope fractionation exponent θapp, the fractionation relationship ln α(25/24Mg)/ln α(26/24Mg).