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Organisationseinheit der BAM
For the determination of main and minor components in silicon-based ceramic powders, a decomposition by a combustion with elemental fluorine and separation of the volatile fluorination products by a carrier-gas distillation with a subsequent detection by quadrupole mass spectrometry is described. The necessity and success of the separation step is demonstrated for the determination of boron as a minor constituent in SiC, where the spectral interferences of silicon on the boron signals are decreased considerably. The method developed is shown to be directly applicable to determination of silicon in Si3N4, SiC, and SiO2. The determination of nitrogen in Si3N4 requires additional effort, to separate nitrogen from the excess of fluorine. For the determination of boron, a complete mobilization of BF3 is assured by the presence of an adequate amount of GeF4. Analysis results obtained with different types of calibration show a precision of 30 g for boron at the milligram-per-gram level and a precision between 0.5 and 2% (m/m) for the main components, silicon and nitrogen. Within these standard deviations, the results agree well with the values expected from the stoichiometry, with the results for silicon and boron obtained by wet chemical decomposition and slurry techniques in combination with ICP-OES and with the results for nitrogen obtained by carrier gas heat extraction.
Isotope amount ratios of krypton were measured on subsamples from one large batch of high purity krypton separated from the atmosphere. Synthetic mixtures of enriched krypton isotopes were used to calibrate the measurements with small uncertainties. The result is a primary isotopic gas standard (PIGS) IRMM-2030 with certified values for isotope ratios, isotopic composition, and molar mass of krypton with small combined uncertainties uc, evaluated according to the ISO/BIPM Guide (GUM). It is commercially available from IRMM-Geel or from MESSER (Duisburg, D). The certified krypton isotope amount ratios in the PIGS IRMM-2030 are as follows: n(78Kr)/n(84Kr)=0.006 232 5(55), n(80Kr)/n(84Kr)=0.040 107(17), n(82Kr)/n(84Kr)=0.203 43(12), n(83Kr)/n(84Kr)=0.201 79(11), and n(86Kr)/n(84Kr)=0.303 205(59) with expanded uncertainty U=kuc and coverage factor k=2. The molar mass of Kr in this sample is M(Kr)=83.798 02(16) g/mol. These values are in good agreement with published measurements of atmospheric krypton but have smaller combined uncertainties and are calibrated by means of synthetic isotope mixtures. The values of the PIGS are traceable to the SI. Measurements of isotope amount ratios of krypton in other samples can be linked to SI using this PIGS.
A new approach was developed for quantitative calibration in GD-MS which can afford reliable and metrologically traceable results for many trace elements and was exemplified for pure copper and pure iron. It can be assumed that the technique can be further improved and applied to the analysis of other pure metals. Pressed copper and iron powder samples were used to calibrate the glow discharge mass spectrometry applied to the analysis of pure copper and iron. The new type of glow discharge mass spectrometerthe Element GD (Thermo Electron Corporation)was used with a Grimm-type discharge cell for flat samples. Two series of powder samples were prepared for each of the copper and iron matrixes. The powders were quantitatively doped with solutions of graduated and defined concentrations of 40 or 20 analytes, respectively. The mass fractions of the analytes in the dried and homogenized metal powder samples ranged from µg/kg levels up to 10 mg/kg levels. A special technique was developed to press the samples and to form mechanically stable pellets with low risk of contamination. Ion beam ratios of analyte ions to matrix ions were used as measurands. The calibration curves were determined and the linear correlation coefficients were calculated for different intervals of the curves. The linear correlation coefficients are very satisfactory for most of the calibration curves, which include the higher segments of mass fractions; however, they are less satisfactory for the lower segments of the calibration curves. Nevertheless, in many cases rather acceptable and rather promising values were achieved even for these lower segments, representing mass fractions of analytes at ultra-trace level. The comparison of the certified values of different reference materials with the measured values based on calibrations with the pressed powder samples led to deviations less than 30% for most of the considered examples.
The International Temperature Scale of 1990 (ITS-90) is based on thermodynamic equilibrium states of ideally pure substances. The largest contribution to the uncertainty budgets of most metallic fixed points is the influence of impurities on the fixed-point temperature. Therefore, a traceable chemical analysis of the remaining impurities with small uncertainty is the basis of further progress. Further requirements are better knowledge of the phase diagrams at very low impurity contents, impurity segregation, and the quantification and correction of thermal effects during a fixed-point realization. In this article, current and future activities at PTB and BAM in order to develop improved metallic fixed-point cells of the ITS-90 are reviewed.
Solution-doped metal powder pellets as well as aspirated liquids were used as calibration samples to analyze pure copper and zinc certified reference materials (CRMs) by femtosecond laser ablation ICP-MS. It was demonstrated that calibration by copper pellets resulted in relative deviations up to 20%, whereas fs-LA-ICP-MS among copper-based CRMs led to inaccuracies in the same range unless nominal mass fractions were chosen to be <3 mg/kg. Calibration by zinc pellets generally provided better accuracy. Depending on the analyte considered, deviations below 10% were obtained even for mass fractions close to the limit of quantification. Our data, therefore, indicate solution-doped metal powder pellets to be suitable as calibration samples for fs-LA-ICP-MS of metals. Furthermore, the utilization of liquid standards for calibration was found to result in stronger deviations of up to 50% for both copper and zinc samples which, in addition, turned out to be dependent on the plasma conditions.
Determining the influence of impurities on the fixed-point emperatures
of the ITS-90 requires the completion of several tasks. In this paper, the Progress made at Physikalisch-Technische Bundesanstalt (PTB) and BAMFederal Institute for Materials Research and Testing is presented and remaining questions are discussed.
The projected characterization procedure at PTB, which is based on the established SIE method (sum of the individual estimates), using a new indium fixed-point cell is described as an example. This procedure includes an SI-traceable chemical Analysis of the material in the fixed-point cell with sufficiently low uncertainties, the individual experimental determination of the influence of the quantified impurities on the fixed-point temperature, and the establishment of direct links to the phase-transition temperatures of the national standard and of an assumed material of ideal purity.
A characteristic difference to the common practice is the chemical analysis of the fixed-point metal being done after determining the cells freezing temperature. This allows for the detection and consideration of contamination and purification effects due to the filling process, or due to the contact with the carbon crucible and other parts of the fixed-point cell. A chemical analysis of an indium fixed-point was carried out by BAM with relative measurement uncertainties below 30 % which have not been previously achieved. The results provide evidence for the precipitation of some impurities,
which is apparently inconsistent with the corresponding binary phase diagrams, but was explained in a recent publication. Implications for the use of the SIE method shall be described briefly at the end.
KEY COMPARISON
High purity elements can serve as a realization of the SI unit amount of substance for the specific element. Solutions prepared from high purity metals by applying gravimetric preparation and the concept of molar mass are used as 'calibration' solutions in many fields of analytical chemistry and provide the metrological basis in elemental analysis. Since ideal purity does not exist for real materials, the actual purity of the high purity material must be known with a specified uncertainty. As required uncertainties around 10-4 relative on the purity statement are not accessible in almost all cases by a direct measurement of the element in itself, the indirect approach is followed, where all elements excepting the matrix element itself are measured and their sum is subtracted from the value for ideal purity, which is 1 kg/kg.
It was the aim of this comparison to demonstrate the capability of national metrology institutes and designated institutes to determine the purity of pure elements. In order to limit the effort within this comparison, only six metallic impurities (Ag, Al, Cd, Cr, Ni, Tl) in the low mg/kg range are considered in a zinc matrix. It has to be underlined here that the task was to measure the purity of zinc based on the determination of six analytes. The task is not trace analysis of specific analytes in zinc. This subtle distinction defines different measurands.
The sample, pure Zn, was cut in pieces of cubic geometry for wet chemical analysis or of pin geometry for GDMS analysis and was sent to the participants. The comparison was run under the auspices of the Inorganic analysis Working Group (IAWG) of the CCQM and was piloted by the BAM Federal Institute for Materials Research and Testing, Berlin, Germany. The majority of the participants applied ICP-MS techniques and only two participants used additionally atomic absorption spectrometry. GDMS was used only by one participant.
The observed spreads for the measurement results reported by the participants were significantly lower than those of the preceding study CCQM-P107 and were well below the target uncertainty of 30% relative. As a consequence, comparability within the participating laboratories is demonstrated to be established. The individual measurement results, mean values and medians derived were in all cases very consistent with the reference values obtained by IDMS and so the accuracy of the measurement results for the participating laboratories is as well demonstrated to be established. Especially with the results of CCQM-P62 and CCQM-P107 in mind, the outcome of CCQM-K72 can be considered as a big step forward in the community.
CCQM is aware of the difference between a characterization based on only six analytes and a complete characterization. Therefore, the pilot study CCQM-P149 has been initiated and already started, which focuses on the fit-for-purpose approaches for the purity determination of metals (here: zinc) to be used as primary standards in elemental analysis. Another follow-up in the form of a pilot study on non-metal impurities is mandatory, because non-metal impurities such as oxygen, nitrogen and sulfur often make up the largest contributions.
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 the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).
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.
This key comparison aims to assess the capabilities of the participants to determine the amount-of-substance fraction oxygen in nitrogen. The GAWG has classified this as a track B comparison, due to the unexpected 50 μmol/mol argon mole fraction content of the transfer standards, which effects the achievable performance of some measurement techniques such a GC-TCD. The separation of oxygen and argon is challenging, and not all systems in use are equally well designed for it. As this analytical challenge due to a substantial fraction of argon in the transfer standards became a reality, the Gas Analysis Working Group (GAWG) decided to qualify this key comparison as a regular key comparison and not as a core comparison, which may be used to support calibration and measurement capabilities (CMCs) for oxygen in nitrogen, or for oxygen in nitrogen mixtures containing argon only (see also the section on support to CMCs).
This key comparison aims to assess the core capabilities of the participants in gas analysis. Such competences include, among others, the capabilities to prepare Primary Standard gas Mixtures (PSMs), perform the necessary purity analysis on the materials used in the gas mixture preparation, the verification of the composition of newly prepared PSMs against existing ones, and the capability of calibrating the composition of a gas mixture. According to the Strategy for Key Comparisons of the Gas Analysis Working Group, this key comparison is classified as an RMO track A key comparison.
The artefacts were binary mixtures of propane in nitrogen at a nominal amount-of-substance fraction level of 1000 μmol/mol. The values and uncertainties from the gravimetric gas mixture preparation were used as key comparison reference values (KCRVs). Each transfer standard had its own KCRV. The results are generally good. All results are within ± 1 % of the KCRV.