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Eingeladener Vortrag
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Accurate and precise isotope ratio measurements of heavy elements are playing an increasinglyimportant role in modern analytical sciences and have numerous applications. Today, isotope ratio measurements are typically performed with two principal techniques: thermal ionization mass spectrometry (TIMS) and multiple collector-inductively coupled plasma mass spectrometry (MC-ICP-MS). To obtain accurate results by mass spectrometry, isotopic certified reference materials (iCRMs) are needed for mass bias correction and for the validation of the method used for analysis.Thus, it is of paramount importance to achieve measurement comparability of all data reported, and to assess measurement capability of each CRM producer/National Metrology Institute (NMI). Therefore, the international comparison (CCQM-P213) was performed to assess the analytical capabilities of NMIs for the accurate determination of copper isotope ratio delta values in high purity materials. The study was proposed by the coordinating laboratories, National Research Council Canada (NRC), National Institute of Standards and Technology (NIST), Bundesanstalt für Materialforschung und -prüfung (BAM) and Physikalisch-Technische Bundesanstalt (PTB), as an activity of the Isotope Ratio Working Group (IRWG) of the Consultative Committee for Amount of Substance - Metrology in Chemistry and Biology (CCQM). Participants included six NMIs and one designated institute (DI) from the six countries. Although no measurement method was prescribed by the coordinating laboratories, MC-ICP-MS with either standard-sample bracketing (SSB) or combined SSB with internal normalization (C-SSBIN) models for mass bias correction were recommended. Results obtained from the six NMIs and one DI were in good agreement.
Isotope amount ratios (hereafter referred to as simply isotope ratios) are proving useful in an ever increasing array of applications that range from studies unravelling transport processes, to pinpointing the provenance of specific samples as well as trace element quantification by using isotope dilution mass spectrometry (IDMS). These expanding applications encompass fields as diverse as archaeology, food chemistry, forensic science, geochemistry, medicine and metrology. However, to be effective tools, the isotope ratio data must be reliable and traceable to enable the comparability of measurement.
The importance of traceability and comparability in isotope ratio analysis has already been recognized by the Inorganic Analysis Working Group (IAWG) within the CCQM. Three pilot studies have focused on the quality of isotope ratio determinations (P48 “U isotope ratios in urine”, P75 “stable isotopes in Methionine”, P105 “87Sr/86Sr in wine”). Moreover, isotope ratio measurements are fundamental to IDMS amount of substance determinations. For example, when Pb quantification using IDMS is undertaken, this requires the measurements of Pb isotope ratios. While the requirements for isotope ratio accuracy and precision in the case of of IDMS are generally quite modest, “absolute” Pb isotope ratio measurements for geochemical age dating and source rock characterization as well as forensic provenance and fingerprinting studies require Pb isotope ratio measurements of the highest quality. To support present and future CMCs on isotope ratio determinations, a Key Comparison was urgently needed. Therefore, it was decided at the IAWG meeting in Paris in April 2011 that a Key Comparison on the determination of Pb isotope ratios in a pure Pb solution and in a bronze sample should be organized and accompanied by a pilot study.
Measuring Pb isotope amount ratios in a pure Pb solution, while seemingly straight forward, rigorously tests the ability of analyst to correct for any instrumental effects (such as mass discrimination and blank correction) on the measured ratios. Pb, present in trace amounts in a metal matrix sample (e.g. Pb in bronze), provides a real world test of the whole chemical and instrumental procedure, from chemical separation and sample purification to analysis and subsequent correction of appropriate instrumental effects on the separated samples.
A suitable bronze material with a Pb mass fraction between 10 and 100 mg·kg-1 was available at BAM. A high purity solution of Pb with a mass fraction of approximately 100 mg·kg-1 was also available. By comparing the Pb isotope ratio results obtained for the bronze sample with the Pb isotope ratio results from the Pb solution, potential biases arising from the processing of the bronze sample could be effectively identified and separated from the instrumental effects arising from the measurement and data processing protocol.
Blei weist in der Natur die vier stabilen Isotope 204Pb, 206Pb, 207Pb und 208Pb auf, mit den entsprechenden Isotopenhäufigkeiten von 1,4%, 24,1 %, 22,1 % und 52,4 %686. Dabei sind 206Pb, 207Pb und 208Pb radiogene Isotope, welche als Endglieder der natürlichen Zerfallsreihen von 238U, 235U und 232Th entstanden sind. 204Pb ist ein sogenanntes primordiales Isotop, welches bereits vor Entstehung der Erde vorhanden war. Aufgrund dieser Prozesse weisen Gesteine, Mineralien und Erze eine charakteristische Bleiisotopenzusammensetzung auf, die von der Ausgangskonzentration an Blei, Thorium und Uran sowie von der seit der Gesteinsgenese vergangenen Zeit abhängt. Diese Eigenschaft bleibt auch während der Bleigewinnung aus dem Erz, meist Galenit, erhalten, auch wenn das Silber vorher durch Kupellation abgetrennt wurde687. Das in Silbererzen enthaltene Blei war ein willkommenes Nebenprodukt der Erzverhüttung und wurde zur Herstellung von Bleiobjekten verwendet. Artefakte, die aus dem so gewonnenen Blei gefertigt wurden, spiegeln daher die Bleiisotopenzusammensetzung der verwendeten Rohmaterialien wider.
Dieser Bericht beschreibt die Herstellung und Zertifizierung einer Cadmiumlösung mit natürlicher Isotopenzusammensetzung. Für das Isotopen-Referenzmaterial sind sowohl die Isotopenverhältnisse als auch Isotopenhäufigkeiten und die molare Masse von Cadmium zertifiziert. Die Unsicherheiten sind erweiterte Messunsicherheiten U = k.uc mit k = 2. Der Cadmium-Massenanteil in der Lösung wird als indikativer Wert angegeben mit einer erweiterten Messunsicherheit U = k.uc mit k = 4,5.
Dieser Bericht beschreibt die Zertifizierung einer synthetischen Isotopenmischung aus natürlicher und angereicherter Borsäure. Dieses Isotopen-ZRM liegt als konzentrierte, wässerige Borsäure-Lösung vor und ist bezüglich ihrer Isotopenzusammensetzung zertifiziert. Der Bor-Gesamtgehalt ist als Richtwert angegeben. Die Unsicherheiten sind erweiterte Messunsicherheiten mit k=2. Sie sind in Klammern angegeben und beziehen sich auf die letzten beiden Stellen. Das vorliegende Isotopen-ZRM ist bezüglich der Isotopenzusammensetzung mit erweiterten relativen Messunsicherheiten kleiner 0,08% zertifiziert. Die absolute, kombinierte Standardmessunsicherheit des prozentualen Stoffmengenanteils der Isotope (Isotopenhäufigkeit) liegt bei 0,014. In diesem Zertifizierungsbericht wird nur auf die Kapitel eingegangen, die abweichend vom bestehenden Zertifizierungsreport für die Referenzmaterialien BAM I001 bis BAM I003 sind und Einfluss auf die Endergebnisse haben. Dabei handelt es sich um die Kapitel 3.3, 3.4, 4, 6, 7 und 8, Die Kapitel 1, 2, 3, 3.1, 3.2, 5, 9 und 10 des alten Zertifizierungsberichts für die Referenzmaterialien BAM I001 bis BAM I003 behalten unverändert ihre Gültigkeit und sind nicht noch einmal aufgeführt worden.
Isotope reference materials
(2022)
The variation of isotope ratios is increasingly used to unravel natural and technical questions. In the past, the investigation and interpretation of such variations was the field of a limited number of experts. With new upcoming techniques and research topics in the last decades, such as provenance or food authenticity studies, the number of published isotope data strongly increased. Instrumental developments such as the enhancement of inductively coupled plasma mass spectrometers (ICP-MS) from an instrument for simple quantitative analysis to highly sophisticated isotope ratio machines influenced this process significantly. While in former times only experts in mass spectrometry were able to produce reliable isotope data, nowadays many laboratories, never been in touch with mass spectrometry before, produce isotope data with an ICP-MS. Isotope reference materials (iCRM) are indispensable to enable a reliable method validation or in rare cases even SI-traceability. The fast development and the broad availability of ICP-MS also lead to an expansion of the classical research areas and new elements are under investigation. Irrespective of the investigated element or the knowledge of the user all isotope ratio applications require reference materials either for correction of instrumental isotope fractionation, for method validation or to provide a common accepted basis as needed for delta measurements. This presentation will outline the basic principles and illustrate the urgent need for new iCRMs. Consequently, the production and certification of iCRMs will be discussed and illustrated by examples of already completed certification projects. Finally, plans for future iCRMs to be produced at BAM will be presented.
HIIRM final report
(2002)
Accurate analytical results have become more and more an absolute essential tool for further progresses in technology and science. Precision however, often used in this context as a quality criterion for analytical methods, is not a replacement for accuracy in any way. Therefore, analytical procedures are necessary, which will generate reliable and accurate results and can be used for evaluation of other analytical procedures and certification of reference materials (RM). Especially the certification of reference materials for the amount content of trace elements requires highly accurate results with a small combined uncertainty. The best example for an analytical procedure having this capability is undoubtedly Isotope Dilution Mass Spectrometry, for inorganic as well as for organic applications. Applied on Thermal Ionization Mass Spectrometry (TIMS) isotope dilution provides results of highest quality and proven high accuracy especially in the field of RM certifications for more than 30 years.
The major drawback of this approach often is the necessity for a complex chemical separation step. A major advantage of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is the potential to cope with a simplified or an on-line-carried-out sample preparation. A key requirement of isotope dilution analysis however is the accurate and precise determination of isotope ratios, because of its limiting factor for the accuracy and the total uncertainty of the result. Very precise isotope ratios can be determined in combination with a detection system that is capable of simultaneous detection of different isotopes, a so-called multi-collector system.
The objective of the presented project is to evaluate the advantages of such a multi-collector ICP-MS for the application of IDMS in the field of trace elemental analysis in reference materials. The focus is on the development of simple and fast procedures for isotope ratio measurements in general and IDMS analysis in particular. Furthermore, it is an objective to develop and evaluate on-line spiking procedures.
One of the first conclusions, which had a severe impact on the HIIRM project, arose during the early days of the project. Currently most certification experiments are accomplished by certification campaign based on a variety of participants’ results. The sample preparation, sending, analyzing and data evaluation however require lot of resources and the process may take a long time. An alternative way is given by using fewer participants with methods of higher metrological quality. For elemental amount contents such a method undoubtedly is isotope dilution analysis and multi-collector ICP-MS offers the necessary precision and matrix robustness and ruggedness as said above. Therefore, a multiple collector ICP-MS was manufactured as part of the project instead of the originally planned high-resolution instrument. Due to the novelty of the multicollector ICP-MS instrument, a lot of additional fundamental research became necessary. Especially interferences needed further attention. Fortunately, the IsoProbe is additionally equipped with a high-resolution option and with a brand new collison cell. The high-resolution option however can only be used to identify interferences but cannot be used to quantify them. On the other hand, the use of the collision cell proved to be a vital tool to overcome interferences. This is especially true because high resolution would have reduced the available sensitivity by a factor of ~100. Applying the collision cell technology the high sensitivity was maintained whilst most interferences were eliminated. However finding the right parameters of operation for the collision cell became a huge part of the method development process. A variety of gases like argon, helium, hydrogen, deuterium, nitrogen, krypton and xenon were tested for their suitability. Especially the tests accomplished for iron determinations revealed that carefully selected parameters have an incredible influence on the quality of the final results. Nonetheless collision cell technology will undoubtedly be widely used for newer ICP-MS instrumentation and contribute to eliminate wrong analytical values caused by interferences.
The enhanced sensitivity of the instrument revealed also problems not detectable by other means. The control of contaminations seems to be one of the major analytical challenges in the future. Sector field based ICP-MS will give the user an opportunity to trace sources of contamination back to their origin and eliminate them. Even lower limits of detection will be the result as well as an enhanced reliability for analysts dealing with higher amount contents. The analyst will also benefit in controlling blank levels when the method of isotopically spiked procedure blanks is used as demonstrated in this project.
Another main part of the project was the validation of the developed procedures. The first validation step was successfully accomplished by applying these procedures on the standard solutions provided by NRG. The main part of this validation however consisted of the evaluation of the fundamental parameters of the isotope dilution process and related measurements. This approach turned out to be superior compaired to the first step as a highly reliable uncertainty calculation can be easily performed. The best reputation and acceptance of the methods can however only be accomplished by participating in certification campaigns or in interlaboratory comparisons on highest metrological level. The results of the participations in general were brilliant. Not only the results but also the obtained realistic uncertainties were of superior quality. Direct comparisons at BAM with the results obtained by a multicollector TIMS applying the same calculation schemes showed the benefits of using multicollector ICP-MS. The main benefits of ICP-MS were identified, of which the first one is the enhanced precision of the isotope ratio measurement. For a variety of elements the values obtained by multicollector ICP-MS are even more precise than the ones obtainable by multicollector TIMS. The second important advantage identified was the simplified sample preparation. The laborious and time consuming analyte-matrix-separation step as necessary for TIMS measurements can either be omitted or at least be drastically reduced. Therefore, the advantages regarding time and enhanced sample throughput will result in further spread of ICP-MS in general and multicollector ICP-MS in particular.
The third major part in terms of the method development process was mainly focused on developing an online isotope dilution system. The volumetric instead of the gravimetric IDMS approach proved difficult, if the high demands necessary for RM definition measurements are to be fulfilled. The major obstacle in this case is the missing stability and precision in terms of mass flow. Consequently, the initial development failed as the system made up of two piezoelectric droplet injectors was unable to reach the required stability as well as precision. The thereupon-created system consisting of two HPLC pumps proved to be far more valuable regarding automation and particularly the quantification of transient signals. Nevertheless, such a system will probably be rarely used in certification campaigns, as the gain of time is too small compared to the loss of reliability. However, this system proved most valuable in terms of species-specific elemental analysis as preliminary investigations showed. In this context this system will be suitable for certification measurements, as the major uncertainty contributions derive from sampling, sample treatment and species distribution and as moreover all availabel methods demonstrate far beyond.
During the whole project, one of the main concerns regarding the multicollector ICP-MS instrument were software issues caused by the early development state of the original control programs. A lot of effort was necessary to accomplish the necessary data manipulations externally. The development of a new software suite by Micromass for the HIIRM project has solved this issue almost completely. With the new software suite and the validated parameters of measurement a step forward for institutes dealing with reference material certifications was achieved.
Future certifications campaigns for minor elemental contents in different matrices should be performed by a small number of participants from highly qualified institutes. These institutes should apply very reliable methods of measurement like the ones developed in the HIIRM project. Many resources may be saved that way while the outcome of the certifications may easily be improved. ICP-MC-MS has the potential to be an important method in this context. Stable isotope dilution analysis in combination with a multi-detector ICP-MS, equipped with a hexapole collision cell for the suppression of important spectral interferences and for enhanced sensitivity, proved to be an advanced method of elemental analysis with a high potential for matrix independent measurements. Since ICP mass spectrometers of this type have only recently been introduced, no systematic evaluation of the capabilities of this specific application of the ICP-IDMS method has yet been made.