Chemische Charakterisierung und Spurenanalytik
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Polyethylene (PE) frits were used to quantify sulphur in copper and its alloys by isotope dilution combined with LA-ICP-MS as an alternative approach to conventional sample preparation: the copper samples were spiked, the spiked samples were dissolved, the resulting solutions were absorbed in the PE frits and finally the PE frits were analysed by LA-ICP-MS. A prerequisite for such a support material is a low sulphur blank and thus PE was selected for this purpose. The absorption efficiency of the PE frits was studied for varying sulphur amounts ranging from 2 mg S to 80 mg S showing that more than 99.5% of the loaded sulphur was absorbed by the frit. The so prepared PE frits were measured by LA-ICP-MS and yielded a good linearity (R2 ¼ 0.999) for the sulphur ion intensities corresponding to sulphur amounts up to 40 mg S; the associated sensitivity is approximately 3.4 x 10⁴ cps μg⁻¹ for ³²S. For the validation of the developed procedure the reference materials BAM-M376a, BAM-228 and BAM-227 were applied such that 2 μg S, 5 μg S and 11 μg S were absorbed in the PE frits, respectively. These samples were pre-quantified for the adsorbed sulphur amount by external calibration LA-ICP-MS yielding sulphur amounts of 0.9 μg, 5.1 μg and 8.5 μg (quantified for ³²S only), respectively. Relative Standard deviations of the isotope ratios were below 5% in average (n ¼ 3 lines) in all cases (except for the pure spike solution). These samples were then analysed by LA-ICP-IDMS and the measurement results were validated by comparing them with the results obtained by conventional ICP-IDMS. The obtained relative expanded measurement uncertainties ranged between 10% and 26%. Pearson's coefficient was used to express the correlation between both techniques; the obtained value was 0.999 demonstrating a strong correlation. Contrary to most published LA-ICP-IDMS procedures, the developed procedure enables SI-traceability for the measurement results. The metrological traceability to the SI for the sulphur mass fractions in copper was established by an unbroken chain of comparisons, each accompanied by an uncertainty budget. Thus, the measurement results are considered reliable, acceptable and comparable within the stated measurement uncertainty. The metrological traceability chain from the kg down to mass fraction in the samples obtained by LA-ICP-IDMS is presented as well.
Previously applied methods for the quantification of sulphur in copper and other pure metals revealed a lack of SI-traceability and additionally showed inconsistent results, when different methods were compared.
Therefore, a reference procedure is required which allows SI-traceable values accompanied by a Sound uncertainty budget. In this study a procedure was developed for the quantification of total sulphur in copper at low concentration levels using inductively coupled plasma-isotope dilution mass spectrometry (ICP-IDMS). The major part of the copper matrix was separated by adding ammonia which forms a complex with the copper while releasing the sulphur followed by chromatographic separation using a weak cation resin. After that the sulphur fraction was further purified by chromatographic means using first an anion exchange method and second a chelating resin. The developed procedure shows high performance, especially concerning high efficiency in matrix removal (>99.999%) while keeping the recovery of sulphur above 80%. Procedure blanks are in the order of 3–53 ng resulting in LOD and LOQ values of 0.2 mg g1 and 0.54 mg g1, respectively. The procedure is sufficient to facilitate value assignment of the total sulphur mass fraction in reference materials. Additionally, relative measurement uncertainties were calculated to be below 1% and the measurement results were traceable to the SI. The procedure reported in this study is a new reference procedure for sulphur measurement in copper, being fit for two major purposes, certification of reference materials and assignment of reference values for inter-laboratory comparison.
For the first time, an international comparison was conducted on the determination of the purity of a high purity element. Participants were free to choose any analytical approach appropriate for their institute’s applications and services. The material tested was a high purity zinc, which had earlier been assessed for homogeneity and previously used in CCQM-K72 for the determination of six defined metallic impurities. Either a direct metal assay of the Zn mass fraction was undertaken by EDTA titrimetry, or an indirect approach was used wherein all impurities, or at least the major ones, were determined and their sum subtracted from ideal purity of 100 %, or 1 kg/kg. Impurity assessment techniques included glow discharge mass spectrometry, inductively coupled plasma mass spectrometry and carrier gas hot extraction/combustion analysis. Up to 91 elemental impurities covering metals, non-metals and semi-metals/metalloids were quantified. Due to the lack of internal experience or experimental capabilities, some participants contracted external laboratories for specific analytical tasks, mainly for the analysis of non-metals. The reported purity, expressed as zinc mass fraction in the high purity zinc material, showed excellent agreement for all participants, with a relative standard deviation of 0.011 %. The calculated reference value, w(Zn) = 0.999 873 kg/kg, was assigned an asymmetric combined uncertainty of + 0.000025 kg/kg and – 0.000028 kg/kg. Comparability amongst participating metrology institutes is thus demonstrated for the purity determination of high purity metals which have no particular difficulties with their decomposition / dissolution process when solution-based analytical methods are used, or which do not have specific difficulties when direct analysis approaches are used. Nevertheless, further development is required in terms of uncertainty assessment, quantification of non-metals and the determination of purity of less pure elements and/or for those elements suffering difficulties with the decomposition process.
Sulfur is one of the major impurity elements in copper. Previously applied methods for the quantification of sulfur in copper and other pure metals revealed a lack of traceability and showed inconsistent result. Therefore, in this study a procedure was developed for the quantification of total sulfur in copper at low concentration levels using inductively coupled plasma-isotope dilution mass spectrometry (ICP-IDMS). A major challenge for the quantification of sulfur in copper (alloyed/unalloyed) by ICPMS is the copper matrix itself, causing matrix effects and making an extensive cleaning (cones, extraction lens) necessary after measurements. Matschat et al investigated the analysis of high-purity metals (including copper) by high resolution ICP-MS and found that the copper matrix shows strong matrix effects on the sensitivity resulting from Cu deposition on the cones. Therefore, the major part of the copper matrix has to be separated, which was accomplished by adding ammonia which forms a complex with the copper while releasing the sulfur. This was followed by a chromatographic separation using a weak cation resin. After that the sulfur fraction was further purified by chromatographic means using an anion exchange method followed by a chelating resin.
The anion exchange resin (AG1X8), however, is selective to sulfate and sulfite but less-selective to sulfide. Therefore, when quantifying total sulfur in copper, the different species of sulfur need to be oxidized to sulfate prior to the sulfur-matrix separation on the AG1X8 resin in order to avoid any measurement bias. When applying the HPA oxidation with concentrated HNO3 and H2O2 a complete conversion from sulfide and sulfite to sulfate could be achieved. The recovery of all investigated sulfur species is quantitative within measurement uncertainties. The copper samples investigated in this study contain copper in the range of 0.85-0.99 kg·kg-1 and zinc from <10 to 300 g·kg-1. Approximately 0.10-0.25 g of these samples were used to perform the sulfur-copper separation. After applying the complete three stage separation procedure the mass fractions of both elements were significantly reduced to below 400 ng·g-1 for copper and below 50 ng·g-1 for zinc, respectively. The developed procedure shows high performance, especially concerning high efficiency in matrix removal (> 99.999%) while keeping the recovery of sulfur above 80%.
The procedure blank was determined by IDMS as well and yielded values for the individual IDMS measurement sequences ranging from 3 ng to 53 ng. The average of these individual procedure blanks (n=22) was calculated and yielded a total procedure blank of 14 ng sulphur with standard deviation of 12 ng. The limit of detection (LOD, blank+3SD) calculated on this basis was 0.20 µg·g-1 while the limit of quantification (LOQ, blank+10SD) was 0.54 µg·g-1, when considering a sample weight of 0.25 g.
The quantification of low sulfur contents (< 15 µg/g) by conventional IDMS is hindered by the very high Cu/S ratio, which clearly affects the separation in a negative way: The recovery of sulfur dropped to about 30 % for four replicates, while two further replicates even showed recoveries below 10%. To enable measurement without completely changing the separation procedure, an exact amount of sulfur was added prior to spiking, such that the sulfur mass fraction was shifted to the optimum working range of the separation procedure. Thus exact amounts of sulfur were added to enhance the mass fraction of sulfur from 15 µg·g-1 to 40 µg·g-1, then the IDMS analysis was performed as usual and finally the added sulfur amount was subtracted. The so obtained measurement result agreed well with the certified value within the uncertainties. The relative expanded measurement uncertainties for conventional IDMS are below 1%. When applying the modified IDMS procedure, where back-spike is added to the sample before spiking, the relative expanded measurement uncertainties are larger and up to 5%. With the presented sulfur-matrix procedure a working range from approximately 15 µg·g-1 to 1500 µg·g-1 can be achieved.
The developed procedure for the quantification of low sulfur amounts in copper has been validated here via three different routes: first an inter-laboratory comparison at highest metrological level, second a step-by-step validation by checking each single step of the procedure and third the setup of a complete uncertainty budget.
The procedure is sufficient to facilitate value assignment of total sulfur mass fraction in reference materials. Additionally, relative measurement uncertainties were calculated below 1 % and the measurement results are traceable to the SI, which is clearly demonstrated in this work. The procedure reported in this study is a new reference procedure for sulfur measurement in copper, well meeting the requirements of the two major purposes: the certification of reference materials and the assignment of reference values for inter-laboratory comparison.
Sulfur is one of the major impurity elements in copper. Previously applied methods for the quantification of sulfur in copper and other pure metals revealed a lack of traceability and showed inconsistent result. Therefore, in this study a procedure was developed for the quantification of total sulfur in copper at low concentration levels using inductively coupled plasma-isotope dilution mass spectrometry (ICP-IDMS). A major challenge for the quantification of sulfur in copper (alloyed/unalloyed) by ICPMS is the copper matrix itself, causing matrix effects and making an extensive cleaning (cones, extraction lens) necessary after measurements. Matschat et al investigated the analysis of high-purity metals (including copper) by high resolution ICP-MS and found that the copper matrix shows strong matrix effects on the sensitivity resulting from Cu deposition on the cones. Therefore, the major part of the copper matrix has to be separated, which was accomplished by adding ammonia which forms a complex with the copper while releasing the sulfur. This was followed by a chromatographic separation using a weak cation resin. After that the sulfur fraction was further purified by chromatographic means using an anion exchange method followed by a chelating resin.
The anion exchange resin (AG1X8), however, is selective to sulfate and sulfite but less-selective to sulfide. Therefore, when quantifying total sulfur in copper, the different species of sulfur need to be oxidized to sulfate prior to the sulfur-matrix separation on the AG1X8 resin in order to avoid any measurement bias. When applying the HPA oxidation with concentrated HNO3 and H2O2 a complete conversion from sulfide and sulfite to sulfate could be achieved. The recovery of all investigated sulfur species is quantitative within measurement uncertainties. The copper samples investigated in this study contain copper in the range of 0.85-0.99 kg·kg-1 and zinc from <10 to 300 g·kg-1. Approximately 0.10-0.25 g of these samples were used to perform the sulfur-copper separation. After applying the complete three stage separation procedure the mass fractions of both elements were significantly reduced to below 400 ng·g-1 for copper and below 50 ng·g-1 for zinc, respectively. The developed procedure shows high performance, especially concerning high efficiency in matrix removal (> 99.999%) while keeping the recovery of sulfur above 80%.
The procedure blank was determined by IDMS as well and yielded values for the individual IDMS measurement sequences ranging from 3 ng to 53 ng. The average of these individual procedure blanks (n=22) was calculated and yielded a total procedure blank of 14 ng sulphur with standard deviation of 12 ng. The limit of detection (LOD, blank+3SD) calculated on this basis was 0.20 µg·g-1 while the limit of quantification (LOQ, blank+10SD) was 0.54 µg·g-1, when considering a sample weight of 0.25 g.
The quantification of low sulfur contents (< 15 µg/g) by conventional IDMS is hindered by the very high Cu/S ratio, which clearly affects the separation in a negative way: The recovery of sulfur dropped to about 30 % for four replicates, while two further replicates even showed recoveries below 10%. To enable measurement without completely changing the separation procedure, an exact amount of sulfur was added prior to spiking, such that the sulfur mass fraction was shifted to the optimum working range of the separation procedure. Thus exact amounts of sulfur were added to enhance the mass fraction of sulfur from 15 µg·g-1 to 40 µg·g-1, then the IDMS analysis was performed as usual and finally the added sulfur amount was subtracted. The so obtained measurement result agreed well with the certified value within the uncertainties. The relative expanded measurement uncertainties for conventional IDMS are below 1%. When applying the modified IDMS procedure, where back-spike is added to the sample before spiking, the relative expanded measurement uncertainties are larger and up to 5%. With the presented sulfur-matrix procedure a working range from approximately 15 µg·g-1 to 1500 µg·g-1 can be achieved.
The developed procedure for the quantification of low sulfur amounts in copper has been validated here via three different routes: first an inter-laboratory comparison at highest metrological level, second a step-by-step validation by checking each single step of the procedure and third the setup of a complete uncertainty budget.
The procedure is sufficient to facilitate value assignment of total sulfur mass fraction in reference materials. Additionally, relative measurement uncertainties were calculated below 1 % and the measurement results are traceable to the SI, which is clearly demonstrated in this work. The procedure reported in this study is a new reference procedure for sulfur measurement in copper, well meeting the requirements of the two major purposes: the certification of reference materials and the assignment of reference values for inter-laboratory comparison.
In chemical elements with three or more stable isotopes, mass-dependent stable isotope fractionation is expressed by co-varying isotope ratios. In the three-isotope space ((δ’m2/δ’m1)/(( δ’m3/δ’m1)) these plot along a line with a slope (β), the so called ‘terrestrial fractionation line’. This partitioning of stable isotopes results from both kinetic and equilibrium reactions that are characterized by specific β-values.
For the natural range of isotope ratios of ‘novel’ stable isotope systems such as Si, Mg, Fe, Zn, Cu this information cannot be accessed because samples fall close to the delta-zero standard where the current measurement precision is too low to resolve small differences in β. We present a new approach to resolve deviations from a reference slope β by standard-sample bracketing against material offset from the natural range. We use this approach to explore the isotope fractionation mechanism in the mammalian food web. We have analyzed Mg stable isotope ratios in bone bioapatite of herbivore, omnivore and carnivore mammals. Positive shifts in δ26/24Mg along the trophic chain (from herbivore to carnivore) together with β= 0.513 suggest the presence of two isotope fractionation mechanisms operating during biomineralization. While positive shifts in δ26/24Mg are in favor of equilibrium isotope fractionation process, the proximity of β to the theoretically calculated β(kinetic) of typically 0.511 suggests the presence of a second component that fractionates stable isotopes kinetically. The herein presented approach is applicable to any element with 3+ stable isotopes analyzed by multi-collector inductively coupled plasma mass spectrometry.
For the first time polyethylene (PE) frits were used to quantify sulphur in copper metal and its alloys by isotope dilution combined with LA-ICP-MS: an alternative approach for sample preparation. The properties of the PE frit meet the requirements for isotope dilution LA-ICPMS which are porous material, thermal and chemical resistance and high absorption efficiency. The breakthrough, however, as a support material, is the low sulphur blank, which is only two times of the gas flow blank (2.3-4.0 x 10⁴ cps). Additionally, the porosity of the frit was considered, as it directly affects the absorption efficiency for the sample solution, which is present in the cavities of the frit.
The absorption efficiency was studied by loading sulphur standards with varying sulphur amounts (0 - 80 µg S) onto the frits. The remaining sulphur which was not absorbed by the frit was rinsed off and was measured by ICP-MS. This indirect method shows that more than 99.5 % of the loaded sulphur was absorbed by the frit. The so prepared frits with increasing sulphur amount were measured by LA-ICP-MS showing a good linearity between 0 µg S and 40 µg S with a coefficient of determination, r2 of 0.9987 and a sensitivity of 3.4x10⁴ cps µgˉ¹ for 32S.
Three copper reference materials produced by BAM (BAM-M376a, BAM-228 and BAM-227) were selected to develop and validate the LA-ICP-IDMS procedure. The IDMS technique was applied to these samples as follows: the samples were spiked, dissolved, digested and then the digested solution was absorbed on the frits. The dried frit samples were then analyzed by LA-ICP-IDMS and it could be demonstrated that the sample solution dispersed on the frits did not influence the 32S/34S ratio significantly even though the sulphur intensities were fluctuating along the scan lines. Relative standard deviations of the isotope ratios were below 5 % in average between 3 lines (except for the pure spike solution and procedure blank). The measurement results were validated by comparing them with the results obtained by conventional ICP-IDMS. Plotting the mass fraction of sulphur in copper obtained by LA-ICP-IDMS versus those obtained by ICP-IDMS yields a linear curve with a correlation coefficient of 0.9999 showing a strong agreement between both techniques.
The metrological traceability to the SI for the mass fraction of sulphur in copper is established by an unbroken chain of comparisons, each accompanied by an uncertainty budget. Thus, the measurement results are considered reliable, acceptable and comparable within the stated measurement uncertainty. The metrological traceability chain from the kg down to the final mass fraction in the samples obtained by LA-ICP-IDMS is illustrated in this presentation.
This is the first time that PE frits were used to quantify sulfur in copper and its alloys by isotope dilution combined with LA-ICP-MS: an alternative approach for sample preparation. The following properties of the PE frit meet the requirements for isotope dilution LA-ICPMS:: porous material, thermo plastic (melting point >100oC), chemical resistance (nitric acid >70%) and high adsorption efficiency. The breakthrough, however, as a support material, especially when comparing the PE frit with other materials such gelatin or sodium silicate is the low blank, which is only two times of the gas flow blank (2.3-4.0 x 104 cps). Additionally, the porosity of the frit was considered, as it directly affects the adsorption efficiency for the sample solution, which is present in the cavities of the frit.
Adsorption efficiency was studied by depositing sulfur standards with varying sulfur amounts (0, 2, 5, 10, 20, 40 and 80 µg S) on the frits. The remaining sulfur which was not absorbed by the frit was rinsed off and was measured by ICP-MS. This indirect method shows that more than 99.5 % of the loaded sulfur was absorbed by the frit. Such high absorption efficiency is completely sufficient for a support material to be used in LA-ICP-IDMS. The so prepared frits whith increasing sulfur amount were measured by LA-ICP-MS showing a good linearity between 0 µg S and 40 µg S whit a correlation coefficient r2 of 0.9987 and sensitivy of 3.4x104 cpsµg-1 for 32S.
Three copper reference materials produced by BAM were selected to develop and validate the LA-ICP-IDMS procedure. The IDMS technique was applied to these samples as follows: the samples were spiked, dissolved, digested and then the digest was adsorbed on the frits.
A set of 59 ancient magical artefacts, mainly made of lead, was selected from the collections of the Staatliche Museen zu Berlin in order to unravel their origins. All the selected artefacts have been studied for their Pb isotope compositions, which covered the whole range of the Mediterranean ore deposits. However, the majority (≈86%) were made of lead matching the small compositional range of the Laurion ore deposits. Only eight out of the 59 artefacts were made of recycled lead or lead from other ore deposits.
Additionally, all but two were approximately dated based on their inscriptions. The lead isotopic composition together with information obtained from the inscriptions, the resulting dating, the context of the find and the known history of each item allowed us to gain more detailed information about the origins of these magical artefacts. The Attic provenance of 36 curse tablets was confirmed, whereas for 11 curse tablets previously classified as non-Attic, the provenance was either confirmed and specified (six artefacts) or changed to Attic (five artefacts). Surprisingly, the majority (six out of eight) of the analysed curse tablets from the Egyptian collection showed a lead isotopic composition closely matching that of Laurion.
A Laurion-like lead isotopic composition was also observed for three of the four analysed oracular tablets from Dodona. Together with the dating information, this points to Laurion as the major and dominant lead source in the Aegean, at least during the fourth–third century B.C. The few curse tablets from earlier than the fourth–third century B.C. point to the use of multiple and thus isotopically more variable lead sources compared with the Roman times.
Small variations in the isotopic composition of some elements have been used as proof of provenance of mineral and biological samples, to describe geological processes, and to estimate a contamination source. Routinely, isotope compositions are measured by mass spectrometry; the working horse of the isotope analysis. However, mass spectrometric methods are expensive, time-consuming and they require a high qualified analyst.
Here, an alternative faster and low-cost optical method for isotope ratio determination is investigated: high-resolution continuum source molecular absorption spectrometry (HR-CS-MAS). Stable isotope amount composition of X = Li, B, Mg, Ca and Sr have been determined by monitoring the absorption spectra of their in situ generated hydrides (XH) in graphite furnace HR-CS-MAS. For example, the system of two stable isotopes of boron (10B and 11B) was studied via its hydride for the electronic transition X1Σ+ → A1Π (Fig. 1a). The spectrum of a given sample is a linear combination of the 10BH molecule and its isotopologue 11BH. Therefore, isotopic composition of samples and reference materials are calculated by a partial least square regression (PLS). For this, a spectral library is built by using samples with known isotope composition. Results are metrologically compatible with those reported by mass spectrometric methods. [1] Similar results are obtained for n isotope systems like Mg (24Mg, 25Mg, and 26Mg), where isotope shift of their isotopologues can be resolved as shown in Fig.1 b. The extension of this methodology to other elements like Li, Ca and Sr is discussed [2].
References:
[1] C. Abad, S. Florek, H. Becker-Ross, M.-D. Huang, H.-J. Heinrich, S. Recknagel, J. Vogl, N. Jakubowski, U. Panne, Determination of boron isotope ratios by high-resolution continuum source molecular absorption spectrometry using graphite furnace vaporizers, Spectrochim. Acta, Part B, 136 (2017) 116-122.
[2] C. Abad et al., unpublished results, 2018.