Chemische Charakterisierung und Spurenanalytik
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An interlaboratory comparison (ILC)was organised to characterise 87Sr/86Sr isotope ratios in geological and industrial reference materials by applying the so-called conventional method for determining 87Sr/86Sr isotope ratios. Four cements (VDZ 100a,VDZ 200a, VDZ 300a, IAG OPC-1), one limestone (IAG CGL ML-3) and one slate (IAG OU-6) reference materials were selected, covering a wide range of naturally occurring Sr isotopic signatures. Thirteen laboratories received aliquots of these six reference materials together with a detailed technical protocol. The consensus values for the six reference materials and their associated measurement uncertainties were obtained by applying a Gaussian, linear mixed effects model fitted to all the measurement results. By combining the consensus values and their uncertainties with an uncertainty contribution for potential heterogeneity, reference values ranging from 0.708134 mol mol-1 to 0.729778 mol mol-1 were obtained with relative expanded uncertainties of ≤ 0.007 %. This study represents an ILC on conventional 87Sr/86Sr isotope ratios, within which metrological principles were considered and the compatibility of measurement results obtained by MC-ICP-MS and by MC-TIMS is demonstrated. The materials characterised in this study can be used as reference materials for validation and quality control purposes and to estimate measurement uncertainties in conventional 87Sr/86Sr isotope ratio measurement.
Introduction: The influence of copper, iron and zinc concentrations on the formation of ß-amyloid plaques and neurofibrillary tangles in Alzheimer’s disease (AD) is widely discussed in the community. The results from human and animal studies so far are mixed with some studies showing a correlation and others not. From a number of studies, it is known that disease state and isotopic composition of essential elements can be coupled.
Aim: The aim of the study was to identify changes in element content and isotopic composition in two transgenic mouse models used in AD research compared to their genetic WT relatives and to establish whether element content and isotopic signature between different laboratories is comparable.
Methods: ß-amyloid (5xFAD) and tau overexpressing (L66) mice together with their matching wild-types were bred at dedicated facilities in accordance with the European Communities Council Directive (63/2010/EU). Serum and brain were sampled after sacrifice and the samples distributed among the participants of the study. The tissues were acid digested for total element determination and high-precision isotope ratio determination. Element content was determined by either sector-field or quadrupole-based inductively coupled plasma mass spectrometry (ICPMS). For the determination of isotope ratios multi-collector ICPMS was used.
Results: Total copper content was significantly higher for L66 and their matched WT compared to 5xFAD and WT. Brains of L66 mice contained more Fe in brain than their WT, Zn and Cu were not significantly different between L66 and WT. Whereas 5xFAD mice had a slightly lower Cu and slightly higher Zn concentration in brain compared to WT. The isotopic signature in brain of L66 mice for Fe was different from their controls, whereas Zn isotope ratios were influenced in 5xFAD mice compared to their WT. The Cu isotopic ratio did not seem to be influenced in either strain. In serum, the shifts were less pronounced.
Conclusion: Even though neither Tau-protein nor amyloid precursor protein are known to be metal-dependent / -containing proteins, the overexpression of both influences the Fe, Cu and Zn metabolism in brain and to some extent also in serum as can be seen not only using total element determination but probably more clearly studying the isotopic signature of Fe, Cu and Zn.
In most chemical reactions, stable isotopes are fractionated in a mass-dependent manner, yielding correlated isotope ratios in elements with three or more stable isotopes. The proportionality between isotope ratios is set by the triple isotope fractionation exponent θ that can be determined precisely for, e.g., sulfur and oxygen by IRMS, but not for metal(loid) elements due to the lower precision of MC-ICP-MS analysis and smaller isotopic variations. Here, using Mg as a test case, we compute a complete metrologically robust uncertainty budget for apparent θ values and, with reference to this, present a new measurement Approach that reduces uncertainty on θ values by 30%. This approach, namely, direct educt-product bracketing (sample−sample bracketing), allows apparent θ values of metal(loid) isotopes to be determined precisely enough to distinguish slopes in three-isotope space. For the example of Mg, we assess appropriate quality Control standards for interference-to-signal ratios and Report apparent θ values of carbonate−seawater pairs. We determined apparent θ values for marine biogenic carbonates, where the foraminifera Globorotalia menardii yields 0.514 ± 0.005 (2 SD), the coral Porites, 0.515 ± 0.006 (2 SD), and two specimens of the giant clam Tridacna gigas, 0.508 ± 0.007 (2 SD) and 0.509 ± 0.006 (2 SD), documenting differences in the uptake pathway of Mg among marine calcifiers. The capability to measure apparent θ values more precisely adds a new dimension to metal(loid) δ values, with the potential to allow us to resolve different modes of fractionation in industrial and natural processes.
Trace humidity was measured by using miniaturized planar coulometric sensors in technical gases such as hydrogen, nitrogen, helium, nitrous oxide, and synthetic air. Frost point temperatures tf in the gases ranged from −60 °C to −30 °C, which is equivalent to a vapour mole fraction xv from 10 μmol mol−1 to 376 μmol mol−1. In addition, the generated humidity was determined by using a precision dew point hygrometer as reference. Nonlinear calibration functions were calculated that correlated electric current (sensor signal) and reference humidity. Parameters of functions were tested with one-way analysis of variances (ANOVA) to prove if all used sensors had a similar behavior in the same gas during experiments. Results of ANOVA confirmed that averaged functions can be applied for trace humidity measurement in nitrogen, helium, nitrous oxide, and synthetic air. The calculated functions were negligibly different for nitrogen, helium, and synthetic air. In humidified nitrous oxide, a minor change of parameters was observed due to lower electrical currents. In total contrast to that, the measured sensor signals were significantly higher in humidified hydrogen and each sensor required its own calibration function. The reason was a recombination effect that favoured multiple measurements of water molecules. Nevertheless, it was possible to measure continuously trace humidity in all tested gases by using coulometric sensors with an expanded uncertainty below 2 K (k = 2).
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.
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.
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.
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.