Investigation of mass-scale drift effects in the milli-mass range: Influence on high mass resolution mode multicollector-inductively coupled plasma mass spectrometer isotope ratio measurements.

The consequences, possible origins, and prevention of mass-scale drifts in the high mass resolution mode (HR, M/ΔM ≈ 8000) under constant conditions were investigated and simulated in case of a multicollector-inductively coupled plasma mass spectrometer (MC-ICP-MS) using silicon enriched in 28 Si as the main element in this survey. A drifting mass scale strongly impairs the precise and accurate determination of isotope ratios, depending especially on the peak/plateau width. For example, 29 Si+ in Si highly enriched in 28 Si has an extremely small mass plateau width of ΔM ≤ 4 × 10-3 u, compare ΔM(56 Fe+ ) ≈ 18 × 10-3 u, which is to our knowledge one of the smallest plateaus routinely investigated in isotope ratio measurements, thus requiring extreme stability. During warm-up of the double-focusing sector field mass spectrometer, a mass drift up to ΔM/Δt ≥ 0.006 u/hr has been observed. Long-term studies on mass scale stability and simulations concerning fluctuations of the magnetic field B, acceleration voltage Uacc and ESA voltage UESA are reported. A change of one of these quantities of 0.01% induces changes of the mass scale of 6 × 10-3 u, 3 × 10-3 u, and 1 × 10-3 u in the case of B, Uacc , and UESA , respectively. After identifying electrical charging/discharging effects in the mass spectrometer affecting the mass scale stability, the instrument was completely dismantled and carefully reinstalled. Additional stability tests using silicon, strontium, and lead finally yielded a mass drift of ΔM/Δt ≤ 0.001 u/8 h in the case of silicon. This enhanced stability guarantees measurements of isotope ratios on smallest plateaus with lowest uncertainty. The importance of a stable mass scale is pointed out and the relevant quantities of a typical magnetic sector field mass spectrometer are discussed.

(238)U/(235)U isotope ratios of crustal material, rivers and products of hydrothermal alteration: new insights on the oceanic U isotope mass balance.

In this study, the U isotope composition, n((238)U)/n((235)U), of major components of the upper continental crust, including granitic rocks of different age and post-Archaean shales, as well as that of rivers (the major U source to the oceans) was investigated. Furthermore, U isotope fractionation during the removal of U at mid-ocean ridges, an important sink for U from the oceans, was investigated by the analyses of hydrothermal water samples (including low- and high-temperature fluids), low-temperature altered basalts and calcium carbonate veins. All analysed rock samples from the continental crust fall into a limited range of δ(238)U between -0.45 and -0.21 ‰ (relative to NBL CRM 112-A), with an average of -0.30 ± 0.15 ‰ (2 SD, N = 11). Despite differences in catchment lithologies, all major rivers define a relatively narrow range between -0.31 and -0.13 ‰, with a weighted mean isotope composition of -0.27 ‰, which is indistinguishable from the estimate for the upper continental crust (-0.30 ‰). Only some tributary rivers from the Swiss Alps display a slightly larger range in δ(238)U (-0.29 to +0.01 ‰) and lower U concentrations (0.87-3.08 nmol/kg) compared to the investigated major rivers (5.19-11.69 nmol/kg). These findings indicate that only minor net U isotope fractionation occurs during weathering and transport of material from the continental crust to the oceans. Altered basalts display moderately enriched U concentrations (by a factor of 3-18) compared to those typically observed for normal mid-ocean ridge basalts. These, and carbonate veins within altered basalts, show large U isotope fractionation towards both heavy and light U isotope compositions (ranging from -0.63 to +0.27 ‰). Hydrothermal water samples display low U concentrations (0.3-1 nmol/kg) and only limited variations in their U isotope composition (-0.43 ± 0.25 ‰) around the seawater value. Nevertheless, two of the investigated fluids display significantly lower δ(238)U (-0.55 and -0.59 ‰) than seawater (-0.38 ‰). These findings, together with the heavier U isotope composition observed for some altered basalts and carbonate veins support a model, in which redox processes mostly drive U isotope fractionation. This may result in a slightly heavier U isotope composition of U that is removed from seawater during hydrothermal seafloor alteration compared to that of seawater. Using the estimated isotope compositions of rivers and all U sinks from the ocean (of this study and the literature) for modelling of the isotopic U mass balance, this gives reasonable results for recent estimates of the oceanic U budget. It furthermore provides additional constraints on the relative size of the diverse U sinks and respective net isotope fractionation during U removal.

Gravimetric preparation and characterization of primary reference solutions of molybdenum and rhodium.

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 U rel(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.

The iron stable isotope fingerprint of the human diet.

The stable isotopes of iron disclose the metabolic pathways of iron within the human food chain. We have measured with precise multicollector ICP-MS the iron concentrations and stable isotope composition of 60 food products that are representative of the average German diet. We find that vegetables fall within the range typical of strategy I plants (-0.1 to -1.4‰ in δ(56)Fe), crop products and processed crop foods into the range typical of strategy II plants (-0.6 to +0.4‰), and animal products into the (54)Fe-enriched range known for animal tissue and blood (-1.1 to -2.7‰). Weighting these isotope compositions by the average iron dietary sources, we find a representative composition of European vegetarian diet of -0.45‰, whereas that of omnivores is -0.82‰. For human blood, known to be enriched in light iron isotopes, we find fractionation factors for iron absorption of -2.0 and -2.3‰ for vegetarians (female and male, respectively) and -1.3 and -1.5‰ for omnivores (female and male, respectively). Knowing these fractionation factors is a prerequisite for using stable iron isotope ratios in blood as monitors of intestinal iron uptake regulation.