Li isotopic seasonality in a small catchment at the northeastern Tibetan Plateau: Roles of hydrology and temperature dependency.

Silicate weathering is critical to sustain our habitable planet. Lithium (Li) isotopes enable us to investigate the nature of silicate weathering. A number of riverine Li isotope (δ7Lirw) investigations have been made from polar to equatorial terrains, but there remains no consensus about the controlling mechanisms of both weathering and δ7Lirw. Here we investigated δ7Lirw response to climate by collecting weekly river water samples in a small catchment (the Buha River within the Lake Qinghai basin) on the northeastern Tibetan Plateau, with stable tectonic, lithology, and topography. In the hydrology year of 2007 of the Buha River, we find that during the dry seasons, δ7Lirw ratios show temperature dependency typically, when the groundwater fed the river. During the monsoon seasons, δ7Lirw were obviously lower than the temperature dependency predicted values, when abundant rock dissolved and thereby fresh Li release into rivers. We propose that the hydrology and temperature dependency together play important roles in regulating δ7Lirw ratios in such an alpine small catchment. The mechanism is that long residence time facilitates the equilibrium chemical and Li isotopic fractionation during the dry seasons, so a temperature dependency of δ7Lirw is achieved. In contrast, rapid erosion and weathering contribution of fresh rock-like δ7Li to river water would significantly decrease δ7Lirw ratios during the monsoon seasons. This hypothesis can better interpret previously reported data of seasonal δ7Lirw variation, as a superposition between temperature dependency and hydrology regulation on silicates weathering in the small catchments besides tectonics.

Quantifying the impact of recovery during chromatographic purification on the accuracy of lithium isotopic determination by multi-collector inductively coupled plasma mass spectrometry.

RATIONALE: Lithium (Li) isotopes have increasingly been applied as tracers in Earth and planetary sciences and their effectiveness relies upon accurate and precise Li isotopic data. Nowadays, multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) combined with chromatographic purification is the most common strategy for obtaining Li isotopic ratios in natural samples, with a long-term internal precision better than 0.3‰ in most laboratories. However, there is a large discrepancy in the Li isotopic compositions of the same reference materials determined by MC-ICP-MS among international laboratories (e.g. ca 3.5‰ difference for measurements of homogeneous seawater), which has been attributed to insufficient recovery of Li during chromatographic purification. Despite this recognition, the exact impact of Li recovery during purification on Li isotopic determinations by MC-ICP-MS has never been quantified. METHODS: We employed a normal distribution function to model Li elution curves and quantified the Li isotopic fractionation resulting from Li recovery during chromatographic purification. Furthermore, we compared the calculated and measured relative recovery (R) with the Li isotopic ratios determined by ICP-MS to validate our theoretical calculation. RESULTS: The theoretical calculations showed that R should be higher than 99.8% in order to avoid observable Li isotopic fractionation during chromatographic purification at IEECAS. This idea is further supported by the better long-term external precisions for data with R ≥ 99.8% compared with previous values of 99.5% ≤ R < 99.8%. Our results indicated that the large differences in the reported Li isotopic ratios for homogeneous seawater among international laboratories are probably attributable to Li isotopic fractionation occurring during ion exchange chromatography. CONCLUSIONS: Our theoretical calculation via R is the first quantitative and convenient approach for monitoring Li isotopic fractionation during sample purification, ensuring that R ≥ 99.8% can avoid observable Li isotopic fractionation during purification, which will improve the accuracy of Li isotopic measurements by MC-ICP-MS and the comparability among laboratories.

Determination of Barium Isotopic Ratios in River Water on the Multiple Collector Inductively Coupled Plasma Mass Spectrometer.

Barium (Ba) isotopes have been booming while providing ubiquitous traceability to various geochemical/oceanographical processes in recent years. Accurate and precise determinations of Ba isotopes is the main precondition to apply them to tracing various processes. However, a particular determination method is lacking for Ba isotopic ratios in river water. Compared to the double-spike method which needs expensive spikes, intensive laboratory operation, and complex calibration, sample-standard bracketing (SSB) is an easier and more convenient method to obtain isotopic data on the multiple collector inductively coupled plasma mass spectrometer. In this study, to obtain the Ba isotopic ratio of river-water samples with a rather lower Ba concentration than igneous rock, we experimented with K, Ca, Na, Mg, and Ce doping tests. The acidity and concentration matches to explore suitable SSB method conditions for river water Ba isotopic determinations on an MC-ICP-MS. The results showed no obvious matrix effects to Ba isotopes when K/Ba, Ca/Ba, Na/Ba, and Mg/Ba <5, and Ce/Ba <0.2, indicating that the matrix effects are negligible after column purification of Ba. HNO3 concentration match tests showed that within a 25% acidity difference, there would be also no systematic effects to a Ba isotopic determination. However, Ba isotopic determinations are highly sensitive to the Ba concentration difference between the standard and the sample. We observed a mismatch between the Ba concentration and its isotopic ratios, even when the concentration was strictly within a 5% difference. Nevertheless, the on-peak zero strategy (i.e. subtraction of HNO3, gas, and instrumental blanks) worked well to solve the mismatch situation. With this easy strategy, the Ba isotopic ratios of batch natural rive-water samples with various Ba concentrations will become easily accessible.

Effects of cone combinations on accurate and precise Mg-isotopic determination using multi-collector inductively coupled plasma mass spectrometry.

RATIONALE: High-precision determination of magnesium (Mg) isotopes can now be routinely achieved by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The analytical sensitivity and instrumental mass discrimination behavior of this method are, however, sensitive to the types of sample and skimmer cones used in these measurements, so it is important that these parameters should be investigated. METHODS: Using the sample-standard-bracketing method in the wet-plasma mode, four available combinations of sample and skimmer cones [Jet sample cone + H skimmer cone (Jet + H), standard sample cone + H skimmer cone (Standard + H), standard sample cone + X skimmer cone (Standard + X), and Jet sample cone + X skimmer cone (Jet + X)] were systematically investigated for peak shape, sensitivity, mass discrimination, accuracy, and precision in Mg-isotopic ratio determination using a Neptune plus MC-ICP mass spectrometer. RESULTS: The results showed that different cone combinations do not affect peak shapes but would significantly change the sensitivities for Mg-isotopic determinations. Compared with using the Standard + H, the sensitivities of Mg-isotopic determinations were enhanced by approximately a factor of 1.3, 1.4, and 1.9 by using the Standard + X, the Jet + H, and the Jet + X combinations, with the most stable mass discrimination behaviors obtained by the Jet + H. The instrumental mass fractionation slope for any combination of a modified cone geometry (i.e. Standard + X, Jet + X, and Jet + H) is 0.500, while it is 0.510 for the Standard + H. In addition, the mass discrimination behavior is related to Mg concentrations once the combination is set, indicating the necessity of concentration match during Mg-isotopic determination. CONCLUSIONS: The precision and accuracy of the Jet + H combination are better than those of the other combinations, and this is further supported by the validation of the Mg-isotope data for four international reference materials: Cambridge-1, NASS-6, AGV-2, and BHVO-2. As the Jet + H combination also provides a high signal, this combination gives the most robust strategy for the highly precise and accurate determination of Mg isotopes.

High beam quality in two directions and high efficiency output of a diode laser array by spectral-beam-combining.

Improvement is made in the traditional spectral beam combining structure by adding beam shaping element, namely Beam Transformation System (BTS). Spectral beam combination is performed in a horizontal direction by the external cavity after beam shaping. The effect of smile and the divergence of the slow axis are reduced. A standard semiconductor laser array is used in this experiment. A CW output power of 58.8 W and an electro-optic conversion efficiency of 51% are achieved. The spectral line-width is 12.7 nm. M(2) of 1.3 × 11.6 in horizontal and vertical directions are obtained. The beam quality of the output is close to that of a single emitter of the array in two directions.