Isotope analysis of trace and ultra-trace elements in environmental matrices
Abstract: High precision isotope ratio measurements have found increasing application in various branches of science, from classical isotope geochronology to complex multi-tracer experiments in environmental studies. Progress in analytical technologies in recent years has allowed higher quality data to be obtained through mass spectrometry. Major pitfalls lie in challenges to obtain accurate and reliable isotopic data for trace and ultra-trace element by inductively coupled plasma sector field mass spectrometry (ICP-SFMS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Sources of errors must therefore be accurately evaluated and avoided at every procedural step. The present study has been focused on analytical method development, optimization and evaluation (including sample preparation, matrix separation, instrumental analysis and data evaluation stages) for isotopic and multi-elemental analyses in environmental samples at trace and ultra-trace levels. The first method tested and optimized has allowed obtaining both osmium concentration and isotopic ratio (187Os/188Os) data together with broad multi-elemental characterization of samples. The procedure was as follows: digestion of small biological samples spiked with Os (enriched in 190Os) followed by concentration and Os isotope measurements by ICP- SFMS, operated with solution nebulization (SN) introduction system and methane addition to the plasma. A specific gas-phase introduction (GPI) system was adopted for samples with low analyte content (<500 pg) for isotopic measurements. Memory effects were avoided with implementation of disposable plastic lab ware. This resulted in a relatively inexpensive and rapid method with high sample throughput capability. SN method limit of detection (MLOD) did not improve compared to previous studies (2 pg g-1), while a MLOD of 0.2 pg g-1 obtained with GPI represents an important improvement, especially taking into account potential further gains due to refined digestion stage. Method reproducibility for 187Os/188Os evaluated using in-house control samples was better than 1.5% RSD. The method, successively applied to a large scale bio-monitoring case, confirmed the presence of an anthropogenic Os in animals from an area affected by emissions from a stainless steel foundry. The second method has been focused on Cd concentration and isotope ratio measurements. Different analytical stages were critically evaluated and optimized for processing carbon-rich environmental samples. Several digestion methods were tested (high pressure ashing, microwave, ultrawave and ashing) followed by ion-exchange chromatography to separate analyte from matrix. Concentrations were measured by ICP-SFMS and subsequently isotopic ratio measurements were performed using Neptune Plus MC-ICP-MS. The latter was tested with two different introduction configurations: a standard SN system and a high sensitivity setup (Aridus II desolvating nebulizer). Overall reproducibility of the method was assessed by replicate preparation and Cd isotope ratio measurements in various environmental matrices (soil, sediments, Fe-Mn nodules, sludge, kidney, liver, leaves, mushroom, lichen) and was found to be better than 0.1‰ (2σ for δ114Cd/110Cd) for the majority of samples. Purification performances were evaluated by analyzing the different elution fractions collected at each separation step by ICP-SFMS. Cadmium recovery in the purified fraction was above 95% for all samples analyzed. Spectrally interfering elements such as Mo and Sn might partially co-elute in the Cd fraction and their effect on isotope ratios was carefully investigated. Though the use of a desolvating nebulizer increases the instrumental sensitivity and reduces oxide formation, at the same time it resulted in degraded in-run precision because of poorer signal stability. Accuracy was difficult to assess due to the absence of certified Cd isotopic values. Therefore, Cd isotope data for several commercially-available reference materials are presented and compared with previously published resultswhere available. The method was then applied to variety of biological samples including birch leaves (Betula pubescenes) collected from various locations and different seasons. Birch leaves showed fractionation towards heavier isotopes with a mean δ114Cd/110Cd of 0.7‰ and found to be influenced by seasonal variations. The reason for such fractionation is assumed to be derived from sample uptake through the root system and translocation in the plant. Cd isotopic patterns were compared with other isotopic systems (Zn, Os, Pb) to provide a more comprehensive view of the observed variations.
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