Cadmium isotopic composition of biogenic certified reference materials determined by thermal ionization mass spectrometry with double spike correction

Efficient Cd separation protocols for high-precision cadmium isotope analyses of diverse samples by double spike MC-ICP-MS

Cadmium isotope analyses are applied for research in planetary, Earth, environmental and life sciences. However, there is still a lack of efficient methods for the separation of the trace element Cd from the different types of samples that are of interest for isotopic analyses. This study presents new and improved Cd separation and purification techniques for meteorite, diverse terrestrial and seawater samples prior to Cd isotope measurements by multiple collector ICP-MS using the double spike approach for mass bias correction. The first separation stage for meteorites and terrestrial samples employs an existing anion exchange chromatography method that uses AG MP-1M resin, whilst AG 1-X8 and Nobias Chelate PA-1 resins were applied for the initial isolation of Cd from small and large seawater samples, respectively. Teflon microcolumns and AG MP-1M anion exchange resin was then employed for further Cd purification of all sample types. The methods consistently isolate Cd from matrix elements more efficiently than previous protocols, whilst achieving consistently high Cd recoveries, of >95 % for meteorite and terrestrial samples and >80 % for seawater samples, combined with low procedural blanks of <0.08 ng. The performance of the new protocols is demonstrated by repeated Cd isotope analyses of well-characterized geological and biological reference materials, which produced data that are in excellent agreement with results that were obtained with previous, more laborious separation methods. Furthermore, precise Cd isotope compositions (δ114Cd) are presented for the Murchison meteorite and, for the first time, the ordinary chondrite GRO 95504. Finally, analyses of seven seawater samples, with Cd concentrations between 0.25 and 0.91 nmol kg-1 using sample volumes of up to 2 L, generated results consistent with those of previous studies. Overall, the new methods thus enable unbiased δ114Cd measurements for diverse sample types with a 2SD precision of better than ±0.07 ‰, even for samples with 10 ng or less of natural Cd.

Precise Determination of Cd Isotope Ratios at 3-10 ng Level by Thermal Ionization Mass Spectrometry Using a Molybdenum Silicide Emitter

Thermal ionization mass spectrometry (TIMS) combined with the double spike technique has excellent analytical precision for Cd isotopic ratio analysis. However, because of the low ionization efficiency of Cd by TIMS, it is still not possible to obtain high precision Cd isotope ratios for small sample size (<100 ng) due to the lack of a highly sensitive emitter for Cd. A new loading method using molybdenum silicide (MoSi2) emitter has been developed for Cd isotope ratio measurements. This emitter produces a significant enhancement in the ionization efficiency of Cd and thus significantly reduces the required sample size to the 3-10 ng level. Analyses of δ114/110Cd for the NIST SRM 3108 using 108Cd-116Cd double spike method show excellent reproducibility (2 SD) that reaches ±0.032‰, ±0.042‰, and ±0.051‰ for 10, 5, and 3 ng of Cd, respectively. This method was further verified with a suite of geological reference materials. Replicate digestions and analyses (n = 8, 2 SD) of δ114/110Cd for NIST SRM 2711a, NOD A-1, and GBW08401 demonstrated good external reproducibility with results of 0.596 ± 0.024‰ for NIST SRM 2711a, 0.150 ± 0.036‰ for NOD A-1, and -0.665 ± 0.084‰ for GBW08401. These data clearly indicate that MoSi2 is an excellent alternative for traditional silica gel to Cd isotopic measurements, especially for samples with a low content of Cd.

Stable isotopic signature of cadmium in tracing the source, fate, and translocation of cadmium in soil: A review

Cadmium (Cd), one of the most severe environmental pollutants in soil, poses a great threat to food safety and human health. Understanding the potential sources, fate, and translocation of Cd in soil-plant systems can provide valuable information on Cd contamination and its environmental impacts. Stable Cd isotopic ratios (δ114/110Cd) can provide "fingerprint" information on the sources and fate of Cd in the soil environment. Here, we review the application of Cd isotopes in soil, including (i) the Cd isotopic signature of soil and anthropogenic sources, (ii) the interactions of Cd with soil constituents and associated Cd isotopic fractionation, and (iii) the translocation of Cd at soil-plant interfaces and inside plant bodies, which aims to provide an in-depth understanding of Cd transport and migration in soil and soil-plant systems. This review would help to improve the understanding and application of Cd isotopic techniques for tracing the potential sources and (bio-)geochemical cycling of Cd in soil environment.

Stable Isotope Analyses Reveal Impact of Fe and Zn on Cd Uptake and Translocation by Theobroma cacao

High concentrations of toxic cadmium (Cd) in soils are problematic as the element accumulates in food crops such as rice and cacao. A mitigation strategy to minimise Cd accumulation is to enhance the competitive uptake of plant-essential metals. Theobroma cacao seedlings were grown hydroponically with added Cd. Eight different treatments were used, which included/excluded hydroponic or foliar zinc (Zn) and/or iron (Fe) for the final growth period. Analyses of Cd concentrations and natural stable isotope compositions by multiple collector ICP-MS were conducted. Cadmium uptake and translocation decreased when Fe was removed from the hydroponic solutions, while the application of foliar Zn-EDTA may enhance Cd translocation. No significant differences in isotope fractionation during uptake were found between treatments. Data from all treatments fit a single Cd isotope fractionation model associated with sequestration (seq) of isotopically light Cd in roots and unidirectional mobilisation (mob) of isotopically heavier Cd to the leaves (ε114Cdseq-mob = -0.13‱). This result is in excellent agreement with data from an investigation of 19 genetically diverse cacao clones. The different Cd dynamics exhibited by the clones and seen in response to different Fe availability may be linked to similar physiological processes, such as the regulation of specific transporter proteins.

Cadmium accumulation and isotope fractionation in typical protozoa Tetrahymena: A new perspective on remediation of Cd pollution in wastewater

Cadmium (Cd) pollution threatens water safety and human health, which has raised serious public concern. Tetrahymena is a model protozoan, possessing the potential to remediate Cd contaminated water given the rapid expression of thiols. However, the mechanism of Cd accumulation in Tetrahymena has not been well understood, which hinders its application in environmental remediation. This study elucidated the pathway of Cd accumulation in Tetrahymena using Cd isotope fractionation. Our results showed that Tetrahymena preferentially absorb light Cd isotopes, with Δ^114/110Cd_{Tetrahymena-solution} = -0.20 ± 0.02‰ ∼ - 0.29 ± 0.02‰, which implies that the intracellular Cd is probably in the form of Cd-S. The fractionation generated by Cd complexation with thiols is constant (Δ^114/110Cd_{Tetrahymena-remaining solution} ∼ -0.28 ± 0.02‰), which is not affected by the concentrations of Cd in intracellular and culture medium, nor by the physiological changes in cells. Furthermore, the detoxification process of Tetrahymena results in an increase in cellular Cd accumulation from 11.7% to 23.3% with the elevated Cd concentrations in batch Cd stress culture experiments. This study highlights the promising application of Cd isotope fractionation in Tetrahymena for the remediation of heavy metal pollution in water.

Double Spike-Standard Addition Technique and Its Application in Measuring Isotopes

Double spike (DS) method has been extensively used in determining stable isotope ratios of many elements. However, challenges remain in obtaining high-precision isotope data for ultra-trace elements owing to the limitations of instrumental signal-to-noise ratios and the systematics of precision of DS-based measurements. Here, the DS-standard addition (SA) (DSSA) technique is proposed to improve measurements of isotope compositions of ultra-trace elements in natural samples. According to the U-shaped relationship between DS measurement uncertainty and the spike/sample ratio, theoretical equations and an error propagation model (EPM) were constructed comprehensively. In our method, a spiked secondary standard solution with a high, precisely known spike/sample ratio is mixed with samples such that the mixtures have spike/sample ratios within the optimal range. The abundances of the samples relative to the added standards (sample fraction; f_spl) and the samples' isotope ratios can then be obtained exactly using a standard DS data reduction routine and the isotope binary mixing model. The accuracy and precision of the DSSA approach were verified by measurements of cadmium and molybdenum isotopes at as low as 5 ng levels. Compared with traditional DS measurements, the sample size for isotope analysis is reduced to 1/6-1/5 of the original with no loss of measurement precision. The optimal mixing range f_spl = 0.15-0.5 is recommended. The DSSA method can be extended to isotope measurement of more than 33 elements where the DS method is applicable, especially for the ultra-trace elements such as platinum group and rare earth element isotopes.

Cadmium in the hyperaccumulating mushroom Thelephora penicillata: Intracellular speciation and isotopic composition

Thelephora penicillata is an ectomycorrhizal mushroom that can accumulate extraordinarily high concentrations of Cd, As, Cu, and Zn in its fruit-bodies. To better understand its element accumulation ability, we compared the element concentrations in T. penicillata with 10 distinct ectomycorrhizal mushroom species growing at the same site (Karlina Pila, Czech Republic). On average, T. penicillata accumulated 330, 2130, 26, and 4 times more Cd, As, Cu, and Zn, respectively, than other mushrooms. Size-exclusion chromatography and an electrophoretic analysis of T. penicillata cell extracts indicate that intracellular Cd may be present mainly in >1 kDa, presumably compartmentalized, Cd species, and partially binding with 6-kDa cysteinyl-containing peptide(s) resembling metallothioneins. The cadmium isotopic composition of mushroom fruit-bodies, soil digests, and soil extracts was investigated by thermal ionization mass spectrometry (TIMS) with double spike correction. The isotopic composition (δ^114/110Cd) of ectomycorrhizal mushrooms from Karlina Pila varied in a wide range of -0.37 to +0.14 ‰. However, remarkably low δ^114/110Cd values were observed in the majority of the investigated mushrooms when compared to the relatively homogeneous Cd isotopic composition of bulk soil (δ^114/110Cd = +0.09 ‰) and the comparatively heavy isotopic composition of soil extracts (mean δ^114/110Cd values of +0.11 ± 0.01 ‰ and +0.22 ± 0.01 ‰, depending on the extraction method). The isotopic composition of Cd hyperaccumulated in T. penicillata essentially matched the mycoavailable soil Cd fraction. However, most isotopic data indicates isotopic fractionation at the soil/fruit-body interface, which could be of environmental significance.