High-Precision Cadmium Stable Isotope Measurements by MC-ICP-MS and Cadmium Isotopic Variations in Geochemical Reference Materials of Stream Sediments
|School||China University of Geosciences|
|Keywords||cadmium isotope separation and enrichment MC-ICP-MS|
With the development of analytical techniques, especially since the1990s, MC-ICP-MS was applied to geoscience, identificating the natural stable isotope variations of the transition metals which make their geochemistry studies developing rapidly, and then, the researches of stable isotopes for the transition metal elements become one of the international geoscience frontier researches. Transition metal elements exist in all kinds of rocks, minerals, soils, geological fluids and biological bodies, involve extensively in ore-forming, rock-forming and biological cycle. Studying the geochemical behavior of their isotopes in detail will make us understand geological and biological processes clearly.As a transition metal element, cadmium is also a typical pro-sulfur element which has similar geochemical characteristics with zinc, often occurs in the lead-zinc deposits. In the marine system, cadmium is a nutritional material, its cycling is influenced by the marine organisms. Currently, the evaporation/condensation and biological processes are proven to be two factors which could lead significant cadmium isotope fractionation. With these characteristics, the isotopic system of cadmium has great prospects in the study of cosmogony, genesis and material sources of mineral deposits, ancient marine environment, tracing the cadmium pollution in the environment.In this paper, based on the related literature, relying on the micro-geochemistry laboratory in the State Key Laboratory of Geological Processes and Mineral Resources, we discussed to establish the chemical pre-treatment and the measurement method of the cadmium isotopic compositions in geological samples in the Neptune Plus MC-ICP-MS, and then, used for geochemical reference materials of stream sediments to obtain their cadmium isotopic compositions and characteristics.This paper mainly includes the following aspects:1. Establishing the pre-treatment method of separation and enrichment for cadmium in geological samples. By comparing the leaching effect of different amounts of resin, it confirmed that different elements could completly elute in their specific elution conditions, being effectively separated; Against the problem of Sn incomplete separation in the conventional pre-treatment processes, Increasing the amount of resin and leaching twice had been used, which reduced the relative content of Sn effectively, decreased the range of Sn/Cd content ratio from0.03-0.19to0.004-0.018in the leaching solution for receiving Cd, kept the recovery of Cd greater than90%. The reliability of this optimized separation and enrichment process was confirmed by the geological standard materials of GSS-1and GSD-12. The results showed that the total procedural blank was230pg, which accounted for0.2%of the cadmium in the entire sample, and the reference solution had a δ114/110Cd of-0.03±0.08(N=3,2sd), both of them effectively eliminated the impacts of procedural blank and column chemistry fractionation on the cadmium isotope analysis.2. Establishing the measurement method of the cadmium isotopic compositons for geochemical reference materials of stream sediments using MC-ICP-MSThe mass discrimination of MC-ICP-MS is one of the most important factors affecting the accurate determination of cadmium isotope, this study used a "sample-standard bracketing" technique to controll it, and also investigated the effect of three different principles (the linear, power and exponential law) to calibrate the mass discrimination, the result showed that all of them could calibrate it effectively during the analysis.1) The pre-treatment results showed that there was Sn in ng/g level existing in the leaching solution for receiving Cd, in response to this phenomenon, the study established the cup configuration which detect110Cd,111Cd,112Cd,113Cd,114Cd,116Cd,117Sn,118Sn signal, and correct the interference from Sn to the cadmium isotope by detecting the117Sn signal;2) The interface part (the cone structure)of MC-ICP-MS affects the precision and accuracy of cadmium isotope analysis. This experiment examined three cone combination (J+X, J+H, N+H cone combination), considering the stability of the determination of different cadmium isotope standard solutions with the content range of50～500ng/g, the interfering calibration of Sn, Zn and their long-term reproducibility. The results showed that the type of cone combination directly affected the sensitivity, precision and accuracy; The cone combination of J+X was the optimum one to the cadmium isotope analysis in Neptune Plus multi-collector inductively coupled plasma mass spectrometry, which could accurately determine the solutions with the concentration ranging from50ng/g to500ng/g; it could obtained the exact cadmium isotopic compositions of solutions when Cd/Sn content ratio is greater than10, Cd/Zn content ratio ranging from2to0.5in the solution by the subtracting correction; this method had a precision of0.001～0.002%(RSD), an error range of only0.06～0.14(δ114/110Cd,2σ), and a long-term reproducibility of0.12(δ114/110Cd,2σ), which achieved the analysis level of international cadmium isotope analysis.3) The concentration effect and polyatomic ion interference are often overlooked in cadmium isotope analysis. This experiment investigated their effect on the determination of cadmium isotopes. The results showed that the Cd isotopic compositions are statistically indistinguishable from the reference value when sample/reference ratios are between0.80and1.20; and The elements of Zr and Nb could cause uncorrectable interference to cadmium isotope analysis when the Zr/Cd, Nb/Cd content ratio greater than0.01. Owing to pre-treament, the content of Zr and Nb was less than lOpg in the eluent for receving Cd which can ignore their effect on the cadmium isotope results.3. Determinating the cadmium isotopic compositions of geochemical reference materials of stream sediments. Based on the established measurement method for geological samples of the cadmium isotopic compositions, we obtained the cadmium isotopic compositons of GSD-3a, GSD-5a, GSD-7a, GSD-6, GSD-9, GSD-10, GSD-11, GSD-12, GSD-23and GSS-1which were firstly reported. The results showed that their δ114/110Cd values ranging from-1.43-+0.20, different regions had different cadmium isotope distribution, and cadmium isotope system can be used to identify the different environmental conditions(especially metallogenic environmental conditions), which shows great application prospects in geoscience.