The high-precision cadmium isotope analysis method and the cadmium isotope composition of pelagic sediment and manganese-nodule reference samples

In this paper, a high-precision Cd isotope analysis method on MC-ICP-MS has been established using a multi-collector inductively coupled plasma mass spectrometer (Neptune plus). In the process of analyzing, Standard- Sample Bracketing (SSB) and Ag as an external standard have been combined to correct the mass fractionation of the instrument. Key factors such as the cone combination, the addition of Ag amount, the influence of interfering elements (Pd, Sn, In, and Zn), the concentration effect, and the full-process blanks have been evaluated during the establishment of the method, and the results showed that: 1) The use of the combination of the X cone + the Jet cone is more suitable for the samples with low Cd contents in terms of its sensitivity; 2) The addition of Ag does not affect the results of Cd isotope testing in general, but the amount should be within a reasonable range, and in this study it is determined that the optimal concentration of Ag added should be 30~40ng/mL; 3) The interfering elements In and Zn show hardly any effect on Cd isotope testing, but trace amounts of Sn and Pd might have affected the Cd isotope results, which must be monitored in real time; 4) Concentration effect results show that when the sample/standard ratio of Cd concentration is between 0.25~1.5, satisfactory results could be obtained; 5) The full process blank is less than 42pg, which could meet the needs of high-precision testing of geological samples. The method established in this paper has been utilized for long-term monitoring of NIST SRM 3108 (July 2020~June 2021), yielding the result of 0.00±0.05 (2σ, the same below), which is consistent with the statistically reported values in the literature. Using this method, the Cd isotopic composition of two pelagic sediment elemental content samples GSD-11 (GBW07311) and GSD-12 (GBW07312) and two manganese-nodule samples Nod-A-1 and Nod-P-1 have been analyzed, and the results, -0.29±0.16, -0.06±0.16, 0.09±0.08 and 0.07±0.10 respectively, are consistent with the data and accuracy reported previously, indicating that the method established in this study is accurate and reliable.