High-sensitivity determination of Cd isotopes in low-Cd geological samples by double spike MC-ICP-MS

With new advancements in MC-ICP-MS technology, small Cd isotope fractionations in geological processes have attracted more interests in recent years. However, challenges remain in obtaining high-precision Cd isotope data for geological samples with low Cd concentrations and complex matrices. By an improved Cd purification and cleaning resin scheme for low-Cd samples using AGMP-1M resin with >90% Cd recovery, the ratios of trace elements Sn, Mo, Zr, etc. to Cd in the samples, which could potentially jeopardize the accuracy and precision of Cd isotopes analysis under high-sensitivity setup, were less than 0.001 after purification, and the total procedure blank is ≤0.1 ng. Hence, sample size containing 20-30ng Cd can be employed to purify for isotope measurement. Cadmium isotope fractionation (expressed in δ114/110Cd relative to NIST 3108) during column separation and instrument mass bias was corrected for by a 111Cd-113Cd double spike. NIST 3108 was determined at a concentration of 5 -25 ng mL-1 on Neptune Plus (112Cd signal intensity:~640V/ppm) and 25 -50 ng mL-1 on Nu plasma III (112Cd signal intensity:~280V/ppm) MC-ICP-MS equipped with Aridus II/III desolvator for more than one year, with a totally external analytical precision of 0.034‰ (2SD, N=181). The NIST SRM 2711a, NOD-P-1 and GSS-1 were measured at 5, 10, 20, 25 and 50 ng mL-1 level to be 0.532±0.038‰, 0.133±0.038‰ and 0.098±0.027‰, respectively, indicating good accuracy and precision of Cd isotope analysis can be achieved with an injecting sample size as low as 5 ng Cd. Our method can be reliably applied to various geological reference materials (GRMs). The δ114/110Cd of GRMs with relatively high and low Cd concentrations are consistent with previous studies: NOD-A-1(0.124±0.067‰), GSD-11 (-0.274±0.037‰) and BCR-2 (-0.030±0.063‰). The SGR-1b (0.069±0.049‰), GSH-1(-0.337±0.077‰) and others are reported for the first time in this paper. The δ114/110Cd in basalts and stream sediments is relatively homogenous and overlaps with that of the bulk silicate earth (BSE). The δ114/110Cd in shales and soils varies greatly from -0.694 to 0.532‰. Human hair and other animal organs are enriched in light isotopes relative to BSE, whereas terrestrial plants are enriched in heavy isotopes.